2.A Hydrologic and Hydraulic Processes€¦ · illary and osmotic forces that keep the moisture in...

22.A Hydrologic and Hydraulic Processes

• Where does stream flow come from?• What processes affect or are involved with stream flow?• How fast, how much, how deep, how often and when does water flow?• How is hydrology different in urban stream corridors?

2.B Geomorphic Processes• What factors affect the channel cross section and channel profile?• How are water and sediment related?• Where does sediment come from and how is it transported downstream?• What is an equilibrium channel?• What should a channel look like in cross section and in profile?• How do channel adjustments occur?• What is a floodplain?• Is there an important relationship between a stream and its floodplain?

2.C Chemical Processes• What are the major chemical constituents of water?• What are some important relationships between physical habitat and key

chemical parameters?• How are the chemical and physical parameters critical to the aquatic life in a

stream corridor?• What are the natural chemical processes in a stream corridor and water column?• How do disturbances in the stream corridor affect the chemical characteristics of

stream water?

2.D Biological Processes• What are the important biological components of a stream corridor?• What biological activities and organisms can be found within a stream corridor?• How does the structure of stream corridors support various populations of organisms?• What are the structural features of aquatic systems that contribute to the biological diversity

of stream corridors?• What are some important biological processes that occur within a stream corridor?• What role do fish have in stream corridor restoration?

2.E Stream Corridor Functions and Dynamic Equilibrium• What are the major ecological functions of stream corridors?• How are these ecological functions maintained over time?• Is a stream corridor stable?• Are these functions related?• How does a stream corridor respond to all the natural forces acting on it

(i.e., dynamic equilibrium)?

2

Figure 2.1: A stream corridor inmotion. Processes, characteris-tics, and functions shape streamcorridors and make them lookthe way they do.


2.B Geomorphic Processes

2.C Physical and Chemical Characteristics

2.D Biological Community Characteristics

2.E Functions and Dynamic Equilibrium

hapter 1 provided an overview of stream corridors and the many per-

spectives from which they should beviewed in terms of scale, equilibrium, andspace. Each of these views can be seen asa “snapshot” of different aspects of astream corridor.

Chapter 2 presents the stream corridor inmotion, providing a basic understandingof the different processes that make the

stream corridor look and function the wayit does. While Chapter 1 presented stillimages, this chapter provides “filmfootage” to describe the processes, char-acteristics, and functions of stream corri-dors through time.

Section 2.A: Hydrologic and HydraulicProcesses

Understanding how water flows into andthrough stream corridors is critical torestorations. How fast, how much, how

deep, how often, and when waterflows are importantbasic questions thatmust be answered to

make appropriate decisions aboutstream corridor restoration.

Section 2.B: Geomorphic Processes

This section combines basic hydro-logic processes with physical orgeomorphic functions and charac-teristics. Water flows throughstreams but is affected by the kindsof soils and alluvial features withinthe channel, in the floodplain, andin the uplands. The amount andkind of sediments carried by astream largely determines its equi-librium characteristics, includingsize, shape, and profile. Successfulstream corridor restoration,whether active (requiring directchanges) or passive (managementand removal of disturbance fac-tors), depends on an understandingof how water and sediment are re-lated to channel form and functionand on what processes are involvedwith channel evolution.

Section 2.C: Physical and ChemicalCharacteristics

The quality of water in the streamcorridor is normally a primary ob-jective of restoration, either to im-prove it to a desired condition, orto sustain it. Restoration shouldconsider the physical and chemicalcharacteristics that may not bereadily apparent but that are

nonetheless critical to the functionsand processes of stream corridors.Changes in soil or water chemistryto achieve restoration goals usuallyinvolve managing or altering ele-ments in the landscape or corridor.

Section 2.D: Biological CommunityCharacteristics

The fish, wildlife, plants, and hu-mans that use, live in, or just visitthe stream corridor are key ele-ments to consider in restoration.Typical goals are to restore, create,enhance, or protect habitat to ben-efit life. It is important to under-stand how water flows, howsediment is transported, and howgeomorphic features and processesevolve; however, a prerequisite tosuccessful restoration is an under-standing of the living parts of thesystem and how the physical andchemical processes affect thestream corridor.

Section 2.E: Functions andDynamic Equilibrium

The six major functions of streamcorridors are: habitat, conduit,barrier, filter, source, and sink.The integrity of a stream corridorecosystem depends on how wellthese functions operate. Thissection discusses these functionsand how they relate to dynamicequilibrium.

2–2 Chapter 2: Stream Corridor Processes, Characteristics, and Functions

The hydrologic cycle describes the contin-uum of the transfer of water from pre-cipitation to surface water and groundwater, to storage and runoff, and to theeventual return to the atmosphere bytranspiration and evaporation (Figure2.2).

Precipitation returns water to the earth’ssurface. Although most hydrologicprocesses are described in terms of rain-fall events (or storm events), snowmeltis also an important source of water, es-pecially for rivers that originate in highmountain areas and for continental re-

gions that experience seasonal cycles ofsnowfall and snowmelt.

The type of precipitation that will occuris generally a factor of humidity and airtemperature. Topographic relief and ge-ographic location relative to large waterbodies also affect the frequency andtype of precipitation. Rainstorms occurmore frequently along coastal and low-latitude areas with moderate tempera-tures and low relief. Snowfalls occurmore frequently at high elevations andin mid-latitude areas with colder sea-sonal temperatures.

Hydrologic and Hydraulic Processes 2–3


rain cloudsevaporation

precipitation

infiltration

percolation

soil

ground water

rock ocean

lakestorage

deep percolation

cloud formation

surface runoff

from

vege

tati

onfr

omst

ream

s

fro

mso

il

fro

mo

cean

tran

spir

atio

n

Figure 2.2: The hydrologic cycle. The transfer of water from precipitation to surface water andground water, to storage and runoff, and eventually back to the atmosphere is an ongoing cycle.

Precipitation can do one of three thingsonce it reaches the earth. It can returnto the atmosphere; move into the soil;or run off the earth’s surface into astream, lake, wetland, or other waterbody. All three pathways play a role indetermining how water moves into,across, and down the stream corridor.

This section is divided into two subsec-tions. The first subsection focuses onhydrologic and hydraulic processes inthe lateral dimension, namely, themovement of water from the land intothe channel. The second subsectionconcentrates on water as it moves in thelongitudinal dimension, specifically asstreamflow in the channel.

Hydrologic and HydraulicProcesses Across the StreamCorridor

Key points in the hydrologic cycle serveas organizational headings in this sub-section:

■ Interception, transpiration, andevapotranspiration.

■ Infiltration, soil moisture, andground water.

■ Runoff.

Interception, Transpiration, andEvapotranspiration

More than two-thirds of the precipita-tion falling over the United States evap-orates to the atmosphere rather thanbeing discharged as streamflow to theoceans. This “short-circuiting” of thehydrologic cycle occurs because of thetwo processes, interception and transpi-ration.

Interception

A portion of precipitation never reachesthe ground because it is intercepted byvegetation and other natural and con-structed surfaces. The amount of water

intercepted in this manner is determinedby the amount of interception storageavailable on the above-ground surfaces.

In vegetated areas, storage is a functionof plant type and the form and densityof leaves, branches, and stems (Table2.1). Factors that affect storage inforested areas include:

■ Leaf shape. Conifer needles holdwater more efficiently than leaves.On leaf surfaces droplets run togeth-er and roll off. Needles, however,keep droplets separated.

■ Leaf texture. Rough leaves store morewater than smooth leaves.

■ Time of year. Leafless periods provideless interception potential in thecanopy than growing periods; howev-er, more storage sites are created byleaf litter during this time.

■ Vertical and horizontal density. Themore layers of vegetation that precip-itation must penetrate, the less likelyit is to reach the soil.

■ Age of the plant community. Somevegetative stands become more densewith age; others become less dense.

The intensity, duration, and frequencyof precipitation also affect levels of in-terception.

Figure 2.3 shows some of the pathwaysrainfall can take in a forest. Rainfall at

Forests

Deciduous

Coniferous

Crops

Alfalfa

Corn

Oats

Grasses

Vegetative Type

13

28

36

16

7

10–20

% Precipitation Intercepted

Table 2.1: Percentage of precipitation inter-cepted for various vegetation types.Source: Dunne and Leopold 1978.


the beginning of a storm initially fillsinterception storage sites in the canopy.As the storm continues, water held inthese storage sites is displaced. The dis-placed water drops to the next lowerlayer of branches and limbs and fillsstorage sites there. This process is re-peated until displaced water reaches thelowest layer, the leaf litter. At this point,water displaced off the leaf litter eitherinfiltrates the soil or moves downslopeas surface runoff.

Antecedent conditions, such as mois-ture still held in place from previousstorms, affect the ability to interceptand store additional water. Evaporationwill eventually remove water residingin interception sites. How fast thisprocess occurs depends on climaticconditions that affect the evaporationrate.

Interception is usually insignificant inareas with little or no vegetation. Baresoil or rock has some small imperme-able depressions that function as inter-ception storage sites, but typically mostof the precipitation either infiltrates thesoil or moves downslope as surfacerunoff. In areas of frozen soil, intercep-tion storage sites are typically filledwith frozen water. Consequently, addi-tional rainfall is rapidly transformedinto surface runoff.

Interception can be significant in largeurban areas. Although urban drainagesystems are designed to quickly movestorm water off impervious surfaces, theurban landscape is rich with storagesites. These include flat rooftops, park-ing lots, potholes, cracks, and otherrough surfaces that can intercept andhold water for eventual evaporation.

Transpiration and Evapotranspiration

Transpiration is the diffusion of watervapor from plant leaves to the atmos-phere. Unlike intercepted water, whichoriginates from precipitation, transpired

water originates from water taken in byroots.

Transpiration from vegetation and evap-oration from interception sites andopen water surfaces, such as ponds andlakes, are not the only sources of waterreturned to the atmosphere. Soil mois-ture also is subject to evaporation.Evaporation of soil moisture is, how-ever, a much slower process due to cap-illary and osmotic forces that keep themoisture in the soil and the fact thatvapor must diffuse upward through soilpores to reach surface air at a lowervapor pressure.

Because it is virtually impossible to sep-arate water loss due to transpiration


precipitationcanopyinterceptionand evaporation

litterinterceptionand evaporation

throughfall

stemflow

throughfall

throughfall

net rainfall entering the soilmineral soil

litter

understory

Figure 2.3: Typical pathways for forest rainfall. A portion of precipitation never reaches theground because it is intercepted by vegetationand other surfaces.


30–40 inches

60–70 inches

<20 inches

70–80 inches

20–30 inches

50–60 inches40–50 inches

>80 inches

Figure 2.4: Mean annual lake evaporation for the period 1946–1955.Source: Dunne and Leopold (1978) modified from Kohler et al. (1959).

Water is subject to evaporation whenever it isexposed to the atmosphere. Basically this processinvolves:

■ The change of state of water from liquid tovapor

■ The net transfer of this vapor to the atmosphere

The process begins when some molecules in theliquid state attain sufficient kinetic energy (primari-ly from solar energy) to overcome the forces ofsurface tension and move into the atmosphere.This movement creates a vapor pressure in theatmosphere.

The net rate of movement is proportional to thedifference in vapor pressure between the watersurface and the atmosphere above that surface.Once the pressure is equalized, no more evapora-tion can occur until new air, capable of holdingmore water vapor, displaces the old saturated air.Evaporation rates therefore vary according to lati-tude, season, time of day, cloudiness, and windenergy. Mean annual lake evaporation in theUnited States, for example, varies from 20 inchesin Maine and Washington to about 86 inches inthe desert Southwest (Figure 2.4).

from water loss due to evaporation, thetwo processes are commonly combinedand labeled evapotranspiration. Evapo-transpiration can dominate the waterbalance and can control soil moisturecontent, ground water recharge, andstreamflow.

The following concepts are importantwhen describing evapotranspiration:

■ If soil moisture conditions are limit-ing, the actual rate of evapotranspira-tion is below its potential rate.

■ When vegetation loses water to theatmosphere at a rate unlimited bythe supply of water replenishing theroots, its actual rate of evapotranspi-ration is equal to its potential rate ofevapotranspiration.

The amount of precipitation in a regiondrives both processes, however. Soiltypes and rooting characteristics alsoplay important roles in determining theactual rate of evapotranspiration.

Infiltration, Soil Moisture, andGround Water

Precipitation that is not intercepted orflows as surface runoff moves into thesoil. Once there, it can be stored in theupper layer or move downward throughthe soil profile until it reaches an areacompletely saturated by water called thephreatic zone.

Infiltration

Close examination of the soil surface re-veals millions of particles of sand, silt,and clay separated by channels of differ-ent sizes (Figure 2.5). These macroporesinclude cracks, “pipes” left by decayedroots and wormholes, and pore spacesbetween lumps and particles of soil.

Water is drawn into the pores by gravityand capillary action. Gravity is thedominant force for water moving intothe largest openings, such as worm orroot holes. Capillary action is the domi-


rain

wettedgrains

drygrains

gravitatio

nal

force

rain

wettedgrains

drygrains

capillary

force

drygrains

wettedgrains

capillary

force

Figure 2.5: Soil profile. Water is drawn into thepores in soil by gravity and capillary action.

nant force for water moving into soilswith very fine pores.

The size and density of these poreopenings determine the water’s rate ofentry into the soil. Porosity is the termused to describe the percentage of thetotal soil volume taken up by spaces be-tween soil particles. When all thosespaces are filled with water, the soil issaid to be saturated.

Soil characteristics such as texture andtilth (looseness) are key factors in deter-mining porosity. Coarse-textured, sandysoils and soils with loose aggregatesheld together by organic matter or smallamounts of clay have large pores and,thus, high porosity. Soils that are tightlypacked or clayey have low porosity.

Infiltration is the term used to describethe movement of water into soil pores.The infiltration rate is the amount ofwater that soaks into soil over a givenlength of time. The maximum rate thatwater infiltrates a soil is known as thesoil’s infiltration capacity.

If rainfall intensity is less than infiltra-tion capacity, water infiltrates the soil ata rate equal to the rate of rainfall. If therainfall rate exceeds the infiltration ca-

pacity, the excess water either is de-tained in small depressions on the soilsurface or travels downslope as surfacerunoff (Figure 2.6).

The following factors are important indetermining a soil’s infiltration rate:

■ Ease of entry through the soil surface.

■ Storage capacity within the soil.

■ Transmission rate through the soil.

Areas with natural vegetative cover andleaf litter usually have high infiltrationrates. These features protect the surfacesoil pore spaces from being plugged byfine soil particles created by raindropsplash. They also provide habitat forworms and other burrowing organismsand provide organic matter that helpsbind fine soil particles together. Both ofthese processes increase porosity andthe infiltration rate.

The rate of infiltration is not constantthroughout the duration of a storm.The rate is usually high at the begin-ning of a storm but declines rapidly asgravity-fed storage capacity is filled.A slower, but stabilized, rate of infiltra-tion is reached typically 1 or 2 hoursinto a storm. Several factors are in-


runoff

0.5 inches/hr

rainfall1.5 inches/hr

infiltration1 inch/hr

rainfall.75 inches/hr

infiltration.75 inches/hr

A. Infiltration Rate = rainfall rate, which is less thaninfiltration capacity

B. Runoff Rate = rainfall rate minus infiltration capacity

Figure 2.6: Infiltration and runoff. Surface runoff occurs when rainfall intensity exceeds infiltrationcapacity.

volved in this stabilization process,including the following:

■ Raindrops breaking up soil aggregatesand producing finer material, whichthen blocks pore openings on the sur-face and reduces the ease of entry.

■ Water filling fine pore spaces andreducing storage capacity.

■ Wetted clay particles swelling andeffectively reducing the diameter ofpore spaces, which, in turn, reducestransmission rates.

Soils gradually drain or dry following astorm. However, if another storm occursbefore the drying process is completed,there is less storage space for new water.Therefore, antecedent moisture condi-tions are important when analyzingavailable storage.

Soil Moisture

After a storm passes, water drains out ofupper soils due to gravity. The soil re-mains moist, however, because someamount of water remains tightly held infine pores and around particles by sur-face tension. This condition, called fieldcapacity, varies with soil texture. Likeporosity, it is expressed as a proportionby volume.

The difference between porosity andfield capacity is a measure of unfilledpore space (Figure 2.7). Field capacityis an approximate number, however, be-cause gravitation drainage continues inmoist soil at a slow rate.

Soil moisture is most important in thecontext of evapotranspiration. Terrestrialplants depend on water stored in soil.As their roots extract water from pro-gressively finer pores, the moisture con-tent in the soil may fall below the fieldcapacity. If soil moisture is not replen-ished, the roots eventually reach a pointwhere they cannot create enough suc-tion to extract the tightly held interstitial

pore water. The moisture content of thesoil at this point, which varies depend-ing on soil characteristics, is called thepermanent wilting point because plantscan no longer withdraw water from thesoil at a rate high enough to keep upwith the demands of transpiration, caus-ing the plants to wilt.

Deep percolation is the amount of waterthat passes below the root zone ofcrops, less any upward movement ofwater from below the root zone (Jensenet al. 1990).

Ground Water

The size and quantity of pore openingsalso determines the movement of waterwithin the soil profile. Gravity causes


Pro

po

rtio

n b

y V

olu

me

wiltingpoint

fieldcapacity

porosity

0.60

heavyclay loam

clay

clay loam

silt loam

loam

finesandy loam

sand

fine sand

sandy loamlight clay loam

0.50

0.40

0.30

0.20

0.10

0

unfilledpore space

Figure 2.7: Water-holding properties of varioussoils. Water-holding properties vary by texture.For a fine sandy loam the approximate differ-ence between porosity, 0.45, and field capacity,0.20, is 0.25, meaning that the unfilled porespace is 0.25 times the soil volume. The differ-ence between field capacity and wilting point isa measure of unfilled pore space.Source: Dunne and Leopold 1978.

water to move vertically downward.This movement occurs easily throughlarger pores. As pores reduce in size dueto swelling of clay particles or filling ofpores, there is a greater resistance toflow. Capillary forces eventually takeover and cause water to move in anydirection.

Water will continue to move downwarduntil it reaches an area completely satu-rated with water, the phreatic zone orzone of saturation (Figure 2.8). The topof the phreatic zone defines the groundwater table or phreatic surface. Justabove the ground water table is an areacalled the capillary fringe, so named be-cause the pores in this area are filledwith water held by capillary forces.

In soils with tiny pores, such as clay orsilt, the capillary forces are strong. Con-sequently, the capillary fringe can ex-tend a large distance upward from thewater table. In sandstone or soils withlarge pores, the capillary forces are weakand the fringe narrow.

Between the capillary fringe and the soilsurface is the vadose zone, or the zone of

aeration. It contains air and microbialrespiratory gases, capillary water, andwater moving downward by gravity tothe phreatic zone. Pellicular water is thefilm of ground water that adheres to in-dividual particles above the groundwater table. This water is held above thecapillary fringe by molecular attraction.

If the phreatic zone provides a consis-tent supply of water to wells, it isknown as an aquifer. Good aquifersusually have a large lateral and verticalextent relative to the amount of waterwithdrawn from wells and high poros-ity, which allows water to drain easily.

The opposite of an aquifer is anaquitard or confining bed. Aquitards orconfining beds are relatively thin sedi-ment or rock layers that have low per-meability. Vertical water movementthrough an aquitard is severely re-stricted. If an aquifer has no confininglayer overlying it, it is known as anunconfined aquifer. A confined aquifer isone confined by an aquitard.

The complexity and diversity of aquifersand aquitards result in a multitude of


water table

unconfined aquifer

aquitard

bedrock

confining bed

confiningbed

springwater table

capillaryfringe

ground water(phreatic water)

seep

water tablewell

flowingartesianwell

gainingstream

losingstream

potentimetricsurface

perched watertable and aquifer

confined aquifer

vadose zone

zone ofsaturation

landsurface

Figure 2.8: Groundwater related fea-tures and terminolo-gy. Ground waterelevation along thestream corridor canvary significantly overshort distances,depending on subsur-face characteristics.Source: USGS WaterSupply Paper #1988,972, Definitions of

Selected Ground WaterTerms.

underground scenarios. For example,perched ground water occurs when a shal-low aquitard of limited size preventswater from moving down to thephreatic zone. Water collects above theaquitard and forms a “mini-phreaticzone.” In many cases, perched groundwater appears only during a storm orduring the wet season. Wells tappingperched ground water may experience ashortage of water during the dry season.Perched aquifers can, however, be im-portant local sources of ground water.

Artesian wells are developed in con-fined aquifers. Because the hydrostaticpressure in confined aquifers is greaterthan atmospheric pressure, water levelsin artesian wells rise to a level where at-mospheric pressure equals hydrostaticpressure. If this elevation is above theground surface, water can flow freelyout of the well.

Water also will flow freely where theground surface intersects a confinedaquifer. The piezometric surface is thelevel to which water would rise in wellstapped into confined aquifers if thewells extended indefinitely above theground surface. Phreatic wells drawwater from below the phreatic zone inunconfined aquifers. The water level ina phreatic well is the same as theground water table.

Practitioners of stream corridor restora-tion should be concerned with locationswhere ground water and surface waterare exchanged. Areas that freely allowmovement of water to the phreatic zoneare called recharge areas. Areas where thewater table meets the soil surface orwhere stream and ground water emergeare called springs or seeps.

The volume of ground water and theelevation of the water table fluctuateaccording to ground water rechargeand discharge. Because of the fluctua-tion of water table elevation, a stream

channel can function either as arecharge area (influent or “losing”stream) or a discharge area (effluentor “gaining” stream).

Runoff

When the rate of rainfall or snowmeltexceeds infiltration capacity, excesswater collects on the soil surface andtravels downslope as runoff. Factorsthat affect runoff processes include cli-mate, geology, topography, soil charac-teristics, and vegetation. Average annualrunoff in the contiguous United Statesranges from less than 1 inch to morethan 20 inches (Figure 2.9).

Three basic types of runoff are intro-duced in this subsection (Figure 2.10):

■ Overland flow

■ Subsurface flow

■ Saturated overland flow

Each of these runoff types can occur in-dividually or in some combination inthe same locale.

Overland Flow

When the rate of precipitation exceedsthe rate of infiltration, water collects onthe soil surface in small depressions(Figure 2.11). The water stored in thesespaces is called depression storage. Iteventually is returned to the atmos-phere through evaporation or infiltratesthe soil surface.

After depression storage spaces are filled,excess water begins to move downslopeas overland flow, either as a shallowsheet of water or as a series of smallrivulets or rills. Horton (1933) was thefirst to describe this process in the liter-ature. The term Horton overland flow orHortonian flow is commonly used.

The sheet of water increases in depthand velocity as it moves downhill. As ittravels, some of the overland flow istrapped on the hillside and is called sur-


face detention. Unlike depression stor-age, which evaporates to the atmos-phere or enters the soil, surfacedetention is only temporarily detainedfrom its journey downslope. It eventu-ally runs off into the stream and is stillconsidered part of the total volume ofoverland flow.

Overland flow typically occurs in urbanand suburban settings with paved andimpermeable surfaces. Paved areas andsoils that have been exposed and com-pacted by heavy equipment or vehiclesare also prime settings for overlandflow. It is also common in areas of thinsoils with sparse vegetative cover suchas in mountainous terrain of arid orsemiarid regions.

Subsurface Flow

Once in the soil, water moves in re-sponse to differences in hydraulic head(the potential for flow due to the gradi-ent of hydrostatic pressure at differentelevations). Given a simplified situa-

tion, the water table before a rainstormhas a parabolic surface that slopes to-ward a stream. Water moves downwardand along this slope and into thestream channel. This portion of theflow is the baseflow. The soil below thewater table is, of course, saturated. As-suming the hill slope has uniform soilcharacteristics, the moisture content ofsurface soils diminishes with distancefrom the stream.

During a storm, the soil nearest thestream has two important attributes ascompared to soil upslope—a highermoisture content and a shorter distanceto the water table. These attributes causethe water table to rise more rapidly inresponse to rainwater infiltration andcauses the water table to steepen. Thus anew, storm-generated ground watercomponent is added to baseflow. Thisnew component, called subsurface flow,mixes with baseflow and increasesground water discharge to the channel.


10–20 inches1–10 inches

>20 inches

<1 inch Figure 2.9: Averageannual runoff in thecontiguous UnitedStates. Averageannual runoff varieswith regions.Source: USGS 1986.

In some situations, infiltrated stormwater does not reach the phreatic zonebecause of the presence of an aquitard.In this case, subsurface flow does notmix with baseflow, but also dischargeswater into the channel. The net result,whether mixed or not, is increasedchannel flow.

Saturated Overland Flow

If the storm described above continues,the slope of the water table surface cancontinue to steepen near the stream.Eventually, it can steepen to the pointthat the water table rises above thechannel elevation. Additionally, groundwater can break out of the soil andtravel to the stream as overland flow.This type of runoff is termed quick returnflow.

The soil below the ground water break-out is, of course, saturated. Conse-quently, the maximum infiltration rateis reached, and all of the rain fallingon it flows downslope as overlandrunoff. The combination of this directprecipitation and quick return flow iscalled saturated overland flow. As thestorm progresses, the saturated area ex-

pands further up the hillside. Becausequick return flow and subsurface floware so closely linked to overland flow,they are normally considered part ofthe overall runoff of surface water.

Hydrologic and HydraulicProcesses Along the StreamCorridor

Water flowing in streams is the collectionof direct precipitation and water thathas moved laterally from the land intothe channel. The amount and timing ofthis lateral movement directly influences


precip

itation

precip

itation

saturatedoverlandflow

watertable

Hortonoverland flow

litter layer

shallowsubsurface flow

gro

un

dw

ater flow

Figure 2.10: Flowpaths of water overa surface. The por-tion of precipitationthat runs off orinfiltrates to theground water tabledepends on the soil’spermeability rate;surface roughness;and the amount,duration, and intensi-ty of precipitation.

surfacedetention depth and

velocity of overland flow increase downslope

depression storage (depth of depressions greatly exaggerated)

streamchannel

Figure 2.11: Overland flow and depressionstorage. Overland moves downslope as anirregular sheet.Source: Dunne and Leopold 1978.

the amount and timing of streamflow,which in turn influences ecologicalfunctions in the stream corridor.

Flow Analysis

Flows range from no flow to flood flowsin a variety of time scales. On a broadscale, historical climate records revealoccasional persistent periods of wet anddry years. Many rivers in the UnitedStates, for example, experienced a de-cline in flows during the “dust bowl”decade in the 1930s. Another similar de-cline in flows nationwide occurred inthe 1950s. Unfortunately, the length ofrecord regarding wet and dry years isshort (in geologic time), making it isdifficult to predict broad-scale persis-tence of wet or dry years.

Seasonal variations of streamflow aremore predictable, though somewhatcomplicated by persistence factors. Be-cause design work requires using histor-ical information (period of record) as abasis for designing for the future, flow

information is usually presented in aprobability format. Two formats are es-pecially useful for planning and design-ing stream corridor restoration:

■ Flow duration, the probability a givenstreamflow was equaled or exceededover a period of time.

■ Flow frequency, the probability agiven streamflow will be exceeded(or not exceeded) in a year.(Sometimes this concept is modifiedand expressed as the average numberof years between exceeding [or notexceeding] a given flow.)

Figure 2.12 presents an example of aflow frequency expressed as a series ofprobability curves. The graph displaysmonths on the x-axis and a range ofmean monthly discharges on the y-axis.The curves indicate the probability thatthe mean monthly discharge will beless than the value indicated by thecurve. For example, on about January 1,there is a 90 percent chance that the


Mea

n M

on

thly

Dis

char

ge

(cfs

)

Oct.

Month

5000

0

10000

15000

Nov. Dec. Jan. Feb. Mar. April May June July Aug. Sept.

90%

75%

50%

25%

10%

Figure 2.12: An example of monthly probability curves. Monthly probability that the meanmonthly discharge will be less than the values indicated. Yakima River near Parker, Washington.(Data from U.S. Army Corps of Engineers.)Source: Dunne and Leopold 1978.

FASTFORWARD

Preview Chap-ter 7, SectionA for more de-tailed informa-tion aboutflow durationand frequency.

Geomorphic Processes 2–15

discharge will be less than 9,000 cfsand a 50 percent chance it will be lessthan 2,000 cfs.

Ecological Impacts of Flow

The variability of streamflow is a pri-mary influence on the biotic and abioticprocesses that determine the structureand dynamics of stream ecosystems(Covich 1993). High flows are impor-tant not only in terms of sedimenttransport, but also in terms of recon-necting floodplain wetlands to thechannel.

This relationship is important becausefloodplain wetlands provide spawningand nursery habitat for fish and, later inthe year, foraging habitat for waterfowl.Low flows, especially in large rivers,create conditions that allow tributaryfauna to disperse, thus maintaining

populations of a single species in sev-eral locations.

In general, completion of the life cycleof many riverine species requires anarray of different habitat types whosetemporal availability is determinedby the flow regime. Adaptation to thisenvironmental dynamism allows river-ine species to persist during periodsof droughts and floods that destroyand recreate habitat elements (Poffet al. 1997).

Geomorphology is the study of surfaceforms of the earth and the processesthat developed those forms. The hydro-logic processes discussed in the previ-ous section drive the geomorphicprocesses described in this section. Inturn, the geomorphic processes are theprimary mechanisms for forming thedrainage patterns, channel, floodplain,terraces, and other watershed andstream corridor features discussed inChapter 1.

Three primary geomorphic processesare involved with flowing water, as fol-lows:

■ Erosion, the detachment of soil parti-cles.

■ Sediment transport, the movement oferoded soil particles in flowing water.

■ Sediment deposition, settling of erod-ed soil particles to the bottom of awater body or left behind as waterleaves. Sediment deposition can betransitory, as in a stream channelfrom one storm to another, or moreor less permanent, as in a largerreservoir.

Since geomorphic processes are soclosely related to the movement ofwater, this section is organized intosubsections that mirror the hydrologicprocesses of surface storm water runoffand streamflow:

■ Geomorphic Processes Across theStream Corridor

■ Geomorphic Processes Along theStream Corridor

2.B Geomorphic Processes


Geomorphic Processes Acrossthe Stream Corridor

The occurrence, magnitude, and distrib-ution of erosion processes in water-sheds affect the yield of sediment andassociated water quality contaminantsto the stream corridor.

Soil erosion can occur gradually overa long period, or it can be cyclic orepisodic, accelerating during certainseasons or during certain rainstormevents (Figure 2.13). Soil erosion canbe caused by human actions or by nat-ural processes. Erosion is not a simpleprocess because soil conditions are con-tinually changing with temperature,moisture content, growth stage andamount of vegetation, and the humanmanipulation of the soil for develop-ment or crop production. Tables 2.2and 2.3 show the basic processes thatinfluence soil erosion and the differenttypes of erosion found within the water-shed.

Geomorphic Processes Alongthe Stream Corridor

The channel, floodplain, terraces, andother features in the stream corridor areformed primarily through the erosion,transport, and deposition of sedimentby streamflow. This subsection de-scribes the processes involved withtransporting sediment loads down-stream and how the channel andfloodplain adjust and evolve throughtime.

Sediment Transport

Sediment particles found in the streamchannel and floodplain can be catego-rized according to size. A boulder is thelargest particle and clay is the smallestparticle. Particle density depends on thesize and composition of the particle(i.e., the specific gravity of the mineralcontent of the particle).

No matter the size, all particles in thechannel are subject to being trans-ported downslope or downstream.The size of the largest particle a streamcan move under a given set of hy-draulic conditions is referred to asstream competence. Often, only veryhigh flows are competent to move thelargest particles.

Closely related to stream competence isthe concept of tractive stress, which cre-ates lift and drag forces at the streamboundaries along the bed and banks.Tractive stress, also known as shearstress, varies as a function of flow depthand slope. Assuming constant density,shape, and surface roughness, the largerthe particle, the greater the amount oftractive stress needed to dislodge it andmove it downstream.

The energy that sets sediment particlesinto motion is derived from the effectof faster water flowing past slowerwater. This velocity gradient happensbecause the water in the main body offlow moves faster than water flowing atthe boundaries. This is because bound-

Figure 2.13: Raindrop impact. One of manytypes of erosion.


aries are rough and create friction asflow moves over them which, in turn,slows flow.

The momentum of the faster water istransmitted to the slower boundarywater. In doing so, the faster watertends to roll up the slower water in aspiral motion. It is this shearing mo-tion, or shear stress, that also movesbed particles in a rolling motion down-stream.

Particle movement on the channel bot-tom begins as a sliding or rolling mo-tion, which transports particles alongthe streambed in the direction of flow(Figure 2.14). Some particles also maymove above the bed surface by saltation,a skipping motion that occurs whenone particle collides with another parti-cle, causing it to bounce upward andthen fall back toward the bed.

These rolling, sliding, and skipping mo-tions result in frequent contact of themoving particles with the streambedand characterize the set of moving par-ticles known as bed load. The weight ofthese particles relative to flow velocitycauses them essentially to remain incontact with, and to be supported by,the streambed as they move down-stream.

Surface water runoff

Channelized flow

Wind

Ice

Chemical reactions

Agent

Raindrop impact

Gravity

Sheet, interill, rill, ephemeral gully, classic gully

Rill, ephemeral gully, classic gully, wind, streambank

Wind

Streambank, lake shore

Solution, dispersion

Process

Sheet, interill

Classic gully, streambank, landslide, mass wasting

Table 2.2: Erosion processes.

Interill

Rill

Ephemeral gully

Classic gully

Floodplain scour

Roadside

Streambank

Streambed

Landslide

Wave/shoreline

Urban, construction

Surface mine

Ice gouging

Erosion Type

Sheet and rill

Wind

x

x

Sheet

Erosion/Physical Process

x

x

x

x

x

x

x

x

Concentrated Flow

x

x

x

x

x

MassWasting

Combination

x

x

x

x

x

Table 2.3: Erosion types vs. physical processes.

Direction of shear due to decrease of velocity toward bed.

Tendency of velocity to roll an exposed grain.

Diagram of saltating grains.

Suggested motion of a grain thrown up into turbulent eddies in the flow.

Figure 2.14: Action of water on particles near the streambed. Processes that transport bed loadsediments are a function of flow velocities, particle size, and principles of hydrodynamics.Source: Water in Environmental Planning by Dunne and Leopold © 1978 by W.H. Freeman and Company.Used with permission.


Finer-grained particles are more easilycarried into suspension by turbulent ed-dies. These particles are transportedwithin the water column and are there-fore called the suspended load. Althoughthere may be continuous exchange ofsediment between the bed load andsuspended load of the river, as long assufficient turbulence is present.

Part of the suspended load may be col-loidal clays, which can remain in sus-pension for very long time periods,depending on the type of clay andwater chemistry.

Sediment Transport Terminology

Sediment transport terminology cansometimes be confusing. Because ofthis confusion, it is important to definesome of the more frequently usedterms.

■ Sediment load, the quantity of sedi-ment that is carried past any crosssection of a stream in a specifiedperiod of time, usually a day or ayear. Sediment discharge, the massor volume of sediment passing astream cross section in a unit oftime. Typical units for sediment loadare tons, while sediment dischargeunits are tons per day.

■ Bed-material load, part of the totalsediment discharge that is composedof sediment particles that are thesame size as streambed sediment.

■ Wash load, part of the total sedimentload that is comprised of particlesizes finer than those found in thestreambed.

■ Bed load, portion of the total sedi-ment load that moves on or near thestreambed by saltation, rolling, orsliding in the bed layer.

■ Suspended bed material load, portionof the bed material load that is trans-ported in suspension in the watercolumn. The suspended bed materialload and the bed load comprise thetotal bed material load.

■ Suspended sediment discharge (or sus-pended load), portion of the total sed-iment load that is transported in sus-pension by turbulent fluctuationswithin the body of flowing water.

One way to differentiate the sediment load of a streamis to characterize it based on the immediate source ofthe sediment in transport. The total sediment load in astream, at any given time and location, is divided intotwo parts—wash load and bed-material load. The prima-ry source of wash load is the watershed, including sheetand rill erosion, gully erosion, and upstream streambankerosion. The source of bed material load is primarily thestreambed itself, but includes other sources in the water-shed.

Wash load is composed of the finest sediment particlesin transport. Turbulence holds the wash load in suspen-sion. The concentration of wash load in suspension isessentially independent of hydraulic conditions in thestream and therefore cannot be calculated using mea-sured or estimated hydraulic parameters such as velocityor discharge. Wash load concentration is normally afunction of supply; i.e., the stream can carry as muchwash load as the watershed and banks can deliver (forsediment concentrations below approximately 3000parts per million).

Bed-material load is composed of the sediment of sizeclasses found in the streambed. Bed-material load movesalong the streambed by rolling, sliding, or jumping, andmay be periodically entrained into the flow by turbu-lence, where it becomes a portion of the suspendedload. Bed-material load is hydraulically controlled andcan be computed using sediment transport equationsdiscussed in Chapter 8.

■ Measured load, portion of the totalsediment load that is obtained by thesampler in the sampling zone.

■ Unmeasured load, portion of the totalsediment load that passes beneaththe sampler, both in suspension andon the bed. With typical suspendedsediment samplers this is the lower0.3 to 0.4 feet of the vertical.

The above terms can be combined ina number of ways to give the totalsediment load in a stream (Table 2.4).However, it is important not to com-bine terms that are not compatible.For example, the suspended load andthe bed material load are not compli-mentary terms because the suspendedload may include a portion of the bedmaterial load, depending on the energyavailable for transport. The total sedi-ment load is correctly defined by thecombination of the following terms:

Total Sediment Load =

Bed Material Load + Wash Load

or

Bed Load + Suspended Load

or

Measured Load + Unmeasured Load

Sediment transport rates can be com-puted using various equations or mod-els. These are discussed in the StreamChannel Restoration section of Chapter 8.

Stream Power

One of the principal geomorphic tasksof a stream is to transport particles outof the watershed (Figure 2.15). In thismanner, the stream functions as a trans-porting “machine;” and, as a machine,its rate of doing work can be calculatedas the product of available power multi-plied by efficiency.

Stream power can be calculated as:

ϕ = γ Q S

Where:

ϕ = Stream power (foot-lbs/second-foot)

γ = Specific weight of water (lbs/ft3)

Q = Discharge (ft3/second)

S = Slope (feet/feet)

Sediment transport rates are directly re-lated to stream power; i.e., slope anddischarge. Baseflow that follows thehighly sinuous thalweg (the line thatmarks the deepest points along thestream channel) in a meanderingstream generates little stream power;therefore, the stream’s ability to movesediment, sediment-transport capacity, islimited. At greater depths, the flow fol-lows a straighter course, which increasesslope, causing increased sediment trans-port rates. The stream builds its crosssection to obtain depths of flow andchannel slopes that generate the sedi-ment-transport capacity needed tomaintain the stream channel.

Runoff can vary from a watershed, ei-ther due to natural causes or land usepractices. These variations may changethe size distribution of sediments deliv-ered to the stream from the watershedby preferentially moving particular par-ticle sizes into the stream. It is not un-common to find a layer of sand on topof a cobble layer. This often happenswhen accelerated erosion of sandy soils

Wash load

Tota

l sed

imen

t lo

ad

Suspendedload

Wash load

Suspendedbed-materialload

Bed load Bed load

Bed-materialload

Based onMechanismof Transport

Classification System

Based onParticle Size

Table 2.4: Sediment load terms.



occurs in a watershed and the increasedload of sand exceeds the transport ca-pacity of the stream during events thatmove the sand into the channel.

Stream and Floodplain Stability

A question that normally arises whenconsidering any stream restoration ac-tion is “Is it stable now and will it bestable after changes are made?” The an-swer may be likened to asking an opin-ion on a movie based on only a fewframes from the reel. Although we oftenview streams based on a limited refer-ence with respect to time, it is impor-tant that we consider the long-termchanges and trends in channel crosssection, longitudinal profile, and plan-form morphology to characterize chan-nel stability.

Achieving channel stability requires thatthe average tractive stress maintains astable streambed and streambanks. That

is, the distribution of particle sizes ineach section of the stream remains inequilibrium (i.e., new particles de-posited are the same size and shape asparticles displaced by tractive stress).

Yang (1971) adapted the basic theoriesdescribed by Leopold to explain thelongitudinal profile of rivers, the forma-tion of stream networks, riffles, andpools, and river meandering. All theseriver characteristics and sediment trans-port are closely related. Yang (1971) de-veloped the theory of average streamfall and the theory of least rate of en-ergy expenditure, based on the entropyconcept. These theories state that duringthe evolution toward an equilibriumcondition, a natural stream chooses itscourse of flow in such a manner thatthe rate of potential energy expenditureper unit mass of flow along its course isa minimum.

typicalflow rate

averageparticle sizeon streambottom

Second to Fourth Order StreamFirst Order Stream Fifth to Tenth Order Stream

Figure 2.15: Particle transport. A stream’s total sediment load is the total of all sediment particlesmoving past a defined cross section over a specified time period. Transport rates vary according tothe mechanism of transport.


Corridor Adjustments

Stream channels and their floodplainsare constantly adjusting to the waterand sediment supplied by the water-shed. Successful restoration of degradedstreams requires an understanding ofwatershed history, including both nat-ural events and land use practices, andthe adjustment processes active in chan-nel evolution.

Channel response to changes in waterand sediment yield may occur at differ-ing times and locations, requiring vari-ous levels of energy expenditure. Dailychanges in streamflow and sedimentload result in frequent adjustment ofbedforms and roughness in manystreams with movable beds. Streamsalso adjust periodically to extreme high-and low-flow events, as floods not onlyremove vegetation but create and in-crease vegetative potential along thestream corridor (e.g., low flow periodsallow vegetation incursion into thechannel).

Similar levels of adjustment also maybe brought about by changes in landuse in the stream corridor and the up-land watershed. Similarly, long-termchanges in runoff or sediment yieldfrom natural causes, such as climatechange, wildfire, etc., or human causes,such as cultivation, overgrazing, orrural-to-urban conversions, may lead tolong-term adjustments in channel crosssection and planform that are fre-quently described as channel evolution.

Stream channel response to changes inflow and sediment load have been de-scribed qualitatively in a number ofstudies (e.g., Lane 1955, Schumm1977). As discussed in Chapter 1, oneof the earliest relationships proposedfor explaining stream behavior was sug-gested by Lane (1955), who relatedmean annual streamflow (Q

w) and

channel slope (S) to bed-material sedi-

ment load (Qs) and median particle

size on the streambed (D50

):

Qs

• D50

∼ Qw

• S

Lane’s relationship suggests that a chan-nel will be maintained in dynamicequilibrium when changes in sedimentload and bed-material size are balancedby changes in streamflow or channelgradient. A change in one of these vari-ables causes changes in one or more ofthe other variables such that dynamicequilibrium is reestablished.

Additional qualitative relationshipshave been proposed for interpreting be-havior of alluvial channels. Schumm(1977) suggested that width (b), depth(d), and meander wavelength (L) aredirectly proportional, and that channelgradient (S) is inversely proportional tostreamflow (Q

w) in an alluvial channel:

b, d, LQw

∼ _______S

Schumm (1977) also suggested thatwidth (b), meander wavelength (L),and channel gradient (S) are directlyproportional, and that depth (d) andsinuosity (P) are inversely proportionalto sediment discharge (Q

s) in alluvial

streams:

b, L, SQs

∼ ______d, P

The above two equations may be rewrit-ten to predict direction of change inchannel characteristics, given an in-crease or decrease in streamflow or sedi-ment discharge:

Qw

+ ∼ b+, d+, L+, S–

Qw

– ∼ b–, d–, L–, S+

Qs+ ∼ b+, d–, L+, S+, P–

Qs– ∼ b–, d+, L–, S–, P+

FASTFORWARD

Preview SectionE for a furtherdiscussion ofdynamic equi-librium.


Combining the four equations aboveyields additional predictive relation-ships for concurrent increases or de-creases in streamflow and/or sedimentdischarge:

Qw

+Qs

+ ∼ b+, d+/–, L+, S+/–, P–

Qw

–Qs

– ∼ b–, d+/–, L–, S+/–, P+

Qw

+Qs

– ∼ b+/–, d+, L+/–, S–, P+

Qw

-Qs

+ ∼ b+/–, d–, L+/–, S+, P–

Channel Slope

Channel slope, a stream’s longitudinalprofile, is measured as the difference inelevation between two points in thestream divided by the stream length be-tween the two points. Slope is one ofthe most critical pieces of design infor-mation required when channel modifi-cations are considered. Channel slopedirectly impacts flow velocity, streamcompetence, and stream power. Sincethese attributes drive the geomorphicprocesses of erosion, sediment trans-port, and sediment deposition, channelslope becomes a controlling factor inchannel shape and pattern.

Most longitudinal profiles of streams(See Figs. 1-27 are concave upstream. As described previ-

and 1-28) ously in the discussion of dynamicequilibrium, streams adjust their pro-file and pattern to try to minimize thetime rate of expenditure of potentialenergy, or stream power, present inflowing water. The concave upwardshape of a stream’s profile appears tobe due to adjustments a river makesto help minimize stream power in adownstream direction. Yang (1983)applied the theory of minimum streampower to explain why most longitudinalstreambed profiles are concave upward.In order to satisfy the theory of mini-mum stream power, which is a specialcase of the general theory of minimum

energy dissipation rate (Yang and Song1979), the following equation must besatisfied:

dP dS dQ___ = γQ ___ + S ___ = 0dx dx dx

Where:

P = QS = Stream power

x = Longitudinal distance

Q = Water discharge

S = Water surface or energy slope

γ = Specific weight of water

Stream power has been defined as theproduct of discharge and slope. Sincestream discharge typically increases ina downstream direction, slope mustdecrease in order to minimize streampower. The decrease in slope in a down-stream direction results in the concave-up longitudinal profile.

Sinuosity is not a profile feature, but itdoes affect stream slope. Sinuosity isthe stream length between two pointson a stream divided by the valleylength between the two points. Forexample, if a stream is 2,200 feet longfrom point A to point B, and if a valleylength distance between those twopoints is 1,000 feet, that stream has asinuosity of 2.2. A stream can increaseits length by increasing its sinuosity,resulting in a decrease in slope. Thisimpact of sinuosity on channel slopemust always be considered if channelreconstruction is part of a proposedrestoration.

Pools and Riffles

The longitudinal profile is seldomconstant, even over a short reach. Dif-ferences in geology, vegetation pat-terns, or human disturbances canresult in flatter and steeper reacheswithin an overall profile. Riffles occur


where the stream bottom is higher rel-ative to streambed elevation immedi-ately upstream or downstream. Theserelatively deeper areas are consideredpools. At normal flow, flow velocitiesdecrease in pool areas, allowing finegrained deposition to occur, and in-crease atop riffles due to the increasedbed slope between the riffle crest andthe subsequent pool.

Longitudinal Profile Adjustments

A common example of profile adjust-ment occurs when a dam is constructedon a stream. The typical response todam construction is channel degrada-tion downstream and aggradation up-stream. However, the specific responseis quite complex as can be illustrated byconsidering Lane’s relation. Dams typi-cally reduce peak discharges and sedi-ment supply in the downstream reach.According to Lane’s relation, a decreasein discharge (Q) should be offset by anincrease in slope, yet the decrease insediment load (Q

s) should cause a de-

crease in slope. This response could befurther complicated if armoring occurs(D

50+), which would also cause an in-

crease in slope. Impacts are not limitedto the main channel, but can includeaggradation or degradation on tribu-taries as well. Aggradation often occursat the mouths of tributaries down-stream of dams (and sometimes in theentire channel) due to the reduction ofpeak flows on the main stem. Obvi-ously, the ultimate response will be theresult of the integration of all thesevariables.

Channel Cross Sections

Figure 2.16 presents the type of infor-mation that should be recorded whencollecting stream cross section data. Instable alluvial streams, the high pointson each bank represent the top of thebankfull channel.

The importance of the bankfull channelhas been established. Channel cross sec-tions need to include enough points todefine the channel in relation to a por-tion of the floodplain on each side. Asuggested guide is to include at least onestream width beyond the highest pointon each bank for smaller stream corri-dors and at least enough of the flood-plain on larger streams to clearly defineits character in relation to the channel.

In meandering streams, the channelcross section should be measured inareas of riffles or crossovers. A riffle orcrossover occurs between the apexes oftwo sequential meanders. The effects ofdifferences in resistance to erosion ofsoil layers are prominent in the outsidebends of meanders, and point bars onthe insides of the meanders are con-stantly adjusting to the water and sedi-ment loads being moved by the stream.The stream’s cross section changes muchmore rapidly and frequently in the me-ander bends. There is more variabilityin pool cross sections than in rifflecross sections. The cross section in thecrossover or riffle area is more uniform.

Resistance to Flow and Velocity

Channel slope is an important factor indetermining streamflow velocity. Flowvelocity is used to help predict whatdischarge a cross section can convey. Asdischarge increases, either flow velocity,flow area, or both must increase.

hydrologic floodplain

bankfull width

topographic floodplain

bankfullelevation

bankfull depth

Figure 2.16:Channel cross sec-tion. Informationto record whencollecting streamcross section data.


Roughness plays an important r ole instreams. It helps determine the depth orstage of flow in a stream reach. As flowvelocity slows in a stream reach due toroughness, the depth of flow has to in-crease to maintain the volume of flowthat entered the upstream end of thereach (a concept known as flow conti-nuity). Typical roughness along theboundaries of the stream includes thefollowing:

■ Sediment particles of different sizes.

■ Bedforms.

■ Bank irregularities.

■ The type, amount, and distributionof living and dead vegetation.

■ Other obstructions.

Roughness generally increases with in-creasing particle size. The shape andsize of instream sediment deposits, orbedforms, also contribute to roughness.

Sand-bottom streams are good exam-ples of how bedform roughnesschanges with discharge. At very low dis-charges, the bed of a sand stream maybe dominated by ripple bedforms. Asflow increases even more, sand dunesmay begin to appear on the bed. Eachof these bedforms increases the rough-ness of the stream bottom, which tendsto slow velocity.

The depth of flow also increases due toincreasing roughness. If discharge con-tinues to increase, a point is reachedwhen the flow velocity mobilizes thesand on the streambed and the entirebed converts again to a planar form.The depth of flow may actually decreaseat this point due to the decreasedroughness of the bed. If discharge in-creases further still, antidunes mayform. These bedforms create enoughfriction to again cause the flow depth toincrease. The depth of flow for a givendischarge in sand-bed streams, there-

fore, depends on the bedforms presentwhen that discharge occurs.

Vegetation can also contribute to rough-ness. In streams with boundaries con-sisting of cohesive soils, vegetation isusually the principal component ofroughness. The type and distribution ofvegetation in a stream corridor dependson hydrologic and geomorphicprocesses, but by creating roughness,vegetation can alter these processes andcause changes in a stream’s form andpattern.

Meandering streams offer some resis-tance to flow relative to straightstreams. Straight and meanderingstreams also have different distributionsof flow velocity that are affected by thealignment of the stream, as shown inFigure 2.17. In straight reaches of astream, the fastest flow occurs justbelow the surface near the center of thechannel where flow resistance is lowest(see Figure 2.17 (a) Section G). In me-anders, velocities are highest at the out-side edge due to angular momentum(see Figure 2.17 (b) Section 3). The dif-ferences in flow velocity distribution inmeandering streams result in both ero-sion and deposition at the meanderbend. Erosion occurs at the outside ofbends (cutbanks) from high velocityflows, while the slower velocities at theinsides of bends cause deposition onthe point bar (which also has beencalled the slip-off slope).

The angular momentum of flowthrough a meander bend increases theheight or super elevation at the outsideof the bend and sets up a secondarycurrent of flow down the face of thecut bank and across the bottom of thepool toward the inside of the bend. Thisrotating flow is called helical flow andthe direction of rotation is illustratedon the diagram on the following page bythe arrows at the top and bottom ofcross sections 3 and 4 in the figure.


The distribution of flow velocities instraight and meandering streams is im-portant to understand when planningand designing modifications in streamalignment in a stream corridor restora-tion. Areas of highest velocities generatethe most stream power, so where suchvelocities intersect the stream bound-aries indicates where more durable pro-tection may be needed.

As flow moves through a meander, thebottom water and detritus in the poolare rotated to the surface. This rotationis an important mechanism in movingdrifting and benthic organisms past

predators in pools. Riffle areas are notas deep as pools, so more turbulentflows occur in these shallow zones. Theturbulent flow can increase the dis-solved oxygen content of the water andmay also increase the oxidation andvolatilization of some chemical con-stituents in water.

Another extremely important functionof roughness elements is that they cre-ate aquatic habitat. As one example,the deepest flow depths usually occurat the base of cutbanks. These scourholes or pools create very different

Section G

Section E

Section C

highvelocity

lowvelocity

0

0 2 4 6 8 10 12 14 16 18

1

2

0

1

2

3

0

1

2

3

Dep

th (

feet

)

Horizontal Distance (feet)

Generalized SurfaceStreamlines

Generalized Velocity Distributions

1

3

4

5

2

helical flow

helical flow

helical flow

helical flow

helical flow

helical flow

1

2

34

5

Figure 2.17: Velocity distribution in a(a) straight stream branch and a (b) streammeander. Stream flow velocities are differentthrough pools and riffles, in straight andcurved reaches, across the stream at any point,and at different depths. Velocity distributionalso differs dramatically from baseflow condi-tions through bankfull flows, and flood flows.Source: Leopold et al. 1964. Published by permissionof Dover Publications.

(a) (b)

The quality of water in the stream corri-dor might be a primary objective ofrestoration, either to improve it to a de-sired condition or to sustain it. Estab-lishing an appropriate flow regime andgeomorphology in a stream corridormay do little to ensure a healthy ecosys-tem if the physical and chemical charac-teristics of the water are inappropriate.For example, a stream containing highconcentrations of toxic materials or inwhich high temperatures, low dissolvedoxygen, or other physical/chemicalcharacteristics are inappropriate cannotsupport a healthy stream corridor. Con-versely, poor condition of the streamcorridor—such as lack of riparian shad-ing, poor controls on erosion, or exces-sive sources of nutrients and oxygen-demanding waste—can result in degra-dation of the physical and chemicalconditions within the stream.

This section briefly surveys some of thekey physical and chemical characteristicsof flowing waters. Stream water qualityis a broad topic on which many bookshave been written. The focus here is on

a few key concepts that are relevant tostream corridor restoration. The readeris referred to other sources (e.g.,Thomann and Mueller 1987, Mills et al.1985) for a more detailed treatment.

As in the previous sections, the physicaland chemical characteristics of streamsare examined in both the lateral andlongitudinal perspectives. The lateralperspective refers to the influence of thewatershed on water quality, with partic-ular attention to riparian areas. The lon-gitudinal perspective refers to processesthat affect water quality during trans-port instream.

Physical Characteristics

Sediment

Section 2.B discussed total sedimentloads in the context of the evolution ofstream form and geomorphology. In ad-dition to its role in shaping streamform, suspended sediment plays an im-portant role in water quality, both inthe water column and at the sediment-water interface. In a water quality con-


habitat than occurs in the depositionalenvironment of the slip-off slope.

Active Channels andFloodplains

Floodplains are built by two streamprocesses, lateral and vertical accretion.Lateral accretion is the deposition ofsediment on point bars on the insidesof bends of the stream. The stream lat-erally migrates across the floodplain asthe outside of the meander benderodes and the point bar builds withcoarse-textured sediment. This naturallyoccurring process maintains the crosssection needed to convey water and

sediment from the watershed. Verticalaccretion is the deposition of sedimenton flooded surfaces. This sediment generally is finer textured than pointbar sediments and is considered to bean overbank deposit. Vertical accretionoccurs on top of the lateral accretiondeposits in the point bars; however, lateral accretion is the dominantprocess. It typically makes up 60 to 80percent of the total sediment depositsin floodplains (Leopold et al. 1964). It is apparent that lateral migration ofmeanders is an important naturalprocess since it plays a critical role inreshaping floodplains.

2.C Physical and Chemical Characteristics

Physical and Chemical Characteristics 2–27

text, sediment usually refers to soil par-ticles that enter the water column fromeroding land. Sediment consists of par-ticles of all sizes, including fine clayparticles, silt, and gravel. The term sedi-mentation is used to describe the depo-sition of sediment particles inwaterbodies.

Although sediment and its transportoccur naturally in any stream, changesin sediment load and particle size canhave negative impacts (Figure 2.18).Fine sediment can severely alter aquaticcommunities. Sediment may clog andabrade fish gills, suffocate eggs andaquatic insect larvae on the bottom,and fill in the pore space between bot-tom cobbles where fish lay eggs. Sedi-ment interferes with recreationalactivities and aesthetic enjoyment atwaterbodies by reducing water clarityand filling in waterbodies. Sedimentalso may carry other pollutants into wa-terbodies. Nutrients and toxic chemicalsmay attach to sediment particles onland and ride the particles into surfacewaters where the pollutants may settlewith the sediment or become soluble inthe water column.

Studies have shown that fine sedimentintrusion can significantly impact thequality of spawning habitat (Cooper1965, Chapman 1988). Fine sedimentintrusion into streambed gravels can re-duce permeability and intragravel watervelocities, thereby restricting the supplyof oxygenated water to developingsalmonid embryos and the removal oftheir metabolic wastes. Excessive finesediment deposition can effectivelysmother incubating eggs and entombalevins and fry. A sediment intrusionmodel (Alonso et al. 1996) has beendeveloped, verified, and validated topredict the within-redd (spawning area)sediment accumulation and dissolvedoxygen status.

Sediment Across the Stream Corridor

Rain erodes and washes soil particlesoff plowed fields, construction sites,logging sites, urban areas, and strip-mined lands into waterbodies. Erodingstreambanks also deposit sediment intowaterbodies. In sum, sediment qualityin the stream represents the net resultof erosion processes in the watershed.

The lateral view of sediment is dis-cussed in more detail in Section 2.B.It is worth noting, however, that froma water quality perspective, interest mayfocus on specific fractions of the sedi-ment load. For instance, controllingfine sediment load is often of particularconcern for restoration of habitat forsalmonid fish.

Restoration efforts may be useful forcontrolling loads of sediment and sedi-ment-associated pollutants from thewatershed to streams. These may rangefrom efforts to reduce upland erosionto treatments that reduce sediment de-livery through the riparian zone. Designof restoration treatments is covered inChapter 8.

Figure 2.18: Stream sedimentation. Althoughsediment and its transport occur naturally,changes in sediment load and particle sizehave negative impacts.


Sediment Along the Stream Corridor

The longitudinal processes affectingsediment transport from a water qualityperspective are the same as those dis-cussed from a geomorphic perspectivein Section 2.B. As in the lateral perspec-tive, interest from a water quality pointof view may be focused on specific sedi-ment size fractions, particularly the finesediment fraction, because of its effecton water quality, water temperature,habitat, and biota.

Water Temperature

Water temperature is a crucial factor instream corridor restoration for a numberof reasons. First, dissolved oxygen solu-bility decreases with increasing watertemperature, so the stress imposed byoxygen-demanding waste increases withhigher temperatures. Second, tempera-ture governs many biochemical andphysiological processes in cold-bloodedaquatic organisms, and increased tem-peratures can increase metabolic andreproductive rates throughout the foodchain. Third, many aquatic species cantolerate only a limited range of tempera-tures, and shifting the maximum andminimum temperatures within a streamcan have profound effects on speciescomposition. Finally, temperature alsoaffects many abiotic chemical processes,such as reaeration rate, sorption of or-ganic chemicals to particulate matter,and volatilization rates. Temperature in-creases can lead to increased stress fromtoxic compounds, for which the dis-solved fraction is usually the mostbioactive fraction.

Water Temperature Across theStream Corridor

Water temperature within a streamreach is affected by the temperature ofwater upstream, processes within thestream reach, and the temperature ofinfluent water. The lateral view ad-

dresses the effects of the temperature ofinfluent water.

The most important factor for tempera-ture of influent water within a streamreach is the balance between water ar-riving via surface and ground waterpathways. Water that flows over theland surface to a stream has the oppor-tunity to gain heat through contact withsurfaces heated by the sun. In contrast,ground water is usually cooler in sum-mer and tends to reflect average annualtemperatures in the watershed. Waterflow via shallow ground water pathwaysmay lie between the average annualtemperature and ambient temperaturesduring runoff events.

Both the fraction of runoff arriving viasurface pathways and the temperatureof surface runoff are strongly affectedby the amount of impervious surfaceswithin a watershed. For example, hotpaved surfaces in a watershed can heatsurface runoff and significantly increasethe temperature of streams that receivethe runoff.

Water Temperature Along theStream Corridor

Water also is subject to thermal loadingthrough direct effects of sunlight onstreams. For the purposes of restoration,land use practices that remove overheadcover or that decrease baseflows can in-crease instream temperatures to levelsthat exceed critical thermal maxima forfishes (Feminella and Matthews 1984).Maintaining or restoring normal tem-perature ranges can therefore be an im-portant goal for restoration.

Chemical Constituents

Previous chapters have discussed thephysical journey of water as it movesthrough the hydrologic cycle. Rain per-colates to the ground water table or be-comes overland flow, streams collectthis water and route it toward the

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ocean, and evapotranspiration occursthroughout the cycle. As water makesthis journey, its chemistry changes.While in the air, water equilibrates withatmospheric gases. In shallow soils, itundergoes chemical exchanges with in-organic and organic matter and withsoil gases. In ground water, where transittimes are longer, there are more oppor-tunities for minerals to dissolve. Similarchemical reactions continue alongstream corridors. Everywhere, water in-teracts with everything it touches—air,rocks, bacteria, plants, and fish—and isaffected by human disturbances.

Scientists have been able to define sev-eral interdependent cycles for many ofthe common dissolved constituents inwater. Central among these cycles is thebehavior of oxygen, carbon, and nutri-ents, such as nitrogen (N), phosphorus(P), sulfur (S), and smaller amounts ofcommon trace elements.

Iron, for example, is an essential ele-ment in the metabolism of animals andplants. Iron in aquatic systems may bepresent in one of two oxidation states.Ferric iron (Fe3+) is the more oxidizedform and is very sparingly soluble inwater. The reduced form, ferrous iron(Fe2+), is more soluble by many ordersof magnitude. In many aquatic systems,such as lakes for example, iron can cyclefrom the ferric state to the ferrous stateand back again (Figure 2.19). The oxi-dation of ferrous iron followed by theprecipitation of ferric iron results iniron coatings on the surfaces of somestream sediments. These coatings, alongwith organic coatings, play a substantialrole in the aquatic chemistry of toxictrace elements and toxic organic chemi-cals. The chemistry of toxic organicchemicals and metals, along with thecycling and chemistry of oxygen, nitro-gen, and phosphorus, will be coveredlater in this section.

The total concentration of all dissolvedions in water, also known as salinity,varies widely. Precipitation typicallycontains only a few parts per thousand(ppt) of dissolved solids, while thesalinity of seawater averages about 35ppt (Table 2.5). The concentration ofdissolved solids in freshwater may varyfrom only 10 to 20 mg/L in a pristinemountain stream to several hundredmg/L in many rivers. Concentrationsmay exceed 1,000 mg/L in arid water-sheds. A dissolved solids concentrationof less than 500 mg/L is recommendedfor public drinking water, but thisthreshold is exceeded in many areas ofthe country. Some crops (notably fruittrees and beans) are sensitive to evenmodest salinity, while other crops, suchas cotton, barley, and beets, toleratehigh concentrations of dissolved solids.Agricultural return water from irrigationmay increase salinity in streams, partic-ularly in the west. Recommended salin-ity limits for livestock vary from 2,860mg/L for poultry to 12,900 mg/L foradult sheep. Plants, fish, and otheraquatic life also vary widely in theiradaptation to different concentrationsof dissolved solids. Most species have amaximum salinity tolerance, and fewcan live in very pure water of very lowionic concentration.

organic coatingiron coating

Clay Sand

Figure 2.19: The organic coatings on suspend-ed sediment from streams. Water chemistrydetermines whether sediment will carryadsorbed materials or if stream sedimentswill be coated.


pH, Alkalinity, and Acidity

Alkalinity, acidity, and buffering capac-ity are important characteristics of waterthat affect its suitability for biota andinfluence chemical reactions. The acidicor basic (alkaline) nature of water iscommonly quantified by the negativelogarithm of the hydrogen ion concen-tration, or pH. A pH value of 7 repre-sents a neutral condition; a pH valueless than 5 indicates moderately acidicconditions; a pH value greater than 9indicates moderately alkaline condi-tions. Many biological processes, suchas reproduction, cannot function inacidic or alkaline waters. In particular,aquatic organisms may suffer an os-motic imbalance under sustained expo-sure to low pH waters. Rapid

fluctuations in pH also can stressaquatic organisms. Finally, acidic condi-tions also can aggravate toxic contami-nation problems through increasedsolubility, leading to the release of toxicchemicals stored in stream sediments.

pH, Alkalinity, and Acidity Across theStream Corridor

The pH of runoff reflects the chemicalcharacteristics of precipitation and theland surface. Except in areas with signif-icant ocean spray, the dominant ion inmost precipitation is bicarbonate(HCO

3–). The bicarbonate ion is pro-

duced by carbon dioxide reacting withwater:

H2O + CO

2= H

++ HCO

3

–

This reaction also produces a hydrogenion (H+), thus increasing the hydrogenion concentration and acidity and low-ering the pH. Because of the presenceof CO

2in the atmosphere, most rain is

naturally slightly acidic, with a pH ofabout 5.6. Increased acidity in rainfallcan be caused by inputs, particularlyfrom burning fossil fuels.

As water moves through soils and rocks,its pH may increase or decrease as addi-tional chemical reactions occur. The car-bonate buffering system controls theacidity of most waters. Carbonatebuffering results from chemical equilib-rium between calcium, carbonate, bicar-bonate, carbon dioxide, and hydrogenions in the water and carbon dioxide inthe atmosphere. Buffering causes watersto resist changes in pH (Wetzel 1975).Alkalinity refers to the acid-neutralizingcapacity of water and usually refers tothose compounds that shift the pH inan alkaline direction (APHA 1995, Wet-zel 1975). The amount of buffering isrelated to the alkalinity and primarilydetermined by carbonate and bicarbon-ate concentration, which are introducedinto the water from dissolved calciumcarbonate (i.e., limestone) and similar

Constituent

SiO2

1 2 3 4 5 6

Samples

Al

Fe

Ca

Mg

Na

K

NH4

HCO3

SO4

Cl

NO2

NO3

pH

Total dissolvedsolids

1. Snow, Spooner Summit. U.S. Highway 50, Nevada (east of LakeTahoe) (Feth, Rogers, and Roberson, 1964).

2. Average composition of rain, August 1962 to July 1963, at 27 points in North Carolina and Virginia (Gambell and Fisher, 1966).

3. Rain, Menlo Park, Calif., 7:00 p.m. Jan. 9 to 8:00 a.m. Jan 10, 1958(Whitehead and Feth, 1964).

4. Rain, Menlo Park, Calif., 8:00 a.m. to 2:00 p.m. Jan 10, 1958(Whitehead and Feth, 1964).

5. Average for inland sampling stations in the United States for 1 year. Data from Junge and Werby (1958), as reported by Whitehead and Feth (1964).

6. Average composition of precipitation, Williamson Creek, Snohomish County, Wash., 1973-75. Also reported: As, 0.00045 mg/L; Cu 0.0025 mg/L; Pb, 0.0033 mg/L; Zn, 0.0036 mg/L (Deithier, D.P., 1977, Ph.D. thesis. University of Washington, Seattle).

Table 2.5:Composition, in mil-igrams per liter, ofrain and snow.


minerals present in the watershed. Forexample, when an acid interacts withlimestone, the following dissolutionreaction occurs:

H+

+ CaCO3

= Ca2+

+ HCO3

–

This reaction consumes hydrogen ions,thus raising the pH of the water. Con-versely, runoff may acidify when all al-kalinity in the water is consumed byacids, a process often attributed to theinput of strong mineral acids, such assulfuric acid, from acid mine drainage,and weak organic acids, such as humicand fulvic acids, which are naturallyproduced in large quantities in sometypes of soils, such as those associatedwith coniferous forests, bogs, and wet-lands. In some streams, pH levels canbe increased by restoring degraded wet-lands that intercept acid inputs, such asacid mine drainage, and help neutralizeacidity by converting sulfates from sul-furic acid to insoluble nonacidic metalsulfides that remain trapped in wetlandsediments.

pH, Alkalinity, and Acidity Along theStream Corridor

Within a stream, similar reactions occurbetween acids in the water, atmosphericCO

2, alkalinity in the water column, and

streambed material. An additional char-acteristic of pH in some poorly bufferedwaters is high daily variability in pH lev-els attributable to biological processesthat affect the carbonate buffering sys-tem. In waters with large standing cropsof aquatic plants, uptake of carbon diox-ide by plants during photosynthesis re-moves carbonic acid from the water,which can increase pH by several units.Conversely, pH levels may fall by severalunits during the night when photosyn-thesis does not occur and plants give offcarbon dioxide. Restoration techniquesthat decrease instream plant growththrough increased shading or reductionin nutrient loads or that increase reaera-

tion also tend to stabilize highly vari-able pH levels attributable to high ratesof photosynthesis.

The pH within streams can have impor-tant consequences for toxic materials.High acidity or high alkalinity tend toconvert insoluble metal sulfides to solu-ble forms and can increase the concen-tration of toxic metals. Conversely, highpH can promote ammonia toxicity. Am-monia is present in water in two forms,unionized (NH

3) and ionized (NH

4

+).Of these two forms of ammonia, un-ionized ammonia is relatively highlytoxic to aquatic life, while ionized am-monia is relatively negligibly toxic. Theproportion of un-ionized ammonia isdetermined by the pH and temperatureof the water (Bowie et al. 1985)—as pHor temperature increases, the propor-tion of un-ionized ammonia and thetoxicity also increase. For example, witha pH of 7 and a temperature of 68°F,only about 0.4 percent of the total am-monia is in the un-ionized form, whileat a pH of 8.5 and a temperature of78°F, 15 percent of the total ammoniais in the un-ionized form, representing35 times greater potential toxicity toaquatic life.

Dissolved Oxygen

Dissolved oxygen (DO) is a basic re-quirement for a healthy aquatic ecosys-tem. Most fish and aquatic insects“breathe” oxygen dissolved in the watercolumn. Some fish and aquatic organ-isms, such as carp and sludge worms,are adapted to low oxygen conditions,but most sport fish species, such astrout and salmon, suffer if DO concen-trations fall below a concentration of 3to 4 mg/L. Larvae and juvenile fish aremore sensitive and require even higherconcentrations of DO (USEPA 1997).

Many fish and other aquatic organismscan recover from short periods of low


DO in the water. However, prolongedepisodes of depressed dissolved oxygenconcentrations of 2 mg/L or less can re-sult in “dead” waterbodies. Prolongedexposure to low DO conditions can suf-focate adult fish or reduce their repro-ductive survival by suffocating sensitiveeggs and larvae, or can starve fish bykilling aquatic insect larvae and otherprey. Low DO concentrations also favoranaerobic bacteria that produce thenoxious gases or foul odors often asso-ciated with polluted waterbodies.

Water absorbs oxygen directly from theatmosphere, and from plants as a resultof photosynthesis. The ability of waterto hold oxygen is influenced by temper-ature and salinity. Water loses oxygenprimarily by respiration of aquaticplants, animals, and microorganisms.Due to their shallow depth, large sur-face exposure to air, and constant mo-tion, undisturbed streams generallycontain an abundant DO supply. How-ever, external loads of oxygen-demand-ing wastes or excessive plant growthinduced by nutrient loading followedby death and decomposition of vegeta-tive material can deplete oxygen.

Dissolved Oxygen Across the Stream Corridor

Oxygen concentrations in the water col-umn fluctuate under natural conditions,but oxygen can be severely depleted asa result of human activities that intro-duce large quantities of biodegradableorganic materials into surface waters.Excess loading of nutrients also can de-plete oxygen when plants within astream produce large quantities of plantbiomass.

Loads of oxygen-demanding waste usu-ally are reported as biochemical oxygendemand (BOD). BOD is a measure ofthe amount of oxygen required to oxi-dize organic material in water by bio-logical activity. As such, BOD is an

equivalent indicator rather than a truephysical or chemical substance. It mea-sures the total concentration of DO thateventually would be demanded aswastewater degrades in a stream.

BOD also is often separated into car-bonaceous and nitrogenous compo-nents. This is because the two fractionstend to degrade at different rates. Manywater quality models for dissolved oxy-gen require as input estimates of ulti-mate carbonaceous BOD (CBOD

u) and

either ultimate nitrogenous BOD(NBOD

u) or concentrations of individ-

ual nitrogen species.

Oxygen-demanding wastes can beloaded to streams by point source dis-charges, nonpoint loading, and groundwater. BOD loads from major pointsources typically are controlled andmonitored and thus are relatively easyto analyze. Nonpoint source loads ofBOD are much more difficult to ana-lyze. In general, any loading of organicmaterial from a watershed to a streamresults in an oxygen demand. Excessloads of organic material may arisefrom a variety of land use practices,coupled with storm events, erosion, and washoff. Some agricultural activi-ties, particularly large-scale animaloperations and improper manure appli-cation, can result in significant BODloads. Land-disturbing activities of silvi-culture and construction can result inhigh organic loads through the erosionof organic topsoil. Finally, urban runoffoften is loaded with high concentra-tions of organic materials derived froma variety of sources.

Dissolved Oxygen Along the Stream Corridor

Within a stream, DO content is affectedby reaeration from the atmosphere, pro-duction of DO by aquatic plants as aby-product of photosynthesis, and con-sumption of DO in respiration by

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plants, animals, and, most importantly,microorganisms.

Major processes affecting the DO bal-ance within a stream are summarized inFigure 2.20. This includes the followingcomponents:

■ Carbonaceous deoxygenation

■ Nitrogenous deoxygenation(nitrification)

■ Reaeration

■ Sediment oxygen demand

■ Photosynthesis and respirationof plants.

Reaeration is the primary route for in-troducing oxygen into most waters.Oxygen gas (O

2) constitutes about 21

percent of the atmosphere and readilydissolves in water. The saturation con-centration of DO in water is a measureof the maximum amount of oxygenthat water can hold at a given tempera-ture. When oxygen exceeds the satura-tion concentration, it tends to degas tothe atmosphere. When oxygen is belowthe saturation concentration, it tends todiffuse from the atmosphere to water.The saturation concentration of oxygendecreases with temperature according toa complex power function equation(APHA 1995). In addition to tempera-ture, the saturation concentration is af-fected by water salinity and theatmospheric pressure. As the salinity ofwater increases, the saturation concen-tration decreases. As the atmosphericpressure increases the saturation con-centration also increases.

Interactions between atmospheric andDO are driven by the partial pressuregradient in the gas phase and the con-centration gradient in the liquid phase(Thomann and Mueller 1987). Turbu-lence and mixing in either phase de-crease these gradients and increasereaeration, while a quiescent, stagnantsurface or films on the surface reduce

reaeration. In general, oxygen transferin natural waters depends on the fol-lowing:

■ Internal mixing and turbulence dueto velocity gradients and fluctuation

■ Temperature

■ Wind mixing

■ Waterfalls, dams, and rapids

■ Surface films

■ Water column depth.

carbonaceousdeoxygenation

atmosphericoxygen

algae

ph

oto

syn

thes

is

reaeration

settling

respiratio

n

nitrification

oxygen demand

NH4+

NO2-

NO3-

dis s olv ed ox y gen

Figure 2.20: Interrelationship of major kineticprocesses for BOD and DO as represented bywater quality models. Complex, interactingphysical and chemical processes can sometimesbe simplified by models in order to plan arestoration.


Stream restoration techniques oftentake advantage of these relationships,for instance by the installation of artifi-cial cascades to increase reaeration.Many empirical formulations have beendeveloped for estimating stream reaera-tion rate coefficients; a detailed sum-mary is provided in Bowie et al. (1985).

In addition to reaeration, oxygen is pro-duced instream by aquatic plants.Through photosynthesis, plants captureenergy from the sun to fix carbon diox-ide into reduced organic matter:

6 CO2

+ 6 H2O = C

6H

12O

6+ 6 O

2

Note that photosynthesis also producesoxygen. Plants utilize their simple pho-tosynthetic sugars and other nutrients(notably nitrogen [N], phosphorus [P],and sulfur [S] with smaller amounts ofseveral common and trace elements) tooperate their metabolism and to buildtheir structures.

Most animal life depends on the releaseof energy stored by plants in the photo-synthetic process. In a reaction that isthe reverse of photosynthesis, animalsconsume plant material or other ani-mals and oxidize the sugars, starches,and proteins to fuel their metabolismand build their own structure. Thisprocess is known as respiration andconsumes dissolved oxygen. The actualprocess of respiration involves a seriesof energy converting oxidation-reduc-tion reactions. Higher animals andmany microorganisms depend on suffi-cient dissolved oxygen as the terminalelectron acceptor in these reactions andcannot survive without it. Some mi-croorganisms are able to use other com-pounds (such as nitrate and sulfate) aselectron acceptors in metabolism andcan survive in anaerobic (oxygen-depleted) environments.

Detailed information on analysis andmodeling of DO and BOD in streamsis contained in a number of references

(e.g., Thomann and Mueller 1987), anda variety of well-tested computer mod-els are available. Most stream waterquality models account for CBOD inthe water column separately fromNBOD (which is usually representedvia direct mass balance of nitrogenspecies) and sediment oxygen demand orSOD. SOD represents the oxygen de-mand of sediment organism respirationand the benthic decomposition of or-ganic material. The demand of oxygenby sediment and benthic organismscan, in some instances, be a significantfraction of the total oxygen demand ina stream. This is particularly true insmall streams. The effects may be par-ticularly acute during low-flow andhigh-temperature conditions, as micro-bial activity tends to increase with in-creased temperature.

The presence of toxic pollutants in thewater column can indirectly lower oxy-gen concentrations by killing algae,aquatic weeds, or fish, which providean abundance of food for oxygen-consuming bacteria. Oxygen depletionalso can result from chemical reactionsthat do not involve bacteria. Some pol-lutants trigger chemical reactions thatplace a chemical oxygen demand onreceiving waters.

Nutrients

In addition to carbon dioxide andwater, aquatic plants (both algae andhigher plants) require a variety of otherelements to support their bodily struc-tures and metabolism. Just as with ter-restrial plants, the most important ofthese elements are nitrogen and phos-phorus. Additional nutrients, such aspotassium, iron, selenium, and silica,are needed in smaller amounts andgenerally are not limiting factors toplant growth. When these chemicals arelimited, plant growth may be limited.This is an important consideration in


stream management. Plant biomass(either created instream or loaded fromthe watershed) is necessary to supportthe food chain. However, excessivegrowth of algae and other aquaticplants instream can result in nuisanceconditions, and the depletion of dis-solved oxygen during nonphotosyn-thetic periods by the respiration ofplants and decay of dead plant materialcan create conditions unfavorable toaquatic life.

Phosphorus in freshwater systems existsin either a particulate phase or a dis-solved phase. Both phases include or-ganic and inorganic fractions. Theorganic particulate phase includes livingand dead particulate matter, such asplankton and detritus. Inorganic partic-ulate phosphorus includes phosphorusprecipitates and phosphorus adsorbedto particulates. Dissolved organic phos-phorus includes organic phosphorusexcreted by organisms and colloidalphosphorus compounds. The solubleinorganic phosphate forms H

2PO

4

–,HPO

4

2–, and PO4

3–, collectively knownas soluble reactive phosphorus (SRP) arereadily available to plants. Some con-densed phosphate forms, such as thosefound in detergents, are inorganic butare not directly available for plant up-take. Aquatic plants require nitrogenand phosphorus in different amounts.For phytoplankton, as an example, cellscontain approximately 0.5 to 2.0 µgphosphorus per µg chlorophyll, and 7to 10 µg nitrogen per µg chlorophyll.From this relationship, it is clear thatthe ratio of nitrogen and phosphorusrequired is in the range of 5 to 20 (depending on the characteristics of individual species) to support full utilization of available nutrients andmaximize plant growth. When the ratio deviates from this range, plantscannot use the nutrient present in ex-cess amounts. The other nutrient is then

said to be the limiting nutrient on plantgrowth. In streams experiencing exces-sive nutrient loading, resource man-agers often seek to control loading ofthe limiting nutrient at levels that pre-vent nuisance conditions.

In the aquatic environment, nitrogencan exist in several forms—dissolved ni-trogen gas (N

2), ammonia and ammo-

nium ion (NH3

and NH4

+), nitrite(NO

2

–), nitrate (NO3

–), and organic ni-trogen as proteinaceous matter or indissolved or particulate phases. Themost important forms of nitrogen interms of their immediate impacts onwater quality are the readily availableammonia ions, nitrites, and nitrates. Be-cause they must be converted to a formmore usable by plants, particulate andorganic nitrogen are less important inthe short term.

It may seem unusual that nitrogencould limit plant growth, given that theatmosphere is about 79 percent nitro-gen gas. However, only a few life-forms(for example, certain bacteria and blue-green algae) have the ability to fix nitro-gen gas from the atmosphere. Mostplants can use nitrogen only if it isavailable as ammonia (NH

3, commonly

present in water as the ionic form am-monium, NH

4

+) or as nitrate (NO3

–)(Figure 2.21). However, in freshwatersystems, growth of aquatic plants ismore commonly limited by phospho-rus than by nitrogen. This limitation oc-curs because phosphate (PO

4

3–) formsinsoluble complexes with commonconstituents in water (Ca++ and variableamounts of OH–, Cl–, and F–). Phospho-rus also sorbs to iron coatings on clayand other sediment surfaces and istherefore removed from the water col-umn by chemical processes, resulting inthe reduced ability of the water body tosupport plant growth.


Nutrients Across the Stream Corridor

Both nitrogen and phosphorus aredelivered to surface waters at an ele-vated rate as a result of human activi-ties, including point source dischargesof treated wastewater and nonpointsources, such as agriculture and urbandevelopment. In many developed wa-tersheds, a major source of nutrients

is the direct discharge of treated wastefrom wastewater treatment plants, aswell as combined sewer overflows(CSOs). Such point source dischargesare regulated under the National Pollu-tant Discharge Elimination System(NPDES) and usually are well character-ized by monitoring. The NPDES re-quires permitted dischargers to meet

interstitialwater

ground water dissolved organic nitrogen NO3

nitrogenfixation

particulateorganic matter and associatedmicrobes

biota

cyanobacteriaand microbialpopulations

benthic algae

nitrogenfixation

assimilation

assimilation

assimilationexcretion

nitrificatio

nd

enit

rifi

cati

on

decomposition

decomposition NH3 accum-ulationexcretion

dissolvedorganicnitrogenNH3NO3

dissolvedorganicnitrogenNH3NO3

import fromupstream

export todownstream

stream water

oxygenconcent-

ration

litter inputs

riparian vegetation atmospheric N2

N2

N2

NO3

sediment surface

NH3

NO3

NO2

NH3

NO2

NH3

Figure 2.21: Dynamics and transformations of nitrogen in a stream ecosystem. Nutrient cyclingfrom one form to another occurs with changes in nutrient inputs, as well as temperature andoxygen available.


both numeric and narrative water qual-ity standards in streams. While moststates do not have numeric standardsfor nutrients, point source dischargesof nutrients are recognized as a factorleading to stream degradation and fail-ure to achieve narrative water qualitystandards. As a result, increasingly strin-gent limitations on nutrient concentra-tions in wastewater treatment planteffluent (particularly phosphorus) havebeen imposed in many areas.

In many cases the NPDES program hassignificantly cleaned up rivers andstreams; however, many streams still donot meet water quality standards, evenwith increasingly stringent regulatorystandards. Scientists and regulators nowunderstand that the dominant source ofnutrients in many streams is from non-point sources within the stream’s water-shed, not from point sources such aswastewater treatment plants. Typicalland uses that contribute to the non-point contamination of streams are theapplication of fertilizers to agriculturalfields and suburban lawns, the improperhandling of animal wastes from live-stock operations, and the disposal ofhuman waste in septic systems. Stormrunoff from agricultural fields can con-tribute nutrients to a stream in dissolvedforms as well as particulate forms.

Because of its tendency to sorb to sedi-ment particles and organic matter,phosphorus is transported primarily insurface runoff with eroded sediments.Inorganic nitrogen, on the other hand,does not sorb strongly and can be trans-ported in both particulate and dissolvedphases in surface runoff. Dissolved in-organic nitrogen also can be trans-ported through the unsaturated zone(interflow) and ground water to water-bodies. Table 2.6 presents commonpoint and nonpoint sources of nitrogenand phosphorus loading and shows theapproximate concentrations delivered.Note that nitrates are naturally occur-ring in some soils.

Nutrients Along the Stream Corridor

Nitrogen, because it does not sorbstrongly to sediment, moves easily be-tween the substrate and the water col-umn and cycles continuously. Aquaticorganisms incorporate dissolved andparticulate inorganic nitrogen into pro-teinaceous matter. Dead organisms de-compose and nitrogen is released asammonia ions and then converted tonitrite and nitrate, where the processbegins again.

Phosphorus undergoes continuoustransformations in a freshwater envi-ronment. Some phosphorus will sorb to

Livestock operationsa

Atmosphere (wet deposition)a

50% forestd

90% agricultured

Untreated wastewatera

Treated wastewatera,e

Source

Urban runoffa

90% forestd

6–800b

0.9

0.18–0.34

0.77–5.04

35

30

Total Nitrogen (mg/L)

3–10

0.06–0.19

4–5

0.015c

0.013–0.015

0.085–0.104

10

10

Total Phosphorus (mg/L)

0.2–1.7

0.006–0.012

a Novotny and Olem (1994).b As organic nitrogen.c Sorbed to airborne particulate.d Omernik (1987).e With secondary treatment.

Table 2.6: Sources and concentrations of pollutants from common point and nonpoint sources.


sediments in the water column or sub-strate and be removed from circulation.The SRP (usually as orthophosphate) isassimilated by aquatic plants and con-verted to organic phosphorus. Aquaticplants then may be consumed by detri-tivores and grazers, which in turn ex-crete some of the organic phosphorusas SRP. Continuing the cycle, the SRP israpidly assimilated by aquatic plants.

Toxic Organic Chemicals

Pollutants that cause toxicity in animalsor humans are of obvious concern torestoration efforts. Toxic organic chemi-cals (TOC) are synthetic compoundsthat contain carbon, such as polychlori-nated biphenyls (PCBs) and most pesti-cides and herbicides. Many of thesesynthesized compounds tend to persistand accumulate in the environment be-cause they do not readily break downin natural ecosystems. Some of themost toxic synthetic organics, DDT andPCBs, have been banned from use inthe United States for decades yet con-tinue to cause problems in the aquaticecosystems of many streams.

Toxic Organic Chemicals Across theStream Corridor

TOCs may reach a water body via bothpoint and nonpoint sources. Becausepermitted NPDES point sources mustmeet water quality standards instreamand because of whole effluent toxicityrequirements, continuing TOC prob-lems in most streams are due to non-point loading, recycling of materialsstored in stream and riparian sedi-ments, illegal dumping, or accidentalspills. Two important sources of non-point loading of organic chemicals areapplication of pesticides and herbicidesin connection with agriculture, silvicul-ture, or suburban lawn care, and runofffrom potentially polluted urban and in-dustrial land uses.

The movement of organic chemicalsfrom the watershed land surface to awater body is largely determined by thecharacteristics of the chemical, as dis-cussed below under the longitudinalperspective. Pollutants that tend to sorbstrongly to soil particles are primarilytransported with eroded sediment. Con-trolling sediment delivery from sourcearea land uses is therefore an effectivemanagement strategy. Organic chemi-cals with significant solubility may betransported directly with the flow ofwater, particularly stormflow from im-pervious urban surfaces.

Toxic Organic Chemicals Along theStream Corridor

Among all the elements of the earth,carbon is unique in its ability to form avirtually infinite array of stable covalentbonds with itself: long chains, branchesand rings, spiral helixes. Carbon mole-cules can be so complex that they areable to encode information for the orga-nization of other carbon structures andthe regulation of chemical reactions.

The chemical industry has exploitedthis to produce many useful organicchemicals: plastics, paints and dyes,fuels, pesticides, pharmaceuticals, andother items of modern life. These prod-ucts and their associated wastes and by-products can interfere with the healthof aquatic ecosystems. Understandingthe transport and fate of synthetic or-ganic compounds (SOC) in aquatic envi-ronments continues to challengescientists. Only a general overview ofthe processes that govern the behaviorof these chemicals along stream corri-dors is presented here.

Solubility

It is the nature of the carbon-carbonbond that electrons are distributed rela-tively uniformly between the bondedatoms. Thus a chained or ringed hydro-carbon is a fairly nonpolar compound.


This nonpolar nature is dissimilar tothe molecular structure of water, whichis a very polar solvent.

On the general principle that “like dis-solves like,” dissolved constituents inwater tend to be polar. Witness, for ex-ample, the ionic nature of virtually allinorganic constituents discussed thusfar in this chapter. How does an organiccompound become dissolved in water?There are several ways. The compoundcan be relatively small, so it minimizesits disturbance of the polar order ofthings in aqueous solution. Alterna-tively, the compound may becomemore polar by adding polar functionalgroups (Figure 2.22). Alcohols are or-ganic compounds with -OH groups at-tached; organic acids are organiccompounds with attached -COOHgroups. These functional groups arehighly polar and increase the solubilityof any organic compound. Even moresolubility in water is gained by ionicfunctional groups, such as -COO

-.

Another way that solubility is enhancedis by increased aromaticity. Aromaticity

refers to the delocalized bonding struc-ture of a ringed compound like ben-zene (Figure 2.23). (Indeed, allaromatic compounds can be consideredderivatives of benzene.) Because elec-trons are free to “dance around thering” of the benzene molecule, benzeneand its derivatives are more compatiblewith the polar nature of water.

A simple example will illustrate thefactors enhancing aqueous solubility oforganic compounds. Six compounds,each having six carbons, are shown inTable 2.7. Hexane is a simple hydrocar-bon, an alkane whose solubility is 10mg/L. Simply by adding a single -OHgroup, which converts hexane to the al-cohol hexanol, solubility is increased to5,900 mg/L. You can bend hexane intoa ringed alkane structure called cyclo-hexane. Forming the ring makes cyclo-hexane smaller than hexane andincreases its solubility, but only to 55mg/L. Making the ring aromatic byforming the six-carbon benzene mole-cule increases solubility all the way to1,780 mg/L. Adding an -OH to benzeneto form a phenol leads to another dra-

C-O-C

-OH

-NH2

C-O-RO

Relative Aqueous Solubility

carboxylate

amine

hydroxyl

carboxyl

carbonyl

ester

ether

10,0001,000100101

=

-C-O=

-C-OHO=

-C-OO=

Figure 2.22: Relative aqueous solubility of different functional groups. The solubility of acontaminant in water largely determines the extent to which it will impact water quality.


matic increase in solubility (to 82,000mg/L). Adding a chloride atom to thebenzene ring diminishes its aromaticcharacter (chloride inhibits the dancingelectrons), and thus the solubility ofchlorobenzene (448 mg/L) is less thanbenzene.

Sorption

In the 1940s, a young pharmaceuticalindustry sought to develop medicinesthat could be transported in digestivefluids and blood (both of which areessentially aqueous solutions) andcould also diffuse across cell mem-branes (which have, in part, a rathernonpolar character). The industry devel-oped a parameter to quantify the polarversus nonpolar character of potentialdrugs, and they called that parameterthe octanol-water partition coefficient.Basically they put water and octanol(an eight-carbon alcohol) into a vessel,added the organic compound of inter-est, and shook the combination up.After a period of rest, the water and oc-

tanol separate (neither is very soluble inthe other), and the concentration of theorganic compound can be measured ineach phase. The octanol-water partitioncoefficient, or K

ow, is defined simply as:

Kow

= concentration in octanol /concentration in water

The relation between water solubilityand K

owis shown in Figure 2.24. Gener-

ally we see that very insoluble com-pounds like DDT and PCBs have veryhigh values of K

ow. Alternatively, organic

acids and small organic solvents likeTCE are relatively soluble and have lowK

owvalues.

The octanol-water partition coefficienthas been determined for many com-pounds and can be useful in under-standing the distribution of SOCbetween water and biota, and betweenwater and sediments. Compounds withhigh K

owtend to accumulate in fish

tissue (Figure 2.25). The sediment-waterdistribution coefficient, often expressedas K

d, is defined in a sediment-water

mixture at equilibrium as the ratio ofthe concentration in the sediment tothe concentration in the water:

Kd

= concentration in sediment / concentration in water

One might ask whether this coefficientis constant for a given SOC. Values of K

d

for two polyaromatic hydrocarbons invarious soils are shown in Figure 2.26.For pyrene (which consists of four ben-zene rings stuck together), the K

dratios

vary from about 300 to 1500. Forphenanthrene (which consists of threebenzene rings stuck together), K

dvaries

from about 10 to 300. Clearly Kd

is not aconstant value for either compound.But, K

ddoes appear to bear a relation to

the fraction of organic carbon in the var-ious sediments. What appears to be con-stant is not K

ditself, but the ratio of K

d

to the fraction of organic carbon in thesediment. This ratio is referred to as K

oc:

H

C

C

H

C

C

H

H

H

H

C

C

H

C

C

H

C

C

H

H

H

H

C

C

Figure 2.23: Aromatic hydrocarbons. Benzeneis soluble in water because of its “aromatic”structure.

Hexanol

Cyclohexane

Phenol

Chlorobenzene

Compound

Hexane

Benzene

5,900 mg/L

55 mg/L

82,000 mg/L

448 mg/L

Solubility

10 mg/L

1,780 mg/L

Table 2.7: Solubility of six-carbon compounds.


n -

Oct

ano

l: W

ater

Par

titi

on

Co

effi

cien

t

Solubility in Water (µmoles/L)

10-3 10-2

102

0

103

104

105

106

107

10-1 1 10 102 103 104 105 106

2,4,5,2',4',5' - PCB

2,4,5,2',5' - PCB

4,4' - PCB

dichlofenthionchlorpyrifos

ronnel

dialifor

phosatone

methyl chlorpyrifos

dicapthon

fenitrothion

malathion

phosmel 2,4-D

tetrachloroethylene

flourobenzenenitrobenzene benzoic acid

phenylacetic acid

diphenyl ether

parathion

iodobenzenebromobenzene

chlorobenzenetoluene

carbon tetrachloridesalicylic acid

benzenechloroform

naphthalenep-dichlorobenzene

DDT

DDE

leptophos

phenoxyzcetic acid

Figure 2.24: Relationship between octanol/H2O partition coefficient and aqueous solubility.

The relative solubility in water is a substance’s “Water Partition Coefficient.”

Kd P

yren

e

Kd P

hen

anth

ren

e

Fraction Organic Carbon

0.0

300

600

900

1200

1500

0

1800

100

200

300

400

500

600

slope = Koc

.005 .010

phenanthrene

pyre

ne

.015 .020 .025

Figure 2.26: Relationship between pyrene,phenanthrene, and fraction organic carbon.Contaminant concentrations in sediment vs.water (K

d) are related to the amount of organ-

ic carbon available.

Log

BC

F in

Tro

ut

Mu

scle

2 3Log Poct

1

2

3

4

5

4 5 6

diphenylether

biphenyl

P-

tetrachloroethylenecarbontetrachloride

dichloro-benzene

hexachloro-benzene

2,4,2', 4'- PCB

7

Figure 2.25: Relationship between octanol/water partition (P

oct) coefficient and bioaccu-

mulation factor (BCF) in trout muscle. Waterquality can be inferred by the accumulation of contaminants in fish tissue.


Koc

= Kd

/ fraction of organic carbon in sediment

Various workers have related Koc

to Kow

and to water solubility (Table 2.8).

Using Kow

, Koc, and K

dto describe the

partitioning of an SOC between waterand sediment has shown some utility,but this approach is not applicable tothe sorption of all organic molecules inall systems. Sorption of some SOCoccurs by hydrogen bonding, such asoccurs in cation exchange or metalsorption to sediments (Figure 2.27).Sorption is not always reversible; or atleast after sorption occurs, desorptionmay be very slow.

Volatilization

Organic compounds partition fromwater into air by the process ofvolatilization. An air-water distribution

coefficient, the Henry’s Law constant(H), has been defined as the ratio ofthe concentration of an SOC in air inequilibrium with its concentration inwater:

H = SOC concentration in air /SOC concentration in water

“SOC” = synthetic organic compounds

A Henry’s Law constant for an SOC canbe estimated from the ratio of the com-pound’s vapor pressure to its water sol-ubility. Organic compounds that areinherently volatile (generally low mole-cular weight solvents) have very highHenry’s Law constants. But even com-pounds with very low vapor pressurecan partition into the atmosphere. DDTand PCBs for example, have modestHenry’s Law constants because their sol-ubility in water is so low. These SOCalso have high K

dvalues and so may be-

Equationa

log Koc = -0.55 log S + 3.64 (S in mg/L)

No.b

106

r2c

0.71

Chemical Classes Represented

Wide variety, mostly pesticides

log Koc = -0.54 log S + 0.44 (S in mole fraction)

10 0.94 Mostly aromatic or polynuclear aromatics; two chlorinated

log Koc = -0.557 log S + 4.277 (S in µ moles/L)d

15 0.99 Chlorinated hydrocarbons

log Koc = 0.544 log Kow + 1.377 45 0.74 Wide variety, mostly pesticides

log Koc = 0.937 log Kow - 0.006 19 0.95 Aromatics, polynuclear aromatics, triazines, anddinitroaniline herbicides

log Koc = 1.00 log Kow - 0.21 10 1.00 Mostly aromatic or polynuclear aromatics; two chlorinated

log Koc = 0.95 log Kow + 0.02 9 e S-triazines and dinitroaniline herbicides

log Koc = 1.029 log Kow - 0.18 13 0.91 Variety of insecticides, herbicides, and fungicides

log Koc = 0.524 log Kow + 0.855d 30 0.84 Substituted phenylureas and alkyl-N-phenylcarbamates

log Koc = 0.0067 (p - 45N) + 0.237d,f 29 0.69 Aromatic compounds, urea, 1.3.5-triazines, carbamates, and uracils

log Koc = 0.681 log 8CF(f) + 1.963 13 0.76 Wide variety, mostly pesticides

log Koc = 0.681 log 8CF(t) + 1.886 22 0.83 Wide variety, mostly pesticides

a Koc = soil (or sediment) adsorption coefficient; S = water solubility; Kow = octanol-water partition coefficient; BCF(f) = bioconcentration factor from flowing-water tests; BCF(t) = bioconcentration factor from model ecosystems; P = parachor; N = number of sites in molecule which can participate in the formation of a hydrogen bond.

b No. = number of chemicals used to obtain regression equation.c r2 = correlation coefficient for regression equation.d Equation originally given in terms of Kom. The relationship Kom = Koc/1.724 was used to rewrite the equation in terms of Koc.e Not available.f Specific chemicals used to obtain regression equation not specified.

Table 2.8: Regression equations for sediment adsorption coefficients (Koc

) for various contaminants.


come airborne in association with par-ticulate matter.

Degradation

SOC can be transformed into a varietyof degradation products. These degrada-tion products may themselves degrade.Ultimate degradation, or mineraliza-tion, results in the oxidation of organiccarbon to carbon dioxide. Major trans-formation processes include photolysis,hydrolysis, and oxidation-reduction re-actions. The latter are commonly medi-ated by biological systems.

Photolysis refers to the destruction of acompound by the energy of light. Theenergy of light varies inversely with itswavelength (Figure 2.28). Long-wavelight lacks sufficient energy to breakchemical bonds. Short wave light (x-raysand gamma rays) is very destructive;fortunately for life on earth, this type ofradiation largely is removed by ourupper atmosphere. Light near the visi-ble spectrum reaches the earth’s surfaceand can break many of the bonds com-mon in SOC. The fate of organic sol-vents following volatilization is usuallyphotolysis in the earth’s atmosphere.Photolysis also can be important in thedegradation of SOC in stream water.

Hydrolysis refers to the splitting of an or-ganic molecule by water. Essentiallywater enters a polar location on a mole-cule and inserts itself, with an H+ goingto one part of the parent molecule andan OH- going to the other. The twoparts then separate. A group of SOCcalled esters are particularly vulnerableto degradation by hydrolysis. Many es-ters have been produced as pesticidesor plasticizers.

Oxidation-reduction reactions are whatfuels most metabolism in the bios-phere. SOC are generally considered assources of reduced carbon. In such situ-ations, what is needed for degradationis a metabolic system with the appro-

H

C

C

Si

C

Si

HH

HO2C

H ON Si

H O

O

O

OAl Al

RH OO O

O

Al

O

O

O

adsorbents

silica

Organic Bases Organic Acids

alumina

Figure 2.27: Two important types of hydrogenbonding involving natural organic matter andmineral surfaces. Some contaminants are car-ried by sediment particles that are sorbed ontotheir surfaces by chemical bonding.

DissociationEnergies for

Diatomic Molecules

20

Kilocaloriesper Gram • Mole

of Quanta

Infrared

VisibleLight

NearUltraviolet

MiddleUltraviolet

FarUltraviolet

Wavelength(nanometers)

800

600

500

400

350

300

250

200

30

40

50

I • I

Br • Br

Cl • Cl

C • N

C • OH • Br

S • SH • H

O • OC • F

H • Cl

C • Cl

C • S60

70

80

90

100

110

120

130

140

Figure 2.28: Energy of electromagnetic radia-tion compared with some selected bond ener-gies. Light breaks chemical bonds of somecompounds through photolysis.


priate enzymes for the oxidation of thecompound. A sufficient supply of othernutrients and a terminal electron accep-tor are also required.

The principle of microbial infallibility in-formally refers to the idea that givena supply of potential food, microbialcommunities will develop the meta-bolic capability to use that food forbiochemical energy. Not all degrada-tion reactions, however, involve theoxidation of SOC. Some of the mostproblematic organic contaminantsare chlorinated compounds.

Chlorinated SOC do not exist naturally,so microbial systems generally are notadapted for their degradation. Chlorineis an extremely electronegative element.The electronegativity of chlorine refersto its penchant for sucking on electrons.This tendency explains why chloride ex-ists as an anion and why an attachedchloride diminishes the solubility ofan aromatic ring. Given this character,it is difficult for biological systems tooxidize chlorinated compounds. Aninitial step in that degradation, there-fore, is often reductive dechlorination.The chlorine is removed by reducingthe compound (i.e., by giving it elec-trons). After the chlorines are removed,degradation may proceed along oxida-tive pathways. The degradation ofchlorinated SOC thus may require asequence of reducing and oxidizingenvironments, which water may experi-ence as it moves between stream andhyporheic zones.

The overall degradation of SOC oftenfollows complex pathways. Figure 2.29shows a complex web of metabolicreaction for a single parent pesticide.Hydrolysis, reduction, and oxidationare all involved in the degradation ofSOC, and the distribution and behaviorof degradation products can be ex-tremely variable in space and time.

Chemical consequences are rarely theimmediate goal of most restorationactions. Plans that alter chemicalprocesses and attributes are usuallyfocused on changing the physical andbiological characteristics that are vitalto the restoration goals.

Toxic Concentrations ofBioavailable Metals

A variety of naturally occurring metals,ranging from arsenic to zinc, have beenestablished to be toxic to various formsof aquatic life when present in suffi-cient concentrations. The primarymechanisms for water column toxicityof most metals is adsorption at the gillsurface. While some studies indicatethat particulate metals may contributeto toxicity, perhaps because of factorssuch as desorption at the gill surface,the dissolved metal concentration mostclosely approximates the fraction ofmetal in the water column that isbioavailable. Accordingly, current EPApolicy is that dissolved metal concentra-tions should be used to set and mea-sure compliance with water qualitystandards (40 CFR 22228-22236, May4, 1995). For most metals, the dissolvedfraction is equivalent to the inorganicionic fraction. For certain metals, mostnotably mercury, the dissolved fractionalso may include the metal complexedwith organic binding agents (e.g.,methyl mercury, which can be producedin sediments by methanogenic bacteria,is soluble and highly toxic, and can ac-cumulate through the food chain).

Toxic Concentrations of BioavailableMetals Across the Stream Corridor

Unlike synthetic organic compounds,toxic metals are naturally occurring. Incommon with synthetic organics, met-als may be loaded to waterbodies fromboth point and nonpoint sources. Pol-lutants such as copper, zinc, and lead


are often of concern in effluent fromwastewater treatment plants but arerequired under the NPDES program tomeet numeric water quality standards.

Many of the toxic metals are present atsignificant concentrations in most soilsbut in sorbed nonbioavailable forms.Sediment often introduces significantconcentrations of metals such as zincinto waterbodies. It is then a matter ofwhether instream conditions promotebioavailable dissolved forms of themetal.

Nonpoint sources of metals first reflectthe characteristics of watershed soils. Inaddition, many older industrial areashave soil concentrations of certain met-als that are elevated due to past indus-trial practices. Movement of metals fromsoil to watershed is largely a function ofthe erosion and delivery of sediment.

In certain watersheds, a major source ofmetals loading is provided by acid minedrainage. High acidity increases the sol-ubility of many metals, and mines tendto be in mineral-rich areas. Abandonedmines are therefore a continuing sourceof toxic metals loading in many streams.

Toxic Concentrations of BioavailableMetals Along the Stream Corridor

Most metals have a tendency to leavethe dissolved phase and attach to sus-pended particulate matter or form in-soluble precipitates. Conditions thatpartition metals into particulate forms(presence of suspended sediments, dis-solved and particulate organic carbon,carbonates, bicarbonates, and otherions that complex metals) reduce po-tential bioavailability of metals. Also,calcium reduces metal uptake, appar-ently by competing with metals for ac-tive uptake sites on gill membranes. pHis also an important water quality factorin metal bioavailability. In general,metal solubilities are lower at near neu-

tral pH’s than in acidic or highly alka-line waters.

Ecological Functions of Soils

Soil is a living and dynamic resourcethat supports life. It consists of inor-ganic mineral particles of differing sizes(clay, silt, and sand), organic matter invarious stages of decomposition, nu-merous species of living organisms,

hydrolysis

uv

hydrolysis

hydrolysis

redu

ction

hydrolysis

hydrolysis

oxidation

oxidation

inorganicphosphate

p- aminophenol

p- nitorphenol

paraoxon

NO2

O

O OEtOtE

O

NO2

O

P OEtOtE

O

+

NH2

O

P OEtOtE

S

OH

P OEtOtE

S

OH

P OHHO

S

OH

P OEtOtE

O

OH

P OHOtE

O

OH

P OEtOtE

O

OH

P OEtOtE

S

OH

P OEtHO

S

OH

P OEtHO

O

H3PO4

NO2

O

P OEtOtE

S

NO2

O

P OEtOtE

O

NH2

O

P OEtOtE

ONO2

OH

NH2

OH

NO2

O

P OEtHO

S

NO2

S

P OEtOtE

O

parathion

hydrolysis

hyd

rolysis

redu

ction

hyd

rolysis

redu

ction

Figure 2.29: Metabolic reactions for a singleparent pesticide. Particles break down throughprocesses of hydrolysis, oxidation, reduction,and photolysis.


various water soluble ions, and variousgases and water. These componentseach have their own physical and chem-ical characteristics which can either sup-port or restrict a particular form of life.

Soils can be mineral or organic depend-ing on which material makes up thegreater percentage in the soil matrix.Mineral soils develop in materialsweathered from rocks while organicsoils develop in decayed vegetation.Both soils typically develop horizons orlayers that are approximately parallel tothe soil surface. The extreme variety ofspecific niches or conditions soil cancreate has enabled a large variety offauna and flora to evolve and live underthose conditions.

Soils, particularly riparian and wetlandsoils, contain and support a very highdiversity of flora and fauna both aboveand below the soil surface. A large vari-ety of specialized organisms can befound below the soil surface, outnum-bering those above ground by several or-ders of magnitude. Generally, organismsseen above ground are higher forms oflife such as plants and wildlife. However,at and below ground, the vast majorityof life consists of plant roots having theresponsibility of supporting the aboveground portion of the plant; many in-sects, mollusks, and fungi living on deadorganic matter; and an infinite numberof bacteria which can live on a wide va-riety of energy sources found in soil.

It is important to identify soil bound-aries and to understand the differencesin soil properties and functions occur-ring within a stream corridor in orderto identify opportunities and limita-tions for restoration. Floodplain andterrace soils are often areas of densepopulation and intensive agriculturaldevelopment due to their flat slopes,proximity to water, and natural fertility.When planning stream corridor restora-tion initiatives in developed areas, it is

important to recognize these alterationsand to consider their impacts on goals.

Soils perform vital functions through-out the landscape. One of the most im-portant functions of soil is to provide aphysical, chemical, and biological set-ting for living organisms. Soils supportbiological activity and diversity forplant and animal productivity. Soilsalso regulate and partition the flow ofwater and the storage and cycling of nu-trients and other elements in the land-scape. They filter, buffer, degrade,immobilize, and detoxify organic andinorganic materials and provide the me-chanical support living organisms need.These hydrologic, geomorphic, and bio-logic functions involve processes thathelp build and sustain stream corridors.

Soil Microbiology

Organic matter provides the main sourceof energy for soil microorganisms. Soilorganic matter normally makes up 1 to5 percent of the total weight in a min-eral topsoil. It consists of original tissue,partially decomposed tissue, and humus.Soil organisms consume roots and vege-tative detritus for energy and to buildtissue. As the original organic matter isdecomposed and modified by microor-ganisms, a gelatinous, more resistantcompound is formed. This material iscalled humus. It is generally black orbrown in color and exists as a colloid, agroup of small, insoluble particles sus-pended in a gel. Small amounts ofhumus greatly increase a soil’s ability tohold water and nutrient ions which en-hances plant production. Humus is anindicator of a large and viable popula-tion of microorganisms in the soil and itincreases the options available for vege-tative restoration.

Bacteria play vital roles in the organictransactions that support plant growth.They are responsible for three essentialtransformations: denitrification, sulfur


oxidation, and nitrogen fixation. Micro-bial reduction of nitrate to nitrite andthen to gaseous forms of nitrogen istermed denitrification. A water contentof 60 percent generally limits denitrifi-cation and the process only occurs atsoil temperatures between 5°C and75°C. Other soil properties optimizingthe rate of denitrification include a pHbetween 6 and 8, soil aeration belowthe biological oxygen demand of the or-ganisms in the soil, sufficient amountsof water-soluble carbon compounds,readily available nitrate in the soil, andthe presence of enzymes needed to startthe reaction.

Landscape and TopographicPosition

Soil properties change with topographicposition. Elevation differences generallymark the boundaries of soils anddrainage conditions in stream corridors.Different landforms generally have dif-ferent types of sediment underlyingthem. Surface and subsurface drainagepatterns also vary with landforms.

■ Soils of active channels. The activechannel forms the lowest and usuallyyoungest surfaces in the stream corri-dor. There is generally no soil devel-oped on these surfaces since theunconsolidated materials formingthe stream bottom and banks areconstantly being eroded, transported,and redeposited.

■ Soils of active floodplains. The nexthighest surface in the stream corridoris the flat, depositional surface of theactive floodplain. This surface floodsfrequently, every 2 out of 3 years, soit receives sediment deposition.

■ Soils of natural levees. Natural leveesare built adjacent to the stream bydeposition of coarser, suspended sed-iment dropping out of overbankflows during floods. A gentle back-

slope occurs on the floodplain sideof the natural levee, so the floodplainbecomes lowest at a point far fromthe river. Parent materials decrease ingrain size away from the river due tothe decrease in sediment-transportcapacity in the slackwater areas.

■ Soils of topographic floodplains. Slightlyhigher areas within and outside theactive floodplain are defined as thetopographic floodplain. They areusually inundated less frequentlythan the active floodplain, so soilsmay exhibit more profile develop-ment than the younger soils on theactive floodplain.

■ Soils of terraces. Abandoned flood-plains, or terraces, are the next high-est surfaces in stream corridors. Thesesurfaces rarely flood. Terrace soils, ingeneral, are coarser textured thanfloodplain soils, are more freelydrained, and are separated fromstream processes.

Upon close examination, floodplaindeposits can reveal historical events ofgiven watersheds. Soil profile develop-ment offers clues to the recent and geo-logic history at a site. Intricate andcomplex analysis methods such as car-bon dating, pollen analysis, ratios ofcertain isotopes, etc. can be used topiece together an area’s history. Cyclesof erosion or deposition can at times belinked to catastrophic events like forestfires or periods of high or low precipita-tion. Historical impacts of civilization,such as extensive agriculture or denuda-tion of forest cover will at times alsoleave identifiable evidence in soils.

Soil Temperature and MoistureRelationships

Soil temperature and moisture controlbiological processes occurring in soil.Average and expected precipitation andtemperature extremes are critical pieces


of information when considering goalsfor restoration initiatives. The mean an-nual soil temperature is usually verysimilar to the mean annual air tempera-ture. Soil temperatures do experiencedaily, seasonal, and annual fluctuationscaused by solar radiation, weather pat-terns, and climate. Soil temperatures arealso affected by aspect, latitude, and ele-vation.

Soil moisture conditions change sea-sonally. If changes in vegetation speciesand composition are being consideredas part of a restoration initiative, agraph comparing monthly precipitationand evapotranspiration for the vegeta-tion should be constructed. If the watertable and capillary fringe is below thepredicted rooting depth, and the graphindicates a deficit in available water, ir-rigation may be required. If no supple-mental water is available, different plantspecies must be considered.

The soil moisture gradient can decreasefrom 100 percent to almost zero alongthe transriparian continuum as oneprogresses from the stream bottom,across the riparian zone, and into thehigher elevations of the adjacent up-lands (Johnson and Lowe 1985), whichresults in vast differences in moistureavailable to vegetation. This gradient insoil moisture directly influences thecharacteristics of the ecological commu-nities of the riparian, transitional, andupland zones. These ecological differ-ences result in the presence of two eco-tones along the stream corridor—anaquatic-wetland/riparian ecotone and anon-wetland riparian/floodplain eco-tone—which increase the edge effect ofthe riparian zone and, therefore, the bi-ological diversity of the region.

Wetland Soils

Wet or “hydric” soils present specialchallenges to plant life. Hydric soils are

present in wetlands areas, creating suchdrastic changes in physical and chemicalconditions that most species found inuplands cannot survive. Hence the com-position of flora and fauna in wetlandsare vastly different and unique, espe-cially in wetlands subject to permanentor prolonged saturation or flooding.

Hydric soils are defined as those that aresaturated, flooded, or ponded longenough during the growing season todevelop anaerobic conditions in theupper part. These anaerobic conditionsaffect the reproduction, growth, andsurvival of plants. The driving processbehind the formation of hydric soils isflooding and/or soil saturation near thesurface for prolonged periods (usuallymore the seven days) during the grow-ing season (Tiner and Veneman 1989).

The following focuses primarily onmineral hydric soil properties, but or-ganic soils such as peat and muck maybe present in the stream corridor.

In aerated soil environments, atmos-pheric oxygen enters surface soilsthrough gas diffusion, as soil pores aremostly filled with air. Aerated soils arefound in well drained uplands, and gen-erally all areas having a water table wellbelow the root zone. In saturated soils,pores are filled with water, which diffusegases very slowly compared to the at-mosphere. Only small amounts of oxy-gen can dissolve in soil moisture, whichthen disperses into the top few inches ofsoil. Here, soil microbes quickly depleteall available free oxygen in oxidizing or-ganic residue to carbon dioxide. This re-action produces an anaerobicchemically reducing environment inwhich oxidized compounds are changedto reduced compounds that are solubleand also toxic to many plants. The rateof diffusion is so slow that oxygenatedconditions cannot be reestablishedunder such circumstances. Similar mi-


crobial reactions involving decomposi-tion of organic matter in waterloggedanaerobic environments produce ethyl-ene gas, which is highly toxic to plantroots and has an even stronger effectthan a lack of oxygen. After all free oxy-gen is utilized, anaerobic microbes re-duce other chemical constituents of thesoil including nitrates, manganese ox-ides, and iron oxides, creating a furtherreduced condition in the soil.

Prolonged anaerobic reducing condi-tions result in the formation of readilyvisible signs of reduction. The typicalgray colors encountered in wet soils arethe result of reduced iron, and areknown as gleyed soils. After iron oxidesare depleted, sulfates are reduced to sul-fides, producing the rotten egg odor ofwet soils. Under extremely waterloggedconditions, carbon dioxide can be re-duced to methane. Methane gas, alsoknown as “swamp gas” can be seen atnight, as it fluoresces.

Some wetland plants have evolved spe-cial mechanisms to compensate for hav-ing their roots immersed in anoxicenvironments. Water lilies, for example,force a gas exchange within the entireplant by closing their stomata duringthe heat of the day to raise the air pres-sure within special conductive tissue(aerenchyma). This process tends to in-troduce atmospheric oxygen deep intothe root crown, keeping vital tissuesalive. Most emergent wetland plantssimply keep their root systems close tothe soil surface to avoid anaerobic con-ditions in deeper strata. This is true ofsedges and rushes, for example.

When soils are continually saturatedthroughout, reactions can occur equallythroughout the soil profile as opposedto wet soils where the water level fluctu-ates. This produces soils with littlezonation, and materials tend to bemore uniform. Most differences in tex-

ture encountered with depth are relatedto stratification of sediments sorted bysize during deposition by flowing water.Clay formation tends to occur in placeand little translocation happens withinthe profile, as essentially no watermoves through the soil to transport theparticles. Due to the reactivity of wetsoils, clay formation tends to progressmuch faster than in uplands.

Soils which are seasonally saturated orhave a fluctuating water table result indistinct horizonation within the profile.As water regularly drains through theprofile, it translocates particles andtransports soluble ions from one layerto another, or entirely out of the profile.Often, these soils have a thick horizonnear the surface which is stripped of allsoluble materials including iron; knownas a depleted matrix. Seasonally saturatedsoils usually have substantial organicmatter accumulated at the surface,nearly black in color. The organics addto the cation exchange capacity of thesoil, but base saturation is low due tostripping and overabundance of hydro-gen ions. During non-saturated times,organic materials are exposed to atmos-pheric oxygen, and aerobic decomposi-tion can take place which results inmassive liberation of hydrogen ions.Seasonally wet soils also do not retainbase metals well, and can release highconcentrations of metals in wet cyclesfollowing dry periods.

Wet soil indicators will often remain inthe soil profile for long periods of time(even after drainage), revealing the his-torical conditions which prevailed. Ex-amples of such indicators are rustcolored iron deposits which at one timewere translocated by water in reducedform. Organic carbon distribution frompast fluvial deposition cycles or zonesof stripped soils resulting from wetlandsituations are characteristics which areextremely long lived.


SummaryThis section provides only a brief overview of thediverse and complex chemistry; nevertheless, twokey points should be evident to restoration practi-tioners:

■ Restoring physical habitat cannot restore biologi-cal integrity of a system if there are water qualityconstraints on the ecosystem.

■ Restoration activities may interact in a variety ofcomplex ways with water quality, affecting boththe delivery and impact of water quality stres-sors.

Table 2.9 shows how a sample selection of com-mon stream restoration and watershed manage-ment practices may interact with the water qualityparameters described in this section.

RestorationActivities

FineSedimentLoads

WaterTemperature

Decrease Decrease

Salinity

Decrease

pH

Increase/decrease

DissolvedOxygen

Increase

Nutrients

Decrease

Toxics

DecreaseReduction ofland-disturbingactivities

Decrease Decrease Negligibleeffect

Increase Increase Decrease DecreaseLimit impervioussurface area inthe watershed

Decrease Decrease Decrease Decrease Increase Decrease DecreaseRestore riparianvegetation

Decrease Increase/decrease

Increase/decrease

Increase/decrease

Decrease Increase IncreaseRestore wetlands

Decrease Decrease Decrease Decrease Increase Decrease Negligibleeffect

Stabilize channeland restoreunder-cut banks

Increase Negligibleeffect

Negligibleeffect

Increase/decrease


DecreaseCreate dropstructures

Negligibleeffect

Negligibleeffect

Negligibleeffect

Increase/decrease


Negligibleeffect

Reestablishriffle substrate

Table 2.9: Potential water quality impacts of selected stream restoration and watershed management practices.

Biological Community Characteristics 2–51

Successful stream restoration is basedon an understanding of the relation-ships among physical, chemical, and bi-ological processes at varying time scales.Often, human activities have acceler-ated the temporal progression of theseprocesses, resulting in unstable flowpatterns and altered biological structureand function of stream corridors. Thissection discusses the biological struc-ture and functions of stream corridorsin relation to geomorphologic, hydro-logic, and water quality processes. Theinterrelations between the watershedand the stream, as well as the cause andeffects of disturbances to these interrela-tionships are also discussed. Indicesand approaches for evaluating streamcorridor functions are provided inChapter 7.

Terrestrial Ecosystems

The biological community of a streamcorridor is determined by the character-istics of both terrestrial and aquaticecosystems. Accordingly, the discussionof biological communities in streamcorridors begins with a review of terres-trial ecosystems.

Ecological Role of Soil

Terrestrial ecosystems are fundamen-tally tied to processes within the soil.The ability of a soil to store and cyclenutrients and other elements dependson the properties and microclimate(i.e., moisture and temperature) of thesoil, and the soil’s community of organ-isms (Table 2.10). These factors also de-termine its effectiveness at filtering,buffering, degrading, immobilizing, anddetoxifying other organic and inorganicmaterials.

Terrestrial Vegetation

The ecological integrity of stream corri-dor ecosystems is directly related to theintegrity and ecological characteristicsof the plant communities that make upand surround the corridor. These plantcommunities are a valuable source ofenergy for the biological communities,provide physical habitat, and moderatesolar energy fluxes to and from the sur-rounding aquatic and terrestrial ecosys-tems. Given adequate moisture, light,and temperature, the vegetative com-munity grows in an annual cycle of ac-tive growth/production, senescence, andrelative dormancy. The growth period issubsidized by incidental solar radiation,which drives the photosynthetic processthrough which inorganic carbon is con-verted to organic plant materials. A por-tion of this organic material is stored asabove- and below-ground biomass,while a significant fraction of organicmatter is lost annually via senescence,fractionation, and leaching to the or-ganic soil layer in the form of leaves,twigs, and decaying roots. This organicfraction, rich in biological activity ofmicrobial flora and microfauna, repre-sents a major storage and cycling poolof available carbon, nitrogen, phospho-rus, and other nutrients.

The distribution and characteristics ofvegetative communities are determinedby climate, water availability, topo-graphic features, and the chemical andphysical properties of the soil, includingmoisture and nutrient content. Thecharacteristics of the plant communitiesdirectly influence the diversity and in-tegrity of the faunal communities. Plantcommunities that cover a large area andthat are diverse in their vertical and hor-izontal structural characteristics cansupport far more diverse faunal com-

2.D Biological Community Characteristics

REVERSE

Review SectionC for furtherdiscussion ofthe ecologicalfunctions ofsoils.


munities than relatively homogenousplant communities, such as meadows.As a result of the complex spatial andtemporal relationships that exist be-tween floral and faunal communities,current ecological characteristics of

these communities reflect the recenthistorical (100 years or less) physicalconditions of the landscape.

The quantity of terrestrial vegetation, aswell as its species composition, can di-rectly affect stream channel characteris-tics. Root systems in the streambankcan bind bank sediments and moderateerosion processes. Trees and smallerwoody debris that fall into the streamcan deflect flows and induce erosion atsome points and deposition at others.Thus woody debris accumulation caninfluence pool distribution, organicmatter and nutrient retention, and theformation of microhabitats that are im-portant fish and invertebrate aquaticcommunities.

Streamflow also can be affected by theabundance and distribution of terres-trial vegetation. The short-term effectsof removing vegetation can result in animmediate short-term rise in the localwater table due to decreased evapotran-spiration and additional water enteringthe stream. Over the longer term, how-ever, after removal of vegetation, thebaseflow of streams can decrease andwater temperatures can rise, particularlyin low-order streams. Also, removal ofvegetation can cause changes in soiltemperature and structure, resulting indecreased movement of water into andthrough the soil profile. The loss of sur-face litter and the gradual loss of or-ganic matter in the soil also contributeto increased surface runoff and de-creased infiltration.

In most instances, the functions of veg-etation that are most apparent are thosethat influence fish and wildlife. At thelandscape level, the fragmentation ofnative cover types has been shown tosignificantly influence wildlife, often fa-voring opportunistic species over thoserequiring large blocks of contiguoushabitat. In some systems, relatively

Macro

Animals

Subsisting largely on plant materials

Small mammals—squirrels, gophers, woodchucks, mice, shrews

Insects—springtails, ants, beetles, grubs, etc.

Millipedes

Sowbugs (woodlice)

Mites

Slugs and snails

Earthworms

Largely predatory

Moles

Insects—many ants, beetles, etc.

Mites, in some cases

Centipedes

Spiders

Micro Predatory or parasitic or subsisting on plant residues

Nematodes

Protozoa

Rotifers

Roots of higher plants

Plants

Algae

Fungi

Actinomycetes of many kinds

Bacteria

Aerobic

Anaerobic

Green

Blue-green

Diatoms

Mushroom fungi

Yeasts

Molds

Autotrophic

Heterotrophic

Autotrophic

Heterotrophic

Table 2.10: Groups of organisms commonlypresent in soils.


small breaks in corridor continuity canhave significant impacts on animalmovement or on the suitability ofstream conditions to support certainaquatic species. In others, establishingcorridors that are structurally differentfrom native systems or that are inappro-priately configured can be equally dis-ruptive. Narrow corridors that areessentially edge habitat may encouragegeneralist species, nest parasites, andpredators, and, where corridors havebeen established across historic barriersto animal movement, they can disruptthe integrity of regional animal assem-blages (Knopf et al. 1988).

Landscape Scale

The ecological characteristics and distri-bution of plant communities in a wa-tershed influence the movement ofwater, sediment, nutrients, and wildlife.Stream corridors provide links withother features of the landscape. Linksmay involve continuous corridors be-tween headwater and valley floorecosystems or periodic interactions be-tween terrestrial systems. Wildlife usecorridors to disperse juveniles, to mi-grate, and to move between portions oftheir home range. Corridors of a naturalorigin are preferred and include streamsand rivers, riparian strips, mountainpasses, isthmuses, and narrow straits(Payne and Bryant 1995).

It is important to understand the differ-ences between a stream-riparian ecosys-tem and a river-floodplain ecosystem.Flooding in the stream-riparian ecosys-tem is brief and unpredictable. The ri-parian zone supplies nutrients, water,and sediment to the stream channel,and riparian vegetation regulates tem-perature and light. In the river-flood-plain ecosystem, floods are often morepredictable and longer lasting, the riverchannel is the donor of water, sedi-ment, and inorganic nutrients to the

floodplain, and the influx of turbid andcooler channel water influences lightpenetration and temperature of theinundated floodplain.

Stream Corridor Scale

At the stream corridor scale, the compo-sition and regeneration patterns of veg-etation are characterized in terms ofhorizontal complexity. Floodplains alongunconstrained channels typically arevegetated with a mosaic of plant com-munities, the composition of whichvaries in response to available surfaceand ground water, differential patternsof flooding, fire, and predominantwinds, sediment deposition, and oppor-tunities for establishing vegetation.

A broad floodplain of the southern,midwestern, or eastern United Statesmay support dozens of relatively dis-tinct forest communities in a complexmosaic reflecting subtle differences insoil type and flood characteristics (e.g.,frequency, depth, and duration). Incontrast, while certain western streamsystems may support only a few woodyspecies, these systems may be struc-turally complex due to constant rework-ing of substrates by the stream, whichproduces a mosaic of stands of varyingages. The presence of side channels,oxbow lakes, and other topographicvariation can be viewed as elements ofstructural variation at the stream corri-dor level. Riparian areas along con-strained stream channels may consistprimarily of upland vegetation orga-nized by processes largely unrelated tostream characteristics, but these areasmay have considerable influence on thestream ecosystem.

The River Continuum Concept, as dis-cussed in Chapter 1, is also generallyapplicable to the vegetative componentsof the riparian corridor. Riparian vegeta-tion demonstrates both a transripariangradient (across the valley) and an


intra-riparian (longitudinal, eleva-tional) gradient (Johnson and Lowe1985). In the west, growth of riparianvegetation is increased by the “canyoneffect” resulting when cool moist airspills downslope from higher elevations(Figure 2.30). This cooler air settles incanyons and creates a more moist mi-crohabitat than occurs on the surround-ing slopes. These canyons also serve aswater courses. The combination ofmoist, cooler edaphic and atmosphericconditions is conducive to plant andanimal species at lower than normal al-titudes, often in disjunct populations orin regions where they would not other-wise occur (Lowe and Shannon 1954).

Plant Communities

The sensitivity of animal communitiesto vegetative characteristics is well rec-ognized. Numerous animal species areassociated with particular plant com-munities, many require particular devel-opmental stages of those communities(e.g., old-growth), and some depend onparticular habitat elements within thosecommunities (e.g., snags). The structureof streamside plant communities alsodirectly affects aquatic organisms byproviding inputs of appropriate organicmaterials to the aquatic food web, byshading the water surface and providingcover along banks, and by influencinginstream habitat structure through in-

stream corridorfloodplainchannel

alder-walnut

sycamore-ash

cottonwood- willow

plant and

animal dispersal

canyon effect—downhill drainage of cool, moist air

Figure 2.30: Canyon effect. Cool moist air settles in canyons and creates microhabitat that occurson surrounding slopes.


puts of woody debris (Gregory et al.1991).

Plant communities can be viewed interms of their internal complexity (Fig-ure 2.31). Complexity may include thenumber of layers of vegetation and thespecies comprising each layer; competi-tive interactions among species; and thepresence of detrital components, suchas litter, downed wood, and snags. Veg-etation may contain tree, sapling, shrub(subtree), vine, and herbaceous sub-shrub (herb-grass-forb) layers. Microto-pographic relief and the ability of waterto locally pond also may be regarded ascharacteristic structural components.

Vertical complexity, described in the con-cept of diversity of strata or foliageheight diversity in ecological literature,was important to studies of avian habi-tat by Carothers et al. (1974) along theVerde River, a fifth- or sixth-orderstream in central Arizona. Findingsshowed a high correlation between ri-parian bird species diversity and foliageheight diversity of riparian vegetation(Carothers et al. 1974). Short (1985)demonstrated that more structurally di-verse vegetative habitats support agreater number of guilds (groups ofspecies with closely related niches in acommunity) and therefore a largernumber of species.

Species and age composition of vegeta-tion structure also can be extremely im-portant. Simple vegetative structure,such as an herbaceous layer withoutwoody overstory or old woody ripariantrees without smaller size classes, cre-ates fewer niches for guilds. The fewerguilds there are, the fewer species thereare. The quality and vigor of the vegeta-tion can affect the productivity of fruits,seeds, shoots, roots, and other vegeta-tive material, which provide food forwildlife. Poorer vigor can result in lessfood and fewer consumers (wildlife).

Increasing the patch size (area) of astreamside vegetation type, increasingthe number of woody riparian tree sizeclasses, and increasing the number ofspecies and growth forms (herb, shrub,tree) of native riparian-dependent vege-tation can increase the number ofguilds and the amount of forage, result-ing in increased species richness andbiomass (numbers). Restoration tech-niques can change the above factors.

The importance of horizontal complex-ity within stream corridors to certainanimal species also has been well estab-lished. The characteristic compositional,structural, and topographic complexityof southern floodplain forests, for ex-ample, provides the range of resourcesand foraging conditions required bymany wintering waterfowl to meet par-ticular requirements of their life cyclesat the appropriate times (Fredrickson1978); similar complex relationshipshave been reported for other vertebratesand invertebrates in floodplain habitats(Wharton et al. 1982). In parts of thearid West, the unique vegetation struc-ture in riparian systems contrasts dra-

shrubs

herbaceoussubshrubs

trees

Figure 2.31: Vertical complexity. Complexitymay include a number of layers of vegetation.


matically with the surrounding uplandsand provides essential habitat for manyanimals (Knopf et al. 1988). Evenwithin compositionally simple ripariansystems, different developmental stagesmay provide different resources.

Plant communities are distributed onfloodplains in relation to flood depth,duration, and frequency, as well as vari-ations in soils and drainage condition.Some plant species, such as cottonwood(Populus sp.), willows (Salix sp.), andsilver maple (Acer saccharinum), areadapted to colonization of newly de-posited sediments and may require veryspecific patterns of flood recession dur-ing a brief period of seedfall to be suc-cessfully established (Morris et al. 1978,Rood and Mahoney 1990). The resul-tant pattern is one of even-aged treestands established at different intervalsand locations within the active meanderbelt of the stream. Other species, suchas the bald cypress (Taxodium dis-tichum), are particularly associated withoxbow lakes formed when streams cutoff channel segments, while still othersare associated with microtopographicvariations within floodplains that re-flect the slow migration of a streamchannel across the landscape.

Plant communities are dynamic andchange over time. The differing regener-ation strategies of particular vegetationtypes lead to characteristic patterns ofplant succession following disturbancesin which pioneer species well-adaptedto bare soil and plentiful light are grad-ually replaced by longer-lived speciesthat can regenerate under more shadedand protected conditions. New distur-bances reset the successional process.Within stream corridors, flooding,channel migration, and, in certain bio-mes, fire, are usually the dominant nat-ural sources of disturbance. Restorationpractitioners should understand pat-terns of natural succession in a stream

corridor and should take advantage ofthe successional process by plantinghardy early-successional species to sta-bilize an eroding streambank, whileplanning for the eventual replacementof these species by longer-lived andhigher-successional species.

Terrestrial Fauna

Stream corridors are used by wildlifemore than any other habitat type(Thomas et al. 1979) and are a majorsource of water to wildlife populations,especially large mammals. For example,60 percent of Arizona’s wildlife speciesdepend on riparian areas for survival(Ohmart and Anderson 1986). In theGreat Basin area of Utah and Nevada,288 of the 363 identified terrestrial ver-tebrate species depend on riparianzones (Thomas et al. 1979). Because oftheir wide suitability for upland and ri-parian species, midwestern stream corri-dors associated with prairie grasslandssupport a wider diversity of wildlifethan the associated uplands. Stream cor-ridors play a large role in maintainingbiodiversity for all groups of vertebrates.

The faunal composition of a stream cor-ridor is a function of the interaction offood, water, cover, and spatial arrange-ment (Thomas et al. 1979). These habi-tat components interact in multipleways to provide eight habitat features ofstream corridors:

■ Presence of permanent sources ofwater.

■ High primary productivity and bio-mass.

■ Dramatic spatial and temporal con-trasts in cover types and food avail-ability.

■ Critical microclimates.

■ Horizontal and vertical habitat diver-sity.


■ Maximized edge effect.

■ Effective seasonal migration routes.

■ High connectivity between vegetatedpatches.

Stream corridors offer the optimal habi-tat for many forms of wildlife becauseof the proximity to a water source andan ecological community that consistsprimarily of hardwoods in many partsof the country, which provide a sourceof food, such as nectar, catkins, buds,fruit, and seeds (Harris 1984). Up-stream sources of water, nutrients, andenergy ultimately benefit downstreamlocations. In turn, the fish and wildlifereturn and disperse some of the nutri-ents and energy to uplands and wet-lands during their movements andmigrations (Harris 1984).

Water is especially critical to fauna inareas such as the Southwest or WesternPrairie regions of the U.S. where streamcorridors are the only naturally occur-ring permanent sources of water on thelandscape. These relatively moist envi-ronments contribute to the high pri-mary productivity and biomass of theriparian area, which contrasts dramati-cally with surrounding cover types andfood sources. In these areas, stream cor-ridors provide critical microclimatesthat ameliorate the temperature andmoisture extremes of uplands by pro-viding water, shade, evapotranspiration,and cover.

The spatial distribution of vegetation isalso a critical factor for wildlife. The lin-ear arrangement of streams results in amaximized edge effect that increasesspecies richness because a species cansimultaneously access more than onecover (or habitat) type and exploit theresources of both (Leopold 1933).Edges occur along multiple habitattypes including the aquatic, riparian,and upland habitats.

Forested connectors between habitatsestablish continuity between foresteduplands that may be surrounded by un-forested areas. These act as feeder linesfor dispersal and facilitate repopulationby plants and animals. Thus, connectiv-ity is very important for retaining biodi-versity and genetic integrity on alandscape basis.

However, the linear distribution ofhabitat, or edge effect, is not an effec-tive indicator of habitat quality for allspecies. Studies in island biogeography,using habitat islands rather thanoceanic islands, demonstrate that alarger habitat island supports both alarger population of birds and also alarger number of species (Wilson andCarothers 1979). Although a continu-ous corridor is most desirable, the nextpreferable situation is minimal frag-mentation, i.e., large plots (“islands”)of riparian vegetation with minimalspaces between the large plots.

Reptiles and Amphibians

Nearly all amphibians (salamanders,toads, and frogs) depend on aquatichabitats for reproduction and overwin-tering. While less restricted by the pres-ence of water, many reptiles are foundprimarily in stream corridors and ripar-ian habitats. Thirty-six of the 63 reptileand amphibian species found in west-central Arizona were found to use ripar-ian zones. In the Great Basin, 11 of 22reptile species require or prefer riparianzones (Ohmart and Anderson 1986).

Birds

Birds are the most commonly observedterrestrial wildlife in riparian corridors.Nationally, over 250 species have beenreported using riparian areas duringsome part of the year.

The highest known density of nestingbirds in North America occurs in south-western cottonwood habitats (Carothers


and Johnson 1971). Seventy-three per-cent of the 166 breeding bird species inthe Southwest prefer riparian habitats(Johnson et al. 1977).

Bird species richness in midwesternstream corridors reflects the vegetativediversity and width of the corridor.Over half of these breeding birds arespecies that forage for insects on foliage(vireos, warblers) or species that foragefor seeds on the ground (doves, orioles,grosbeaks, sparrows). Next in abun-dance are insectivorous species that for-age on the ground or on trees(thrushes, woodpeckers).

Smith (1977) reported that the distrib-ution of bird species in forested habi-tats of the Southeast was closely linkedto soil moisture. Woodcock (Scolopaxminor) and snipe (Gallinago gallinago),red-shouldered hawks (Buteo lineatus),hooded and prothonotary warblers(Wilsonia citrina, Protonotaria citrea),and many other passerines in theSoutheast prefer the moist ground con-ditions found in riverside forests andshrublands for feeding. The cypress andmangrove swamps along Florida’s wa-terways harbor many species found al-most nowhere else in the Southeast.

Mammals

The combination of cover, water, andfood resources in riparian areas makethem desirable habitat for large mam-mals such as mule deer (Odocoileushemionus), white-tailed deer (Odocoileusvirginianus), moose (Alces alces), and elk(Cervus elaphus) that can use multiplehabitat types. Other mammals dependon riparian areas in some or all of theirrange. These include otter (Lutracanadensis), ringtail (Bassarisdus astutus),raccoon (Procyon lotor), beaver (Castorcanadensis), muskrat (Ondatra zibethi-cus), swamp rabbit (Sylvilagus aquati-cus), short-tailed shrew (Blarinabrevicauda), and mink (Mustela vison).

Riparian areas provide tall dense coverfor roosts, water, and abundant prey fora number of bat species, including thelittle brown bat (Myotis lucifugus), bigbrown bat (Eptesicus fuscus), and thepallid bat (Antrozous pallidus). Brinsonet al. (1981) tabulated results from sev-eral studies on mammals in riparianareas of the continental U.S. They con-cluded that the number of mammalspecies generally ranges from five to 30,with communities including severalfurbearers, one or more large mammals,and a few small to medium mammals.

Hoover and Wills (1984) reported 59species of mammals in cottonwood ri-parian woodlands of Colorado, secondonly to pinyon-juniper among eightother forested cover types in the region.Fifty-two of the 68 mammal speciesfound in west-central Arizona in Bureauof Land Management inventories use ri-parian habitats. Stamp and Ohmart(1979) and Cross (1985) found that ri-parian areas had a greater diversity andbiomass of small mammals than adja-cent upland areas.

The contrast between the species diver-sity and productivity of mammals inthe riparian zone and that of the sur-rounding uplands is especially high inarid and semiarid regions. However,bottomland hardwoods in the easternU.S. also have exceptionally high habi-tat values for many mammals. For ex-ample, bottomland hardwoods supportwhite-tail deer populations roughlytwice as large as equivalent areas of up-land forest (Glasgow and Noble 1971).

Stream corridors are themselves influ-enced by certain animal activities (For-man 1995). For example, beavers builddams that cause ponds to form within astream channel or in the floodplain. Thepond kills much of the existing vegeta-tion, although it does create wetlandsand open water areas for fish and mi-


gratory waterfowl. If appropriate woodyplants in the floodplain are scarce,beavers extend their cutting activitiesinto the uplands and can significantlyalter the riparian and stream corridors.Over time, the pond is replaced by amudflat, which becomes a meadow andeventually gives way to woody succes-sional stages. Beaver often then build adam at a new spot, and the cycle beginsanew with only a spatial displacement.

The sequence of beaver dams along astream corridor may have major effectson hydrology, sedimentation, and min-eral nutrients (Forman 1995). Waterfrom stormflow is held back, thereby af-fording some measure of flood control.Silts and other fine sediments accumu-late in the pond rather than beingwashed downstream. Wetland areasusually form, and the water table risesupstream of the dam. The ponds com-bine slow flow, near-constant water lev-els, and low turbidity that support fishand other aquatic organisms. Birds mayuse beaver ponds extensively. The wet-lands also have a relatively constantwater table, unlike the typical fluctua-tions across a floodplain. Beavers cut-ting trees diminish the abundance ofsuch species as elm (Ulmus spp.) andash (Fraxinus spp.) but enhance theabundance of rapidly sprouting species,such as alder (Alnus spp.), willow, andpoplar (Populus spp.).

Aquatic Ecosystems

Aquatic Habitat

The biological diversity and speciesabundance in streams depend on thediversity of available habitats. Naturallyfunctioning, stable stream systems pro-mote the diversity and availability ofhabitats. This is one of the primary rea-sons stream stability and the restorationof natural functions are always consid-ered in stream corridor restoration ac-

tivities. A stream’s cross-sectional shapeand dimensions, its slope and confine-ment, the grain-size distribution of bedsediments, and even its planform affectaquatic habitat. Under less disturbedsituations, a narrow, steep-walled crosssection provides less physical area forhabitat than a wider cross section withless steep sides, but may provide morebiologically rich habitat in deep poolscompared to a wider, shallower streamcorridor. A steep, confined stream is ahigh-energy environment that may limithabitat occurrence, diversity, and stabil-ity. Many steep, fast flowing streams arecoldwater salmonid streams of highvalue. Unconfined systems flood fre-quently, which can promote riparianhabitat development. Habitat increaseswith stream sinuosity. Uniform sedi-ment size in a streambed provides lesspotential habitat diversity than a bedwith many grain sizes represented.

Habitat subsystems occur at differentscales within a stream system (Frissellet al. 1986) (Figure 2.32). The grossestscale, the stream system itself, is mea-sured in thousands of feet, while seg-ments are measured in hundreds of feetand reaches are measured in tens offeet. A reach system includes combina-tions of debris dams, boulder cascades,rapids, step/pool sequences, pool/rifflesequences, or other types of streambedforms or “structures,” each of whichcould be 10 feet or less in scale. Fris-sell’s smallest scale habitat subsystemincludes features that are a foot or lessin size. Examples of these microhabitatsinclude leaf or stick detritus, sand or siltover cobbles or other coarse material,moss on boulders, or fine gravelpatches.

Steep slopes often form a step/pool se-quence in streams, especially in cobble,boulder, and bedrock streams. Eachstep acts as a miniature grade stabiliza-tion structure. The steps and pools work


together to distribute the excess energyavailable in these steeply sloping sys-tems. They also add diversity to thehabitat available. Cobble- and gravel-bottomed streams at less steep slopesform pool/riffle sequences, which alsoincrease habitat diversity. Pools providespace, cover, and nutrition to fish andthey provide a place for fish to seekshelter during storms, droughts, andother catastrophic events. Upstream mi-gration of many salmonid species typi-cally involves rapid movements throughshallow areas, followed by periods ofrest in deeper pools (Spence et al.1996).

Wetlands

Stream corridor restoration initiativesmay include restoration of wetlandssuch as riverine-type bottomland hard-wood systems or riparian wetlands.While wetland restoration is a specifictopic better addressed in other references(e.g., Kentula et al. 1992), a general dis-cussion of wetlands is provided here.Stream corridor restoration initiativesshould be designed to protect or restorethe functions of associated wetlands.

A wetland is an ecosystem that dependson constant or recurrent shallow inun-dation or saturation at or near the sur-face of the substrate. The minimumessential characteristics of a wetland arerecurrent, sustained inundation or satu-ration at or near the surface and thepresence of physical, chemical, and bio-logical features that reflect recurrentsustained inundation or saturation.Common diagnostic features of wet-lands are hydric soils and hydrophyticvegetation. These features will be pre-sent except where physicochemical, bi-otic, or anthropogenic factors haveremoved them or prevented their devel-opment (National Academy of Sciences1995). Wetlands may occur in streams,riparian areas, and floodplains of thestream corridor. The riparian area orzone may contain both wetlands andnon-wetlands.

Wetlands are transitional between terres-trial and aquatic systems where thewater table is usually at or near thesurface or the land is covered by shallowwater (Cowardin et al. 1979). For vege-tated wetlands, water creates conditionsthat favor the growth of hydrophytes—plants growing in water or on a sub-

leaf and stickdetritus inmargin

bouldercascade

debris dam

sand-siltover cobbles

transverse bar over cobbles

moss on boulder

fine gravelpatch

Stream Segment Segment System Reach System “Pool/Riffle” System Microhabitat System

Figure 2.32: Hierarchical organization of a stream system and its habitat subsystems.Approximate linear spatial scale, appropriate to second- or third-order mountain stream.


strate that is at least periodically defi-cient in oxygen as a result of excessivewater content (Cowardin et al. 1979)and promotes the development of hy-dric soils—soils that are saturated,flooded, or ponded long enough duringthe growing season to develop anaero-bic conditions in the upper part (Na-tional Academy of Sciences 1995).

Wetland functions include fish andwildlife habitat, water storage, sedimenttrapping, flood damage reduction,water quality improvement/pollutioncontrol, and ground water recharge.Wetlands have long been recognized ashighly productive habitats for threat-ened and endangered fish and wildlifespecies. Wetlands provide habitat for60 to 70 percent of the animal speciesfederally listed as threatened or endan-gered (Lohoefner 1997).

The Federal Geographic Data Commit-tee has adopted the U.S. Fish andWildlife Service’s Classification of Wet-lands and Deepwater Habitats of theUnited States (Cowardin et al. 1979)as the national standard for wetlandsclassification. The Service’s NationalWetlands Inventory (NWI) uses thissystem to carry out its congressionallymandated role of identifying, classify-ing, mapping, and digitizing data onwetlands and deepwater habitats. Thissystem, which defines wetlands consis-tently with the National Academy ofScience’s reference definition, includesMarine, Estuarine, Riverine, Lacustrine,and Palustrine systems. The NWI hasalso developed protocols for classifyingand mapping riparian habitats in the22 coterminous western states.

The riverine system under Cowardin’sclassification includes all wetlands anddeepwater habitats contained within achannel except wetlands dominated bytrees, shrubs, persistent emergents,emergent mosses, or lichens and habi-

tats with water containing ocean-derived salts in excess of 0.5 parts perthousand (ppt).

It is bounded on the upstream end byuplands and on the downstream end atthe interface with tidal wetlands havinga concentration of ocean-derived saltsthat exceeds 0.5 ppt. Riverine wetlands

The riparian zone is a classic example of the maximizedvalue that occurs when two or more habitat types meet.There is little question of the substantial value of riparianhabitats in the United States. The Fish and WildlifeService has developed protocols to classify and mapriparian areas in the West in conjunction with theNational Wetlands Inventory (NWI). NWI will map ripari-an areas on a 100 percent user-pay basis. No formalriparian mapping effort has been initiated. The NWI iscongressionally mandated to identify, classify, and digi-tize all wetlands and deepwater habitats in the UnitedStates. For purposes of riparian mapping, the NWI hasdeveloped a riparian definition that incorporates biologi-cal information consistent with many agencies andapplies information according to cartographic principles.For NWI mapping and classification purposes, a final def-inition for riparian has been developed:

Riparian areas are plant communities contiguous to andaffected by surface and subsurface hydrological featuresof perennial or intermittent lotic and lentic water bodies(rivers, streams, lakes, and drainage ways). Riparian areashave one or both of the following characteristics: (1) dis-tinctly different vegetative species than adjacent areas;and (2) species similar to adjacent areas but exhibitingmore vigorous or robust growth forms. Riparian areasare usually transitional between wetland and upland.

The definition applies primarily to regions of the lower48 states in the arid west where the mean annual pre-cipitation is 16 inches or less and the mean annual evap-oration exceeds mean annual precipitation. For purposesof this mapping, the riparian system is subdivided intosubsystems, classes, subclasses, and dominance types.(USFWS 1997)


are bounded perpendicularly on thelandward side by upland, the channelbank (including natural and manufac-tured levees), or by Palustrine wetlands.In braided streams, riverine wetlandsare bounded by the banks forming theouter limits of the depression withinwhich the braiding occurs.

Vegetated floodplain wetlands of theriver corridor are classified as Palustrineunder this system. The Palustrine sys-tem was developed to group the vege-tated wetlands traditionally called bysuch names as marsh, swamp, bog, fen,and prairie pothole and also includessmall, shallow, permanent, or intermit-tent water bodies often called ponds.Palustrine wetlands may be situatedshoreward of lakes, river channels, orestuaries, on river floodplains, in iso-lated catchments, or on slopes. Theyalso may occur as islands in lakes orrivers. The Palustrine system includes allnontidal wetlands dominated by trees,shrubs, persistent emergents, emergentmosses and lichens, and all such wet-lands that occur in tidal areas wheresalinity due to ocean-derived salts isbelow 0.5 ppt. The Palustrine system isbounded by upland or by any of theother four systems. They may mergewith non-wetland riparian habitatwhere hydrologic conditions cease tosupport wetland vegetation or may betotally absent where hydrologic condi-tions do not support wetlands at all(Cowardin et al. 1979).

The hydrogeomorphic (HGM) approach isa system that classifies wetlands intosimilar groups for conducting functionalassessments of wetlands. Wetlands areclassified based on geomorphology,water source, and hydrodynamics. Thisallows the focus to be placed on agroup of wetlands that function muchmore similarly than would be the casewithout classifying them. Reference wet-lands are used to develop reference

standards against which a wetland isevaluated (Brinson 1995).

Under the HGM approach, riverine wet-lands occur in floodplains and ripariancorridors associated with stream chan-nels. The dominant water sources areoverbank flow or subsurface connec-tions between stream channel and wet-lands. Riverine wetlands lose water bysurface and subsurface flow returning tothe stream channel, ground waterrecharge, and evapotranspiration. At theextension closest to the headwaters,riverine wetlands often are replaced byslope or depressional wetlands wherechannel bed and bank disappear, orthey may intergrade with poorly drainedflats and uplands. Usually forested, theyextend downstream to the intergradewith estuarine fringe wetlands. Lateralextent is from the edge of the channelperpendicularly to the edge of the flood-plain. In some landscape situations,riverine wetlands may function hydro-logically more like slope wetlands, andin headwater streams with little or nofloodplain, slope wetlands may lie adja-cent to the stream channel (Brinson etal. 1995). Table 2.11 summarizes func-tions of riverine wetlands under theHGM approach. The U.S. Fish andWildlife Service is testing an operationaldraft set of hydrogeomorphic type de-scriptors to help bridge the gap betweenthe Cowardin system and the HGM ap-proach (Tiner 1997).

For purposes of regulation under Sec-tion 404 of the Clean Water Act, onlyareas with wetland hydrology, hy-drophytic vegetation, and hydric soilsare classified as regulated wetlands.As such, they represent a subset of theareas classified as wetlands under theCowardin system. However, many areasclassified as wetlands under the Cow-ardin system, but not classified as wet-lands for purposes of Section 404, arenevertheless subject to regulation be-


cause they are part of the Waters of theUnited States.

Aquatic Vegetation and Fauna

Stream biota are often classified in sevengroups—bacteria, algae, macrophytes(higher plants), protists (amoebas, fla-gellates, ciliates), microinvertebrates(invertebrates less than 0.02 inch inlength, such as rotifers, copepods, ostra-cods, and nematodes), macroinverte-brates (invertebrates greater than 0.02inch in length, such as mayflies, stone-flies, caddisflies, crayfish, worms,clams, and snails), and vertebrates(fish, amphibians, reptiles, and mam-mals) (Figure 2.33). The discussionof the River Continuum Concept inChapter 1, provides an overview of themajor groups of organisms found instreams and how these assemblageschange from higher order to lowerorder streams.

Undisturbed streams can contain a re-markable number of species. For exam-ple, a comprehensive inventory ofstream biota in a small German stream,the Breitenbach, found more than 1,300species in a 1.2-mile reach. Lists ofalgae, macroinvertebrates, and fish likelyto be found at potential restoration sitesmay be obtained from state or regionalinventories. The densities of such streambiota are shown in Table 2.12.

Aquatic plants usually consist of algaeand mosses attached to permanentstream substrates. Rooted aquatic vege-tation may occur where substrates aresuitable and high currents do not scourthe stream bottom. Luxuriant beds ofvascular plants may grow in some areassuch as spring-fed streams in Floridawhere water clarity, substrates, nutrients,and slow water velocities exist. Bedrockor stones that cannot be moved easilyby stream currents are often covered bymosses and algae and various forms of

micro- and macroinvertebrates (Ruttner1963). Planktonic plant forms are usu-ally limited but may be present wherethe watershed contains lakes, ponds,floodplain waters, or slow current areas(Odum 1971).

The benthic invertebrate community ofstreams may contain a variety of biota,including bacteria, protists, rotifers, bry-ozoans, worms, crustaceans, aquatic in-sect larvae, mussels, clams, crayfish, andother forms of invertebrates. Aquatic in-vertebrates are found in or on a multi-tude of microhabitats in streamsincluding plants, woody debris, rocks,interstitial spaces of hard substrates, andsoft substrates (gravel, sand, and muck).Invertebrate habitats exist at all verticalstrata including the water surface, thewater column, the bottom surface, anddeep within the hyporheic zone.

Unicellular organisms and microinver-tebrates are the most numerous biota instreams. However, larger macroinverte-brates are important to communitystructure because they contribute signif-icantly to a stream’s total invertebratebiomass (Morin and Nadon 1991,

Hydrologic Dynamic surface water storage

Long-term surface water storage

Subsurface storage of water

Energy dissipation

Moderation of ground-water flow or discharge

Biogeochemical Nutrient cycling

Removal of elements and compounds

Retention of particulates

Organic carbon export

Plant habitat Maintain characteristic plant communities

Maintain characteristic detrital biomass

Animal habitat Maintain spatial habitat structure

Maintain interspersion and connectivity

Maintain distribution and abundance of invertebrates

Maintain distribution and abundance of vertebrates

Table 2.11: Functions of riverine wetlands.Source: Brinson et al. 1995.


Bourassa and Morin 1995). Further-more, the larger species often play im-portant roles in determining communitycomposition of other components ofthe ecosystem. For example, herbivo-rous feeding activities of caddisfly lar-vae (Lamberti and Resh 1983), snails(Steinman et al. 1987), and crayfish(Lodge 1991) can have a significant

effect on the abundance and taxonomiccomposition of algae and periphyton instreams. Likewise, macroinvertebratepredators, such as stoneflies, can influ-ence the abundance of other specieswithin the invertebrate community(Peckarsky 1985).

Collectively, microorganisms (fungiand bacteria) and benthic invertebratesfacilitate the breakdown of organic ma-terial, such as leaf litter, that enters thestream from external sources. Someinvertebrates (insect larvae and am-phipods) act as shredders whose feed-ing activities break down larger organicleaf litter to smaller particles. Other in-vertebrates filter smaller organic mater-ial from the water (blackfly larvae,some mayfly nymphs, and some caddis-fly larvae), scrape material off surfaces

microorganisms

flocculation

microorganisms(e.g., hyphomycete

fungi)

course particulate

organic matter

larger plants(mosses,

red algae)

dissolvedorganic matter

fineparticulate

organicmatter

light

invertebratescrapers

vertebratepredators

invertebratepredators

invertebrateshredders

invertebratecollectors

epilithicalgae

Figure 2.33: Streambiota. Food relation-ships typically foundn streams.

Algae 109 – 1010

Bacteria 1012 – 1013

Protists 108 – 109

Microinvertebrates 103 – 105

Macroinvertebrates 104 – 105

Vertebrates 100 – 102

Biotic Component

Density(Individuals/Square Mile)

Table 2.12: Ranges of densities commonlyobserved for selected groups of stream biota.


(snails, limpets, and some caddisfly andmayfly nymphs), or feed on materialdeposited on the substrate (dipteranlarvae and some mayfly nymphs) (Moss1988). These feeding activities result inthe breakdown of organic matter in ad-dition to the elaboration of invertebratetissue, which other consumer groups,such as fish, feed on.

Benthic macroinvertebrates, particularlyaquatic insect larvae and crustaceans,are widely used as indicators of streamhealth and condition. Many fish speciesrely on benthic organisms as a foodsource either by direct browsing on thebenthos or by catching benthic organ-isms that become dislodged and driftdownstream (Walburg 1971).

Fish are ecologically important instream ecosystems because they are usu-ally the largest vertebrates and often arethe apex predator in aquatic systems.The numbers and species compositionof fishes in a given stream depends onthe geographic location, evolutionaryhistory, and such intrinsic factors asphysical habitat (current, depth, sub-strates, riffle/pool ratio, wood snags,and undercut banks), water quality(temperature, dissolved oxygen, sus-pended solids, nutrients, and toxicchemicals), and biotic interactions (ex-ploitation, predation, and competition).

There are approximately 700 nativefreshwater species of fish in NorthAmerica (Briggs 1986). Fish speciesrichness is highest in the MississippiRiver Basin where most of the adaptiveradiations have occurred in the UnitedStates (Allan 1995). In the Midwest, asmany as 50 to 100 species can occur ina local area, although typically only halfthe species native to a region may befound at any one location (Horwitz1978). Fish species richness generallydeclines as one moves westward acrossthe United States, primarily due to ex-

tinction during and following the Pleis-tocene Age (Fausch et al. 1984). For ex-ample, 210 species are found west of theContinental Divide, but only 40 ofthese species are found on both sides ofthe continent (Minckley and Douglas1991). The relatively depauperate faunaof the Western United States has beenattributed to the isolating mechanismsof tectonic geology. Secondary biologi-cal, physical, and chemical factors mayfurther reduce the species richness of aspecific community (Minckley andDouglas 1991, Allan 1995).

Fish species assemblages in streams willvary considerably from the headwatersto the outlet due to changes in manyhydrologic and geomorphic factorswhich control temperature, dissolvedoxygen, gradient, current velocity, andsubstrate. Such factors combine to de-termine the degree of habitat diversityin a given stream segment. Fish speciesrichness tends to increase downstreamas gradient decreases and stream sizeincreases. Species richness is generallylowest at small headwater streams dueto increased gradient and small streamsize, which increases the frequency andseverity of environmental fluctuations(Hynes 1970, Matthews and Styron1980). In addition, the high gradientand decreased links with tributaries re-duces the potential for colonizationand entry of new species.

Species richness increases in mid-orderto lower stream reaches due to in-creased environmental stability, greaternumbers of potential habitats, and in-creases in numbers of colonizationsources or links between majordrainages. As one proceeds down-stream, pools and runs increase over rif-fles, allowing for an increase in finebottom materials and facilitating thegrowth of macrophytic vegetation.These environments allow for the pres-ence of fishes more tolerant of low oxy-


gen and increased temperatures. Fur-ther, the range of body forms increaseswith the appearance of those specieswith less fusiform body shapes, whichare ecologically adapted to areas typi-fied by decreased water velocities. Inhigher order streams or large rivers thebottom substrates often are typified byfiner sediments; thus herbivores, omni-vores, and planktivores may increase inresponse to the availability of aquaticvegetation and plankton (Bond 1979).

Fish have evolved unique feeding andreproductive strategies to survive in thediverse habitat conditions of NorthAmerica. Horwitz (1978) examined thestructure of fish feeding guilds in 15U.S. river systems and found that mostfish species (33 percent) were benthicinsectivores, whereas piscivores (16 per-cent), herbivores (7 percent), omni-vores (6 percent), planktivores (3percent), and other guilds containedfewer species. However, Allan (1995)indicated that fish frequently changefeeding habits across habitats, lifestages, and season to adapt to changingphysical and biological conditions. Fishin smaller headwater streams tend to beinsectivores or specialists, whereas thenumber of generalists and the range offeeding strategies increases downstreamin response to increasing diversity ofconditions.

Some fish species are migratory, return-ing to a particular site over long dis-tances to spawn. Others may exhibitgreat endurance, migrating upstreamagainst currents and over obstacles suchas waterfalls. Many must move betweensalt water and freshwater, requiringgreat osmoregulatory ability (McKeown1984). Species that return from theocean environment into freshwaterstreams to spawn are called anadromousspecies.

Species generally may be referred to ascold water or warm water, and grada-tions between, depending on their tem-perature requirements (Magnuson et al.1979). Fish such as salmonids are usu-ally restricted to higher elevations ornorthern climes typified by colder,highly oxygenated water. These speciestend to be specialists, with rather nar-row thermal tolerances and rather spe-cific reproductive requirements. Forexample, salmonids typically spawn bydepositing eggs over or within cleangravels which remain oxygenated andsilt-free due to upwelling of currentswithin the interstitial spaces. Reproduc-tive movement and behavior is con-trolled by subtle thermal changescombined with increasing or decreasingday-length. Salmonid populations,therefore, are highly susceptible tomany forms of habitat degradation, in-cluding alteration of flows, temperature,and substrate quality.

Numerous fish species in the U.S. aredeclining in number. Williams andJulien (1989) presented a list of NorthAmerican fish species that the AmericanFisheries Society believed should beclassified as endangered, threatened, orof special concern. This list contains364 fish species warranting protectionbecause of their rarity. Habitat loss wasthe primary cause of depletion for ap-proximately 90 percent of the specieslisted. This study noted that 77 percentof the fish species listed were found in25 percent of the states, with the high-est concentrations in eight southwesternstates. Nehlsen et al. (1991) provided alist of 214 native naturally spawningstocks of depleted Pacific salmon, steel-head, and sea-run cutthroat stocks fromCalifornia, Oregon, Idaho, and Wash-ington. Reasons cited for the declineswere alteration of fish passage and mi-gration due to dams, flow reduction as-sociated with hydropower and


agriculture, sedimentation and habitatloss due to logging and agriculture,overfishing, and negative interactionswith other fish, including nonnativehatchery salmon and steelhead.

The widespread decline in the numbersof native fish species has led to currentwidespread interest in restoring thequality and quantity of habitats for fish.Restoration activities have frequentlycentered on improving local habitats,such as fencing or removing livestockfrom streams, constructing fish pas-sages, or installing instream physicalhabitat. However, research has demon-strated that in most of these cases thesuccess has been limited or question-able because the focus was too narrowand did not address restoration of thediverse array of habitat requirementsand resources that are needed over thelife span of a species.

Stream corridor restoration practition-ers and others are now acutely awarethat fish require many different habitatsover the season and lifespan to fulfillneeds for feeding, resting, avoidingpredators, and reproducing. For exam-ple, Livingstone and Rabeni (1991) de-termined that juvenile smallmouth bassin the Jacks Fork River of southeasternMissouri fed primarily on smallmacroinvertebrates in littoral vegeta-tion. Vegetation represented not only asource of food but a refuge from preda-tors and a warmer habitat, factors thatcan collectively optimize chances forsurvival and growth (Rabeni and Jacob-son 1993). Adult smallmouth bass,however, tended to occupy deeper poolhabitats, and the numbers and biomassof adults at various sites were attributedto these specific deep-water habitats(McClendon and Rabeni 1987). Rabeniand Jacobson (1993) suggested that anunderstanding of these specific habitats,combined with an understanding of thefluvial hydraulics and geomorphology

that form and maintain them, are keyto developing successful stream restora-tion initiatives.

The emphasis on fish communityrestoration is increasing due to manyecological, economic, and recreationalfactors. In 1996 approximately 35 mil-lion Americans older than 16 partici-pated in recreational fishing, resultingin over $36 billion in expenditures(Brouha 1997). Much of this activity isin streams, which justifies stream corri-dor restoration initiatives.

While fish stocks often receive the great-est public attention, preservation ofother aquatic biota may also may be agoal of stream restoration. Freshwatermussels, many species of which arethreatened and endangered, are often ofparticular concern. Mussels are highlysensitive to habitat disturbances andobviously benefit from intact, well-managed stream corridors. The south-central United States has the highestdiversity of mussels in the world. Mus-sel ecology also is intimately linkedwith fish ecology, as fish function ashosts for mussel larvae (glochidia).Among the major threats they face aredams, which lead to direct habitat lossand fragmentation of remaining habi-tat, persistent sedimentation, pesticides,and introduced exotic species, such asfish and other mussel species.

Abiotic and Biotic Interrelationsin the Aquatic System

Much of the spatial and temporal vari-ability of stream biota reflects variationsin both abiotic and biotic factors, in-cluding water quality, temperature,streamflow and flow velocity, substrate,the availability of food and nutrients,and predator-prey relationships. Thesefactors influence the growth, survival,and reproduction of aquatic organisms.While these factors are addressed indi-


vidually below, it is important to re-member that they are often interdepen-dent.

Flow Condition

The flow of water from upstream todownstream distinguishes streams fromother ecosystems. The spatial and tem-poral characteristics of streamflow, suchas fast versus slow, deep versus shallow,turbulent versus smooth, and floodingversus low flows, are described previ-ously in this chapter. These flow charac-teristics can affect both micro- andmacro-distribution patterns of numer-ous stream species (Bayley and Li 1992,Reynolds 1992, Ward 1992). Many or-ganisms are sensitive to flow velocitybecause it represents an importantmechanism for delivering food and nu-trients yet also may limit the ability oforganisms to remain in a stream seg-ment. Some organisms also respond totemporal variations in flow, which canchange the physical structure of thestream channel, as well as increase mor-tality, modify available resources, anddisrupt interactions among species(Resh et al. 1988, Bayley and Li 1992).

The flow velocity in streams determineswhether planktonic forms can developand sustain themselves. The slower thecurrents in a stream, the more closelythe composition and configuration ofbiota at the shore and on the bottomapproach those of standing water (Rut-tner 1963). High flows are cues for tim-ing migration and spawning of somefishes. High flows also cleanse and sortstreambed materials and scour pools.Extreme low flows may limit young fishproduction because such flows oftenoccur during periods of recruitment andgrowth (Kohler and Hubert 1993).

Water Temperature

Water temperature can vary markedlywithin and among stream systems as afunction of ambient air temperature, al-

titude, latitude, origin of the water, andsolar radiation (Ward 1985, Sweeney1993). Temperature governs many bio-chemical and physiological processes incold-blooded aquatic organisms be-cause their body temperature is thesame as the surrounding water; thus,water temperature has an importantrole in determining growth, develop-ment, and behavioral patterns. Streaminsects, for example, often grow and de-velop more rapidly in warmer portionsof a stream or during warmer seasons.Where the thermal differences amongsites are significant (e.g., along latitudi-nal or altitudinal gradients), it is possi-ble for some species to complete two ormore generations per year at warmersites; these same species complete oneor fewer generations per year at coolersites (Sweeney 1984, Ward 1992).Growth rates for algae and fish appearto respond to temperature changes in asimilar fashion (Hynes 1970, Reynolds1992). The relationships between tem-perature and growth, development, andbehavior can be strong enough to affectgeographic ranges of some species(Table 2.13).

Water temperature is one of the mostimportant factors determining the dis-tribution of fish in freshwater streams,due both to direct impacts and influ-ence on dissolved oxygen concentra-tions, and is influenced by localconditions, such as shade, depth andcurrent. Many fish species can tolerateonly a limited temperature range. Suchfish as salmonids and sculpins domi-nate in cold water streams, whereassuch species as largemouth bass, small-mouth bass, suckers, minnows, sun-fishes and catfishes may be present inwarmer streams (Walburg 1971).

Effects of Cover

For the purposes of restoration, landuse practices that remove overhead


cover or decrease baseflows can increaseinstream temperatures to levels that ex-ceed critical thermal maxima for fishes(Feminella and Matthews 1984). Thus,maintenance or restoration of normaltemperature regimes can be an impor-tant endpoint for stream managers.

Riparian vegetation is an important fac-tor in the attenuation of light and tem-perature in streams (Cole 1994). Directsunlight can significantly warm streams,particularly during summer periods oflow flow. Under such conditions,streams flowing through forests warmrapidly as they enter deforested areas,but may also cool somewhat whenstreams reenter the forest. In Pennsylva-nia (Lynch et al. 1980), average dailystream temperatures that increased12ºC through a clearcut area were sub-stantially moderated after flow through1,640 feet of forest below the clearcut.They attributed the temperature reduc-tion primarily to inflows of coolerground water.

A lack of cover also affects stream tem-perature during the winter. Sweeney(1993) found that, while average dailytemperatures were higher in a second-

order meadow stream than in a compa-rable wooded reach from April throughOctober, the reverse was true from No-vember through March. In a review oftemperature effects on stream macroin-vertebrates common to the Pennsylva-nia Piedmont, Sweeney (1992) foundthat temperature changes of 2 to 6 ºCusually altered key life-history charac-teristics of the study species. Riparianforest buffers have been shown to pre-vent the disruption of natural tempera-ture patterns as well as to mitigate theincreases in temperature following de-forestation (Brown and Krygier 1970,Brazier and Brown 1973).

The exact buffer width needed for tem-perature control will vary from site tosite depending on such factors asstream orientation, vegetation, andwidth. Along a smaller, narrow headwa-ter stream, the reestablishment ofshrubs, e.g., willows and alders, mayprovide adequate shade and detritus torestore both the riparian and aquaticecosystems. The planting and/orreestablishment of large trees, e.g., cot-tonwoods, willows, sycamores, ash, andwalnuts (Lowe 1964), along larger,higher order rivers can improve the seg-

Species Max. WeeklyAverage Temp. for Growth (Juveniles)

Max. Temp. forSurvival of ShortExposure (Juveniles)

68ºF 73ºF

Max. WeeklyAverage Temp.for Spawninga

41ºF

Max. Temp.for EmbryoSpawningb

52ºFAtlantic salmon

90ºF 95ºF 77ºF 93ºFBluegill

66ºF 75ºF 48ºF 55ºFBrook trout

70ºF 91ºFCommon carp

90ºF 95ºF 81ºF 84ºFcChannel catfish

90ºF 93ºF 70ºF 81ºFcLargemouth bass

66ºF 75ºF 48ºF 55ºFRainbow trout

84ºF 63ºF 73ºFcSmallmouth bass

64ºF 72ºF 50ºF 55ºFSockeye salmon

a Optimum or mean of the range of spawning temperatures reported for the species.b Upper temperature for successful incubation and hatching reported for the species.c Upper temperature for spawning.

Table 2.13: Maximum weekly average temperatures for growth and short term maximumtemperatures for selected fish (ºF).Source: Brungs and Jones 1977.


ment of the fishery closest to the banks,but has little total effect on light andtemperature of wider rivers.

Heat budget models can accurately pre-dict stream and river temperatures (e.g.,Beschta 1984, Theurer et al. 1984).Solar radiation is the major factor influ-encing peak summer water tempera-tures and shading is critical to theoverall temperature regime of streamsin small watersheds.

Dissolved Oxygen

Oxygen enters the water by absorptiondirectly from the atmosphere and byplant photosynthesis (Mackenthun1969). Due to the shallow depth, largesurface exposure to air and constantmotion, streams generally contain anabundant dissolved oxygen supply evenwhen there is no oxygen production byphotosynthesis.

Dissolved oxygen at appropriate con-centrations is essential not only to keepaquatic organisms alive but to sustaintheir reproduction, vigor, and develop-ment. Organisms undergo stress at re-duced oxygen levels that make them

less competitive in sustaining thespecies (Mackenthun 1969). Dissolvedoxygen concentrations of 3.0 mg/L orless have been shown to interfere withfish populations for a number of rea-sons (Mackenthun 1969, citing severalother sources) (Table 2.14).

Depletion of dissolved oxygen can re-sult in the death of aquatic organisms,including fish. Fish die when the de-mand for oxygen by biological andchemical processes exceeds the oxygeninput by reaeration and photosynthesis,resulting in fish suffocation. Oxygen de-pletion usually is associated with slowcurrent, high temperature, extensivegrowth of rooted aquatic plants, algalblooms, or high concentrations of or-ganic matter (Needham 1969).

Stream communities are susceptible topollution that reduces the dissolvedoxygen supply (Odum 1971). Majorfactors determining the amount of oxy-gen found in water are temperature,pressure, abundance of aquatic plantsand the amount of natural aerationfrom contact with the atmosphere(Needham 1969). A level of 5 mg/L of

11 (8) 6.5

9 (6) 5.5

8 (5) 5.0

7 (4) 4.5

6 (3) 4.0

8 (0) 6.0

6 (0) 5.0

Early life stages (eggs and fry)

Salmonida NonsalmonidLevel of Effect

No production impairment

Slight production impairment

Moderate production impairment

Severe production impairment

Limit to avoid acute mortality

Other life stages

No production impairment

Slight production impairment

5 (0) 4.0Moderate production impairment

4 (0) 3.5Severe production impairment

3 (0) 3.0Limit to avoid acute mortality

a Values for salmonid early life stages are water column concentrations recommended to achieve the required concentration of dissolved oxygen in the gravel spawning substrate (shown in parentheses).

Table 2.14: Summary of dissolved oxygen concentrations (mg/L) generally associated with effectson fish in salmonid and nonsalmonid waters.Source: USEPA 1987.


dissolved oxygen in water is associatedwith normal activity of most fish (Wal-burg 1971). Oxygen analyses of goodtrout streams show dissolved oxygenconcentrations that range from 4.5 to9.5 mg/L (Needham 1969).

pH

Aquatic organisms from a wide range oftaxa exist and thrive in aquatic systemswith nearly neutral hydrogen ion activ-ity (pH 7). Deviations, either toward amore basic or acidic environment, in-crease chronic stress levels and eventu-ally decrease species diversity andabundance (Figure 2.34). One of themore widely recognized impacts ofchanges in pH has been attributed to

increased acidity of rainfall in someparts of the United States, especiallyareas downwind of industrial andurban emissions (Schreiber 1995). Ofparticular concern are environmentsthat have a reduced capacity to neutral-ize acid inputs because soils have a lim-ited buffering capacity. Acidic rainfallcan be especially harmful to environ-ments such as the Adirondack region ofupstate New York, where runoff alreadytends to be slightly acidic as a result ofnatural conditions.

Substrate

Stream biota respond to the many abi-otic and biotic variables influenced bysubstrate. For example, differences in

Mayfly**

Snail**

Crayfish**

Clam**

Spotted salamander* (Ambystoma maculatum)

American toad* (Bufo americanus)

Wood frog* (R. sylvatica)

Bullfrog*(Rana catesbeiana)

Yellow perch (Perca flavescens)

Pumpkinseed sunfish (Lepomis gibbosus)

Flathead minnow (Pimephalus promelas)

Smallmouth bass(Micropterus dolomieu)

Brook trout(Salvelinus fontinalus)

Brown trout (Salmo trutta)

Rainbow trout(Oncorhyncus mykiss)

6.5 5.56.0 5.0

pH

4.5 4.0 3.5 3.0

*embryonic life stage**selected species

Figure 2.34: Effects of acid rain on some aquatic species. As acidity increases (and pH decreases) inlakes and streams, some species are lost.


species composition and abundancecan be observed among macroinverte-brate assemblages found in snags, sand,bedrock, and cobble within a singlestream reach (Benke et al. 1984, Smocket al. 1985, Huryn and Wallace 1987).This preference for conditions associ-ated with different substrates con-tributes to patterns observed at largerspatial scales where different macroin-vertebrate assemblages are found incoastal, piedmont, and mountainstreams (Hackney et al. 1992).

Stream substrates can be viewed in thesame functional capacity as soils in theterrestrial system; that is, stream sub-strates constitute the interface betweenwater and the hyporheic subsurface ofthe aquatic system. The hyporheic zoneis the area of substrate which lies belowthe substrate/water interface, and mayrange from a layer extending onlyinches beneath and laterally from thestream channel, to a very large subsur-face environment. Alluvial floodplainsof the Flathead River, Montana, have ahyporheic zone with significant sur-face water/ground water interactionwhich is 2 miles wide and 33 feet deep(Stanford and Ward 1988). Naiman etal. (1994) discussed the extent and con-nectivity of hyporheic zones aroundstreams in the Pacific Northwest. Theyhypothesized that as one moves fromlow-order (small) streams to high-order(large) streams, the degree of hy-porheic importance and continuityfirst increases and then decreases. Insmall streams, the hyporheic zone islimited to small floodplains, meadows,and stream segments where coarse sedi-ments are deposited over bedrock. Thehyporheic zones are generally not con-tinuous. In mid-order channels withmore extensive floodplains, the spatialconnectivity of the hyporheic zone in-creases. In large order streams, the spa-tial extent of the hyporheic zone is

usually greatest, but it tends to behighly discontinuous because of fea-tures associated with fluvial activitiessuch as oxbow lakes and cutoff chan-nels, and because of complex interac-tions of local, intermediate, andregional ground water systems (Naimanet al. 1994) (Figure 2.35).

Stream substrates are composed of vari-ous materials, including clay, sand,gravel, cobbles, boulders, organic mat-ter, and woody debris. Substrates formsolid structures that modify surface andinterstitial flow patterns, influence theaccumulation of organic materials, andprovide for production, decomposition,and other processes (Minshall 1984).Sand and silt are generally the leastfavorable substrates for supportingaquatic organisms and support thefewest species and individuals. Flat orrubble substrates have the highest den-sities and the most organisms (Odum1971). As previously described, sub-strate size, heterogeneity, stability withrespect to high and baseflow, and dura-bility vary within streams, dependingon particle size, density, and kinetic en-ergy of flow. Inorganic substrates tendto be of larger size upstream than downstreamand tend to be larger in riffles than inpools (Leopold et al. 1964). Likewise,the distribution and role of woody de-bris varies with stream size (Maser andSedell 1994).

In forested watersheds, and in streamswith significant areas of trees in their ri-parian corridor, large woody debris thatfalls into the stream can increase thequantity and diversity of substrate andaquatic habitat or range (Bisson et al.1987, Dolloff et al. 1994). Debris damstrap sediment behind them and oftencreate scour holes immediately down-stream. Eroded banks commonly occurat the boundaries of debris blockages.


Organic Material

Metabolic activity within a stream reachdepends on autochthonous, allochtho-nous, and upstream sources of food andnutrients (Minshall et al. 1985). Au-tochthonous materials, such as algaeand aquatic macrophytes, originatewithin the stream channel, whereas al-lochthonous materials such as wood,leaves, and dissolved organic carbon,originate outside the stream channel.Upstream materials may be of au-tochthonous or allochthonous originand are transported by streamflow todownstream locations. Seasonal flood-ing provides allochthonous input of or-ganic material to the stream channel andalso can significantly increase the rate ofdecomposition of organic material.

The role of primary productivity ofstreams can vary depending on geo-graphic location, stream size, and sea-son (Odum 1957, Minshall 1978). Theriver continuum concept (Vannote et al.1980) (see The River Continuum Conceptin section 1.E in Chapter 1) hypothe-sizes that primary productivity is ofminimal importance in shaded head-water streams but increases in signifi-cance as stream size increases andriparian vegetation no longer limits theentry of light to stream periphyton. Nu-merous researchers have demonstratedthat primary productivity is of greaterimportance in certain ecosystems, in-cluding streams in grassland and desertecosystems. Flora of streams can rangefrom diatoms in high mountain streamsto dense stands of macrophytes in lowgradient streams of the Southeast.

As discussed in Section 2.C, loading ofnitrogen and phosphorus to a streamcan increase the rate of algae andaquatic plant growth, a process knownas eutrophication. Decomposition of thisexcess organic matter can deplete oxy-

gen reserves and result in fish kills andother aesthetic problems in waterbodies.

Eutrophication in lakes and reservoirs isindirectly measured as standing cropsof phytoplankton biomass, usually rep-resented by planktonic chlorophyll aconcentration. However, phytoplanktonbiomass is usually not the dominantportion of plant biomass in smallerstreams, due to periods of energeticflow and high substrate to volume ra-tios that favor the development of peri-phyton and macrophytes on the streambottom. Stream eutrophication can re-sult in excessive algal mats and oxygendepletion at times of decreased flowsand higher temperatures (Figure 2.36).Furthermore, excessive plant growth canoccur in streams at apparently low am-bient concentrations of nitrogen andphosphorus because the stream currentspromote efficient exchange of nutrientsand metabolic wastes at the plant cellsurface.

watertable

permeablelayer

impermeablelayer

ground water

hyporheic zone

Figure 2.35: Hyporheic zone. Summary of thedifferent means of migration undergone bymembers of the stream benthic community.


In many streams, shading or turbiditylimit the light available for algalgrowth, and biota depend highly onallochthonous organic matter, such asleaves and twigs produced in the sur-rounding watershed. Once leaves orother allochthonous materials enter thestream, they undergo rapid changes(Cummins 1974). Soluble organic com-pounds, such as sugars, are removed vialeaching. Bacteria and fungi subse-quently colonize the leaf materials andmetabolize them as a source of carbon.The presence of the microbial biomassincreases the protein content of theleaves, which ultimately represents ahigh quality food resource for shred-ding invertebrates.

The combination of microbial decom-position and invertebrate shredding/scraping reduces the average particlesize of the organic matter, resulting inthe loss of carbon both as respired CO

2

and as smaller organic particles trans-ported downstream. These finer parti-cles, lost from one stream segment,become the energy inputs to the down-

stream portions of the stream. This uni-directional movement of nutrients andorganic matter in lotic systems isslowed by the temporary retention,storage, and utilization of nutrients inleaf packs, accumulated debris, inverte-brates, and algae.

Organic matter processing has beenshown to have nutrient-dependent rela-tionships similar to primary productiv-ity. Decomposition of leaves and otherforms of organic matter can be limitedby either nitrogen or phosphorus, withpredictive N:P ratios being similar tothose for growth of algae and periphy-ton. Leaf decomposition occurs by asequential combination of microbialdecomposition, invertebrate shredding,and physical fractionation. Leaves andorganic matter itself are generally lowin protein value. However, the coloniza-tion of organic matter by bacteria andfungi increases the net content of nitro-gen and phosphorus due to the accu-mulation of proteins and lipidscontained in microbial biomass. Thesecompounds are a major nutritive sourcefor aquatic invertebrates. Decaying or-ganic matter represents a major storagecomponent for nutrients in streams, aswell as a primary pathway of energyand nutrient transfer within the foodweb. Ultimately, the efficiency of reten-tion and utilization is reflected at thetop of the food web in the form of fishbiomass.

Organisms often respond to variationsin the availability of autochthonous, al-lochthonous, and upstream sources. Forexample, herbivores are relatively morecommon in streams having open ripar-ian canopies and high algal productiv-ity compared to streams having closedcanopies and accumulated leaves as theprimary food resource (Minshall et al.1983). Similar patterns can be observedlongitudinally within the same stream(Behmer and Hawkins 1986).

Figure 2.36: Stream eutrophication.Eutrophication can result in oxygen depletion.


Terrestrial and AquaticEcosystem Components forStream Corridor Restoration

The previous sections presented the bio-logical components and functionalprocesses that shape stream corridors.The terrestrial and aquatic environ-ments were discussed separately for thesake of simplicity and ease of under-standing. Unfortunately, this is fre-quently the same approach taken inenvironmental restoration initiatives,with efforts placed separately on theuplands, riparian area, or instreamchannel. The stream corridor must beviewed as a single functioning unit orecosystem with numerous connectionsand interactions between components.Successful stream corridor restorationcannot ignore these fundamental rela-tionships.

The structure and functions of vegeta-tion are interrelated at all scales. Theyare also directly tied to ecosystem dy-namics. Particular vegetation types mayhave characteristic regeneration strate-gies (e.g., fire, treefall gaps) that main-tain those types within the landscape atall times. Similarly, certain topographicsettings may be more likely than othersto be subject to periodic, dramaticchanges in hydrology and related vege-tation structure as a result of massivedebris jams or occupation by beavers.However, in the context of stream corri-dor ecosystems, some of the most fun-damental dynamic interactions relate tostream flooding and channel migration.

Many ecosystem functions are influ-enced by the structural characteristics ofvegetation. In an undeveloped water-shed, the movement of water and othermaterials is moderated by vegetationand detritus, and nutrients are mobi-lized and conserved in complex pat-terns that generally result in balancedinteractions between terrestrial and

aquatic systems. As the character anddistribution of vegetation is altered byremoval of biomass, agriculture, live-stock grazing, development, and otherland uses, and the flow patterns ofwater, sediment, and nutrients are mod-ified, the interactions among systemcomponents become less efficient andeffective. These problems can becomemore pronounced when they are aggra-vated by introductions of excess nutri-ents and synthetic toxins, soildisturbances, and similar impacts.

Stream migration and flooding areprincipal sources of structural andcompositional variation within andamong plant communities in mostundisturbed floodplains (Brinson et al.1981). Although streams exert a com-plex influence on plant communities,vegetation directly affects the integrityand characteristics of stream systems.For example, root systems bind banksediments and moderate erosionprocesses, and floodplain vegetationslows overbank flows, inducing sedi-ment deposition. Trees and smallerwoody debris that fall into the channeldeflect flows, inducing erosion at somepoints and deposition at others, alterpool distribution, the transport of or-ganic material, as well as a number ofother processes. The stabilization ofstreams that are highly interactive withtheir floodplains can disrupt the funda-mental processes controlling the struc-ture and function of stream corridorecosystems, thereby indirectly affectingthe characteristics of the surroundinglandscape.

In most instances, the functions of veg-etation that are most apparent are thosethat influence fish and wildlife. At thelandscape level, the fragmentation ofnative cover types has been shown tosignificantly influence wildlife, often fa-voring opportunistic species over thoserequiring large blocks of contiguous


habitat. In some systems, relativelysmall breaks in corridor continuity canhave significant impacts on animalmovement or on the suitability ofstream conditions to support certainaquatic species. In others, establishmentof corridors that are structurally differ-ent from native systems or inappropri-ately configured can be equallydisruptive. Narrow corridors that are es-sentially edge habitat may encouragegeneralist species, nest parasites, andpredators, and where corridors havebeen established across historic barriersto animal movement, they can disruptthe integrity of regional animal assem-blages (Knopf et al. 1988).

Some riparian dependent species arelinked to streamside riparian areas withfairly contiguous dense tree canopies.Without new trees coming into thepopulation, older trees creating thislinked canopy eventually drop out, cre-ating ever smaller patches of habitat.Restoration that influences tree standsso that sufficient recruitment and patchsize can be attained will benefit thesespecies. For similar reasons, many ripar-ian-related raptors such as the commonblack-hawk (Buteogallus anthracinus),gray hawk (Buteo nitidus), bald eagle(Haliaeetus leucocephalus), Cactus ferrug-inous pygmy-owl (Glaucidium brasil-ianum cactorum), and Cooper’s hawk(Accipiter cooperii), depend upon varioussizes and shapes of woody riparian treesfor nesting substrate and roosts.Restoration practices that attain suffi-cient tree recruitment will greatly bene-fit these species in the long term, andother species in the short term.

Some aspects related to this subjecthave been discussed as ecosystem com-ponents and functions under other sec-tions. Findings from the earliest studiesof the impacts of fragmentation of ri-parian habitats on breeding birds werepublished for the Southwest (Carothers

and Johnson 1971, Johnson 1971,Carothers et al. 1974). Subsequentstudies by other investigators foundsimilar results. Basically, cottonwood-willow gallery forests of the NorthAmerican Southwest supported thehighest concentrations of noncolonialnesting birds for North America. De-struction and fragmentation of these ri-parian forests reduced species richnessand resulted in a nearly straight-line re-lationship between numbers of nestingpairs/acre and number of maturetrees/acre. Later studies demonstratedthat riparian areas are equally impor-tant as conduits for migrating birds(Johnson and Simpson 1971, Stevens etal. 1977).

When considering restoration of ripar-ian habitats, the condition of adjacenthabitats must be considered. Carothers(1979) found that riparian ecosystems,especially the edges, are widely used bynonriparian birds. In addition he foundthat some riparian birds utilized adja-cent nonriparian ecosystems. Carotherset al. (1974) found that smaller breed-ing species [e.g., warblers and the West-ern wood pewee (Contopus sordidulus)]tended to carry on all activities withinthe riparian ecosystem during thebreeding season. However, largerspecies (e.g., kingbirds and doves) com-monly foraged outside the riparianecosystem in adjacent habitats. Largerspecies (e.g., raptors) may forage milesfrom riparian ecosystems, but still de-pend on them in critical ways (Lee et al.1989).

Because of more mesic conditions cre-ated by the canyon effect, canyons andtheir attendant riparian vegetation serveas corridors for short-range movementsof animals along elevational gradients(e.g., between summer and winterranges). Long-range movements thatoccur along riparian zones throughoutNorth America include migration of


birds and bats. Riparian zones alsoserve as stopover habitat for migratingbirds (Stevens et al. 1977). Woody vege-tation is generally important, not onlyto most riparian ecosystems, but also toadjacent aquatic and even uplandecosystems. However, it is important toestablish clear management objectivesbefore attempting habitat modification.

Restoring all of a given ecosystem to its“pristine condition” may be impossible,especially if upstream conditions havebeen heavily modified, such as by adam or other water diversion project.Even if complete restoration is a possi-bility, it may not accomplish or com-plement the restoration goals.

For example, encroachment of woodyvegetation in the channel below severaldams in the Platte River Valley in Ne-

braska has greatly decreased theamount of important wet meadowhabitat. This area has been declaredcritical habitat for the whooping crane(Grus americana) (Aronson and Ellis1979), for piping plover, and for the in-terior least tern. It is also an importantstaging area for up to 500,000 sandhillcranes (Grus canadensis) from late Feb-ruary to late April and supports 150 to250 bald eagles (Haliaeetusleucocephalus). Numerous other impor-tant species using the area include theperegrine falcon (Falco peregrinus),Canada goose (Branta canadensis), mal-lard (Anas platyrhynchos), numerousother waterfowl, and raptors (USFWS1981). Thus, managers here are con-fronted with means of reducing ripariangroves in favor of wet meadows.


2.E Functions and Dynamic Equilibrium

Throughout the past two chapters, thisdocument has covered stream corridorstructure and the physical, chemical,and biological processes occurring instream corridors. This informationshows how stream corridors function asecosystems, and consequently, howthese characteristic structural featuresand processes must be understood inorder to enable stream corridor func-tions to be effectively restored. In fact,reestablishing structure or restoring aparticular physical or biological processis not the only thing that restorationseeks to achieve. Restoration aims toreestablish valued functions. Focusingon ecological functions gives therestoration effort its best chance torecreate a self-sustaining system. Thisproperty of sustainability is what sepa-rates a functionally sound stream, thatfreely provides its many benefits to peo-ple and the natural environment, froman impaired watercourse that cannotsustain its valued functions and may re-main a costly, long-term maintenanceburden.

Section 1.A of Chapter 1 emphasizedmatrix, patch, corridor and mosaic asthe most basic building blocks of physi-cal structure at local to regional scales.Ecological functions, too, can be sum-marized as a set of basic, commonthemes that recur in an infinite varietyof settings. These six critical functionsare habitat, conduit, filter, barrier, source,and sink (Figure 2.37).

In this section, the processes and struc-tural descriptions of the past two chap-ters are revisited in terms of thesecritical ecological functions.

Two attributes are particularly impor-tant to the operation of stream corridorfunctions:

Habitat

Barrier

Conduit

Filter

Source

Sink

Figure 2.37: Critical ecosystem functions. Sixfunctions can be summarized as a set of basic,common themes recurring in a variety of settings.

Barrier—the stoppageof materials, energy,and organisms.

Habitat—the spatialstructure of the envi-ronment which allowsspecies to live, repro-duce, feed, and move.

Conduit—the ability ofthe system to transportmaterials, energy, andorganisms.

Filter—the selectivepenetration of materi-als, energy, and organ-isms.

Source—a settingwhere the output ofmaterials, energy, andorganisms exceedsinput.

Sink—a setting wherethe input of water,energy, organisms and materials exceedsoutput.

■ Connectivity—This is a measure ofhow spatially continuous a corridoror a matrix is (Forman and Godron1986). This attribute is affected bygaps or breaks in the corridor andbetween the corridor and adjacentland uses (Figure 2.38). A streamcorridor with a high degree of con-nectivity among its natural commu-nities promotes valuable functionsincluding transport of materials andenergy and movement of flora andfauna.

■ Width—In stream corridors, this refersto the distance across the stream andits zone of adjacent vegetation cover.Factors affecting width are edges,community composition, environ-mental gradients, and disturbanceeffects of adjacent ecosystems,including those with human activity.Example measures of width include

average dimension and variance,number of narrows, and varyinghabitat requirements (Dramstad etal. 1996).

Width and connectivity interactthroughout the length of a stream corri-dor. Corridor width varies along thelength of the stream and may havegaps. Gaps across the corridor interruptand reduce connectivity. Evaluatingconnectivity and width can providesome of the most valuable insight fordesigning restoration actions that miti-gate disturbances.

The following subsections discuss eachof the functions and general relation-ship to connectivity and width. Thefinal subsection discusses dynamicequilibrium and its relevance to streamcorridor restoration.

Functions and Dynamic Equilibrium 2–79

A B

Figure 2.38: Landscapes with (A) high and (B) low degrees of connectivity. A connected landscapestructure generally has higher levels of functions than a fragmented landscape.


Habitat Functions

Habitat is a term used to describe anarea where plants or animals (includingpeople) normally live, grow, feed, re-produce, and otherwise exist for anyportion of their life cycle. Habitats pro-vide organisms or communities of or-ganisms with the necessary elements oflife, such as space, food, water, andshelter.

Under suitable conditions often pro-vided by stream corridors, many speciescan use the corridor to live, find foodand water, reproduce, and establish vi-able populations. Some measures of astable biological community are popu-lation size, number of species, and ge-netic variation, which fluctuate withinexpected limits over time. To varyingdegrees, stream corridors constructivelyinfluence these measures. The corridor’svalue as habitat is increased by the factthat corridors often connect many smallhabitat patches and thereby createlarger, more complex habitats withlarger wildlife populations and higherbiodiversity.

Habitat functions differ at variousscales, and an appreciation of the scalesat which different habitat functionsoccur will help a restoration initiativesucceed. The evaluation of habitat atlarger scales, for example, may makenote of a biotic community’s size, com-position, connectivity, and shape.

At the landscape scale, the concepts ofmatrix, patches, mosaics and corridorsare often involved in describing habitatover large areas. Stream corridors and

major river valleys together can providesubstantial habitat. North American fly-ways include examples of stream andriver corridor habitat exploited by mi-gratory birds at landscape to regionalscales.

Stream corridors, and other types ofnaturally vegetated corridors as well,can provide migrating forest and ripar-ian species with their preferred restingand feeding habitats during migrationstopovers. Large mammals such asblack bear are known to require large,contiguous wild terrain as home range,and in many parts of the country broadstream corridors are crucial to linkingsmaller patches into sufficiently largeterritories.

Habitat functions within watershedsmay be examined from a somewhat dif-ferent perspective. Habitat types andpatterns within the watershed are signif-icant, as are patterns of connectivity toadjoining watersheds. The vegetation ofthe stream corridor in upper reaches ofwatersheds sometimes has become dis-connected from that of adjacent water-sheds and corridors beyond the divide.When terrestrial or semiaquatic streamcorridor communities are connected attheir headwaters, these connections willusually help provide suitable alternativehabitats beyond the watershed.

Assessing habitat function at the streamcorridor and smaller scales can also beviewed in terms of patches and corri-dors, but in finer detail than in land-scapes and watersheds. It is also at localscales that transitions among the vari-ous habitats within the corridor can be-come more important. Stream corridorsoften include two general types of habi-tat structure: interior and edge habitat.Habitat diversity is increased by a corri-dor that includes both edge and interiorconditions, although for most streams,corridor width is insufficient to provide

Natural Disturbances 2–81

interior

edge

Figure 2.39: Edge and interior habitat of a woodlot.Interior plants and animals differ considerably fromthose that prefer or tolerate the edge’s variability.

Two important habitat characteristics are edgesand interior (Figure 2.39) Edges are critical lines ofinteraction between different ecosystems. Interiorhabitats are generally more stable, sheltered envi-ronments where the ecosystem may remain rela-tively the same for prolonged periods. Edge habi-tat is exposed to highly variable environmental gra-dients. The result is a different species compositionand abundance than observed interior habitat.Edges are important as filters of disturbance tointerior habitat. Edges can also be diverse areaswith a large variety of flora and fauna.

Edges and interiors are scale-independent concepts.Larger mammals known as interior forest speciesmay need to be miles from the forest edge to finddesired habitat, while an insect or amphibian maybe sensitive to the edges and interiors of the micro-habitat under a rotted log. The edges and interiorsof a stream corridor, therefore, depend upon thespecies being considered. As elongated, narrowecosystems that include land/water interfaces andoften include natural/human-made boundaries aswell at the upland fringe, stream corridors have anabundance of edges and these have a pronouncedeffect on their biota.

Edges and interiors are each preferred by differentsets of plant and animal species, and it is inappro-priate to consider edges or interiors as consistently“bad” or “good” habitat characteristics. It may bedesirable to maintain or increase edge in somecircumstances, or favor interior habitats in others.Generally speaking, however, human activity tendsto increase edge and decrease interior, so moreoften it is restoring or protecting interior thatmerits specific management action.

Edge habitat at the stream corridor boundary typi-cally has higher inputs of solar energy, precipita-tion, wind energy, and other influences from theadjacent ecosystems. The difference in environ-mental gradients at the stream corridor’s edgeresults in a diversified plant and animal communityinteracting with adjacent ecosystems. The effect of

edge is more pronounced when the amount ofinterior habitat is minimal.

Interior habitat occurs further from the perimeterof the element. Interior is typified by more stableenvironmental inputs than those found at theedge of an ecosystem. Sunlight, rainfall, and windeffects are less intense in the interior. Many sensi-tive or rare species depend upon a less-disturbedenvironment for their survival. They are thereforetolerant of only “interior” habitat conditions. Thedistance from the perimeter required to createthese interior conditions is dependent upon thespecies’ requirements.

Interior plants and animals differ considerably fromthose that prefer or tolerate the edge’s variability.With an abundance of edge, stream corridorsoften have mostly edge species. Because largeecosystems and wide corridors are becomingincreasingly fragmented in modern landscapes,however, interior species are often rare and henceare targets for restoration. The habitat require-ments of interior species (with respect to distancefrom edge are a useful guide in restoring largerstream corridors to provide a diversity of habitattypes and sustainable communities.


much interior habitat for larger verte-brates such as forest interior birdspecies. For this reason, increasing inte-rior habitat is sometimes a watershedscale restoration objective.

Habitat functions at the corridor scaleare strongly influenced by connectivityand width. Greater connectivity and in-creased width along and across a streamcorridor generally increases its value ashabitat. Stream valley morphology andenvironmental gradients (such as grad-ual changes in soil wetness, solar radia-tion, and precipitation) can causechanges in plant and animal communi-ties. More species generally find suitablehabitat conditions in a wide, contigu-ous, and diverse assortment of nativeplant communities within the streamcorridor than in a narrow, homoge-neous or highly fragmented corridor.

When applied strictly to stream chan-nels, however, this might not be true.Some narrow and deeply incisedstreams, for example, provide thermalconditions that are critical for endan-gered salmonids.

Habitat conditions within a corridorvary according to factors such as climateand microclimate, elevation, topogra-phy, soils, hydrology, vegetation, andhuman uses. In terms of planningrestoration measures, corridor width isespecially important for wildlife. Whenplanning for maintenance of a givenwildlife species, for example, the dimen-sion and shape of the corridor must bewide enough to include enough suit-able habitat that this species can popu-late the stream corridor. Corridors thatare too narrow may provide as much ofa barrier to some species’ movement aswould a complete gap in the corridor.

On local scales, large woody debris thatbecomes lodged in the stream channelcan create morphological changes tothe stream and adjacent streambanks.

Pools may be formed downstream froma log that has fallen across a stream andboth upstream and downstream flowcharacteristics are altered. The structureformed by large woody debris in astream improves aquatic habitat formost fish and invertebrate species.

Riparian forests, in addition to theiredge and interior habitats, may offervertical habitat diversity in their canopy,subcanopy, shrub and herb layers. Andwithin the channel itself, riffles, pools,glides, rapids and backwaters all pro-vide different habitat conditions inboth the water column and thestreambed. These examples, all de-scribed in terms of physical structure,illustrate once again the strong linkagebetween structure and habitat function.

Conduit Function

The conduit function is the ability toserve as a flow pathway for energy, ma-terials, and organisms. A stream corri-dor is above all a conduit that wasformed by and for collecting and trans-porting water and sediment. In addi-tion, many other types of materials andbiota move throughout the system.

The stream corridor can function as aconduit laterally, as well as longitudi-nally, with movement by organisms andmaterials in any number of directions.Materials or animals may further moveacross the stream corridor, from oneside to another. Birds or small mam-mals, for example, may cross a streamwith a closed canopy by movingthrough its vegetation. Organic debrisand nutrients may fall from higher to


lower floodplains and into the streamwithin corridors, affecting the food sup-ply for stream invertebrates and fishes.

Moving material is important because itimpacts the hydrology, habitat, andstructure of the stream as well as the ter-restrial habitat and connections in thefloodplain and uplands. The structuralattributes of connectivity and width alsoinfluence the conduit function.

For migratory or highly mobile wildlife,corridors serve as habitat and conduitsimultaneously. Corridors in combina-tion with other suitable habitats, for ex-ample, make it possible for songbirdsto move from wintering habitat in theneo-tropics to northern, summer habi-tats. Many species of birds can only flyfor limited distances before they mustrest and refuel. For stream corridors tofunction effectively as conduits for thesebirds, they must be sufficiently con-nected and be wide enough to providerequired migratory habitat.

Stream corridors are also conduits forthe movement of energy, which occursin many forms. The gravity-driven en-ergy of stream flow continually sculptsand modifies the landscape. The corri-dor modifies heat and energy from sun-light as it remains cooler in spring andsummer and warmer in the fall. Streamvalleys are effective airsheds, movingcool air from higher to lower elevationsin the evening. The highly productiveplant communities of a corridor accu-mulate energy as living plant material,and export large amounts in the formof leaf fall or detritus. The high levelsof primary productivity, nutrient flow,and leaf litter fall also fuel increaseddecomposition in the corridor, allow-ing new transformations of energy andmaterials. At its outlet, a stream’s out-puts to the next larger water body (e.g.,increased water volume, higher temper-ature, sediments, nutrients, and organ-

isms) are in part the excesses of energyfrom its own system.

One of the best known and studied ex-amples of aquatic species movementand interaction with the watershed isthe migration of salmon upstream forspawning. After maturing in the ocean,the fish are dependent on access totheir upstream spawning grounds. Inthe case of Pacific salmon species, thestream corridor is dependent upon theresultant biomass and nutrient input ofabundant spawning and dying adultsinto the upper reaches of stream sys-tems during spawning. Thus, connectiv-ity is often critical for aquatic speciestransport, and in turn, nutrient trans-port upstream from ocean waters tostream headwaters.

Streams are also conduits for distribu-tion of plants and their establishmentin new areas (Malanson 1993). Flowingwater may transport and deposit seedsover considerable distances. In floodstage, mature plants may be uprooted,relocated, and redeposited alive in newlocations. Wildlife also help redistributeplants by ingesting and transportingseeds throughout different parts of thecorridor.

Sediment (bed load or suspended load)is also transported through the stream.Alluvial streams are dependent on thecontinual supply and transport of sedi-ment, but many of their fish and inver-tebrates can also be harmed by toomuch fine sediment. When conditionsare altered, a stream may become eitherstarved of sediment or choked with sed-iment down-gradient. Streams lackingappropriate amounts of sediment at-tempt to reestablish equilibrium throughdowncutting, bank erosion, and channelerosion. An appropriately structuredstream corridor will optimize timingand supply of sediment to the stream toimprove sediment transport functions.


Local areas in the corridor are depen-dent on the flow of materials from onepoint to another. In the salmonid ex-ample, the local upland area adjacent tospawning grounds is dependent uponthe nutrient transfer from the biomassof the fish into other terrestrial wildlifeand off into the uplands. The localstructure of the streambed and aquaticecosystem are dependent upon the sedi-ment and woody material from up-stream and upslope to create aself-regulating and stable channel.

Stream corridor width is importantwhere the upland is frequently a sup-plier of much of the natural load ofsediment and biomass into the stream.A wide, contiguous corridor acts as alarge conduit, allowing flow laterallyand longitudinally along the corridor.Conduit functions are often more lim-ited in narrow or fragmented corridors.

Filter and Barrier Functions

Stream corridors may serve as barriersthat prevent movement or filters thatallow selective penetration of energy,materials and organisms. In many ways,the entire stream corridor serves benefi-cially as a filter or barrier that reduceswater pollution, minimizes sedi-ment transport, and often provides anatural boundary to land uses, plantcommunities, and some less mobilewildlife species.

Materials, energy, and organisms whichmoved into and through the stream cor-ridor may be filtered by structural attrib-utes of the corridor. Attributes affectingbarrier and filter functions include con-

nectivity (gap frequency) and corridorwidth (Figure 2.40). Elements whichare moving along a stream corridor edgemay also be selectively filtered as theyenter the stream corridor. In these cir-cumstances it is the shape of the edge,whether it is straight or convoluted,which has the greatest effect on filteringfunctions. Still, it is most often move-ment perpendicular to the stream corri-dor which is most effectively filtered orhalted.

Materials may be transported, filtered,or stopped altogether depending uponthe width and connectedness of astream corridor. Material movementacross landscapes toward large river val-leys may be intercepted and filtered bystream corridors. Attributes such as thestructure of native plant communitiescan physically affect the amount ofrunoff entering a stream system throughuptake, absorption, and interruption.Vegetation in the corridor can filter outmuch of the overland flow of nutrients,sediment, and water.

Siltation in larger streams can be re-duced through a network of stream cor-ridors functioning to filter excessivesediment. Stream corridors filter manyof the upland materials from movingunimpeded across the landscape.Ground water and surface water flowsare filtered by plant parts below andabove ground. Chemical elements areintercepted by flora and fauna withinstream corridors. A wider corridor pro-vides more effective filtering, and a con-tiguous corridor functions as a filteralong its entire length.

Breaks in a stream corridor can some-times have the effect of funneling dam-aging processes into that area. Forexample, a gap in contiguous vegetationalong a stream corridor can reduce thefiltering function by focusing increasedrunoff into the area, leading to erosion,


gullying, and the free flow of sedimentsand nutrients into the stream.

Edges at the boundaries of stream corri-dors begin the process of filtering.Abrupt edges concentrate initial filter-ing functions into a narrow area. Agradual edge increases filtering andspreads it across a wider ecologicalgradient (Figure 2.41).

Movement parallel to the corridor isaffected by coves and lobes of an un-even corridor’s edge. These act as barri-ers or filters for materials flowing intothe corridor. Individual plants mayselectively capture materials such aswind-borne sediment, carbon, or pro-pagules as they pass through a convo-luted edge. Herbivores traveling alonga boundary edge, for example, may stopto rest and selectively feed in a shel-tered nook. The wind blows a few seedsinto the corridor, and those suited to

the conditions of the corridor may ger-minate and establish a population. Thelobes have acted as a selective filter col-lecting some seeds at the edge and al-lowing other species to interact at theboundary (Forman 1995).

novegetative buffer

widevegetative buffer

narrowvegetativebuffer

dissolved substances

Figure 2.40: The width of the vegetation buffer influences filter and barrier functions.Dissolved substances, such as nitrogen, phosphorus, and other nutrients, entering a vegetatedstream corridor are restricted from entering the channel by friction, root absorption, clay, andsoil organic matter.Adapted from Ecology of Greenways: Design and Function of Linear Conservation Areas.Edited by Smith and Hellmund. © University of Minnesota Press 1993.

Figure 2.41: Edges can be (a) abrupt or(b) gradual. Abrupt edges, usually causedby disturbances, tend to discourage movementbetween ecosystems and promote movementalong the boundary. Gradual edges usuallyoccur in natural settings, are more diverse,and encourage movement between ecosystems.

(a) (b)


In constantlychangingecosystemslike stream cor-ridors, stabilityis the ability ofa system topersist withina range of con-ditions. Thisphenomenonis referred toas dynamicequilibrium.

Source and Sink Functions

Sources provide organisms, energy ormaterials to the surrounding landscape.Areas that function as sinks absorb or-ganisms, energy, or materials from thesurrounding landscape. Influent and ef-fluent reaches, discussed in Section 1.Bof Chapter 1, are classic examples ofsources and sinks. The influent or “los-ing” reach is a source of water to theaquifer, and the effluent or “gaining”reach is a sink for ground water.

Stream corridors or features within themcan act as a source or a sink of environ-mental materials. Some stream corridorsact as both, depending on the time ofyear or location in the corridor. Stream-banks most often act as a source, forexample, of sediment to the stream. Attimes, however, they can function assinks while flooding deposits new sedi-ments there. At the landscape scale, cor-ridors are connectors to various otherpatches of habitats in the landscape andas such they are sources and conduits ofgenetic material throughout the land-scape.

Stream corridors can also act as a sinkfor storage of surface water, groundwater, nutrients, energy, and sedimentallowing for materials to be temporarilyfixed in the corridor. Dissolved sub-stances, such as nitrogen, phosphorus,and other nutrients, entering a vege-tated stream corridor are restricted fromentering the channel by friction, rootabsorption, clay, and soil organic mat-ter. Although these functions of sourceand sink are conceptually understood,

they lack a suitable body of researchand practical application guidelines.

Forman (1995) offers three source andsink functions resulting from floodplainvegetation:

■ Decreased downstream floodingthrough floodwater moderationand/or uptake

■ Containment of sediments and other materials during flood stage

■ Source of soil organic matter and water-borne organic matter

Biotic and genetic source/sink relation-ships can be complex. Interior forestbirds are vulnerable to nest parasitismby cowbirds when they try to nest intoo small a forest patch. For thesespecies, small forest patches can beconsidered sinks that reduce their pop-ulation numbers and genetic diversityby causing failed reproduction. Largeforest patches with sufficient interiorhabitat, in comparison, support success-ful reproduction and serve as sources ofmore individuals and new genetic com-binations.

Dynamic Equilibrium

The first two chapters of this documenthave emphasized that, although streamcorridors display consistent patterns intheir structure, processes, and functions,these patterns change naturally and con-stantly, even in the absence of humandisturbance. Despite frequent change,streams and their corridors exhibit adynamic form of stability. In constantlychanging ecosystems like stream corri-dors, stability is the ability of a systemto persist within a range of conditions.This phenomenon is referred to asdynamic equilibrium.

The maintenance of dynamic equilib-rium requires that a series of self-cor-recting mechanisms be active in thestream corridor ecosystem. These mech-


anisms allow the ecosystem to controlexternal stresses or disturbances withina certain range of responses therebymaintaining a self-sustaining condition.The threshold levels associated withthese ranges are difficult to identify andquantify. If they are exceeded, the sys-tem can become unstable. Corridorsmay then undergo a series of adjust-ments to achieve a new steady statecondition, but usually after a long pe-riod of time has elapsed.

Many stream systems can accommodatefairly significant disturbances and stillreturn to functional condition in a rea-sonable time frame, once the source ofthe disturbance is controlled or re-moved. Passive restoration is based onthis tendency of ecosystems to healthemselves when external stresses areremoved. Often the removal of stressand the time to recover naturally are aneconomical and effective restorationstrategy. When significant disturbanceand alteration has occurred, however, astream corridor may require severaldecades to restore itself. Even then, therecovered system may be a very differ-ent type of stream that, although atequilibrium again, is of severely dimin-ished ecological value in comparisonwith its previous potential. Whenrestoration practitioners’ analysis indi-cates lengthy recovery time or dubiousrecovery potential for a stream, theymay decide to use active restorationtechniques to reestablish a more func-tional channel form, corridor structure,and biological community in a muchshorter time frame. The main benefit ofan active restoration approach is regain-ing functionality more quickly, but thebiggest challenge is to plan, design, andimplement correctly to reestablish thedesired state of dynamic equilibrium.

This new equilibrium condition, how-ever, may not be the same that existedprior to the initial occurrence of the dis-

turbance. In addition, disturbances canoften stress the system beyond its nat-ural ability to recover. In these instancesrestoration is needed to remove thecause of the disturbance or stress (pas-sive) or to repair damages to the struc-ture and functions of the streamcorridor ecosystem (active).

Stability, as a characteristic of ecosystems, combinesthe concepts of resistance, resilience, and recovery.Resistance is the ability to maintain original form andfunctions. Resilience is the rate at which a system returnsto a stable condition after a disturbance. Recovery is thedegree to which a system returns to its original conditionafter a disturbance. Natural systems have developedways of coping with disturbance, in order to producerecovery and stability. Human activities often superim-pose additional disturbances which may exceed therecovery capability of a natural system. The fact thatchange occurs, however, does not always mean a systemis unstable or in poor condition.

The term mosaic stability is used to denote the stabilityof a larger system within which local changes still takeplace. Mosaic stability, or the lack thereof, illustrates theimportance of the landscape perspective in making site-specific decisions. For example, in a rapidly urbanizinglandscape, a riparian system denuded by a 100-yearflood may represent a harmful break in already dimin-ished habitat that splits and isolates populations of arare amphibian species. In contrast, the same ripariansystem undergoing flooding in a less-developed land-scape may not be a geographic barrier to the amphibian,but merely the mosaic of constantly shifting suitable andunsuitable habitats in an unconfined, naturally function-ing stream. The latter landscape with mosaic stability isnot likely to need restoration while the former landscapewithout mosaic stability is likely to need it urgently.Successful restoration of any stream corridor requires anunderstanding of these key underlying concepts.

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