StreamsStreams

Fluvial (Latin fluvius: river)

Streams 85% sediment transport, Streams 85% sediment transport, glaciers 7%, waves 1.5%, wind 1%glaciers 7%, waves 1.5%, wind 1%

Lab Hose, sand in beaker

Evapotranspiration and DroughtEvapotranspiration and Drought• Globally, something like 64 percent of precipitation occurs over land as a

result of evapo-transpiration from lakes and wetlands and dense vegetation, in particular forests pumping groundwater held in the soils into the air.

Sources of Earth’s waterSources of Earth’s water

(saltwater)

freshwater

InfiltrationInfiltration• Infiltration is controlled by

– Intensity and duration of rainfall– Soil texture– Slope of the land– Nature of the vegetative cover

– Runoff occurs in excess when soil is saturated or when slope is too great.

StreamflowStreamflow

• Factors that determine velocity

–Gradient, or slope–Channel characteristics including

shape, size, and roughness–Discharge – the volume of water

moving past a given point in a certain amount of time

Stream ChangesStream Changes• Changes from upstream to downstream

• Factors that decrease downstream– A. Longitudinal Profile is the side-view of a stream

– From head (headwaters) to mouth of stream profile flattens out

– B. So gradient (slope) decreases downstream – C. channel roughness (grain size) also

decreases downstream• Factors that increase downstream

– A. Velocity– B. Discharge– C. Channel size all get larger

Longitudinal profile of a Longitudinal profile of a streamstream Tributaries

Trunk Stream

Drainage Divide

Distributaries And Delta

Stream capture, decapitation

Base LevelBase Level

• Elevation below which a stream cannot erode

• Local and ultimate

A waterfall results from change A waterfall results from change

in local base levelin local base level

Local change in base level affects river profiles

Graded Streams

Demo: bank collapse, stream cross section reducedPinched hose demo

Graded StreamsGraded Streams

• Erosion and Deposition Equal• When local conditions in a graded stream are

disrupted, velocity changes.• Original conditions are restored.• DEMOS• Pinched hose demo, change in velocity at

restrictions• Hose and sand demo: faster water is better at

erosion

Stream - Sediment TerminologyStream - Sediment Terminology

• Transported material is called sediment load• Types of load

– Dissolved load - ions in solution

– Suspended load - fine clays, etc

– Bed load Moves during flood - high velocity Sandy Portion – Saltation (bouncing)

Boulders – Traction (rolling)

• Capacity –maximum load stream transports

• Competence – Largest particles it can move• Proportional to velocity squared

More important definitions

Suspended load - confluence Green & Colorado Rivers in Canyonlands, Utah

• Competence • Maximum particle size a stream transports

• Proportional to velocity squared –K.E.

Deposition of sediment by a streamCaused by a decrease in velocity

Competence is reducedSediment begins to drop out

• 1. Narrow valleys uplift or drop in base level

• V-shaped • Downcutting toward base level• Can include rapids and waterfalls

• Straighter streams• Typical of steep slopes in

uplifted areas

An Alluvial Fan in Death ValleySudden drop in competence

Alluvial fansAlluvial fansDevelop where a Develop where a high-gradient high-gradient stream leaves a stream leaves a narrow valley and narrow valley and spreads out onto spreads out onto a broad plaina broad plainSlopes outward Slopes outward in a broad arcin a broad arc

2. Wide valleys

–Meanders "Meandering Stream"

• Cut bank and point bar

• Cutoffs and oxbow lakes

– Floodplains, either:

• Erosional floodplains

• Depositional floodplains

Erosion and deposition along Erosion and deposition along a meandering streama meandering stream

Meanders get more extreme with time. Deposition at point bar keeps up with erosion at cutbank

Floodplain

Levee

Discussion: Crack the whip

• Deposition of sediment by a meandering stream

• Channel deposits– Point bars cover bed load as meandering Point bars cover bed load as meandering

stream’s channel loops movestream’s channel loops move

– Floodplain deposits cover point barFloodplain deposits cover point bar

Natural Levees

–form parallel to the stream form parallel to the stream channel by successive floods channel by successive floods over many yearsover many years

Floodplain

MeanderingStream

OxBow

http://hays.outcrop.org/gallery/rivers/arid_meander?full=1

Channel moves in direction of cutbankPointbar advances as crossbed drapes

Cut off loops silt up; called oxbow lakes

Old cut-off loop fills with mud, an oxbow lake

Point Bar Sequence from Meandering StreamPoint Bar Sequence from Meandering Stream

Erosion

Gravel of bed

Crossbeds of Point Bar

Fines of Floodplain

Formation of natural levees by flooding

Flood over banks, sediment carrying water into wide area, greater drag, lower velocity, load drops

Meandering Stream

• Incised meanders • Meanders in steep, narrow valleys

– Caused by a drop in base level or uplift of region

Delaware Water Gap

River meanders across floodplain.

Base level drops,or region uplifts. River cuts into bed, cutoffs abandon large loops,leaving oxbows

Stream less wide, uses narrower floodplain

Old floodplain is a terrace

Stream Terraces

A flight of river terraces

3. Braided Streams3. Braided Streams

• Occur where bed load is very high. Often Occur where bed load is very high. Often big boulders in the stream. At headwaters big boulders in the stream. At headwaters and near glaciersand near glaciers

• Many channels because flow is Many channels because flow is intermittent/seasonal - old channel banks intermittent/seasonal - old channel banks collapse in dry season , redirect flow next collapse in dry season , redirect flow next flood seasonflood season

• "mid-channel bars" between channels"mid-channel bars" between channels

DeltasDeltas

• Occur where stream hits slow water• Velocity, competence, bedload drop • Deposits build out into lake or sheltered

sea, extending the length of the river

• Consists of three types of beds Topset beds Foreset beds Bottomset beds

Deltas have strata deposited in a characteristic geometryTopsets, Foresets and Bottomsets

Actually much more complex: many distributary channel deposits

River delta distributary channels change location with time

General trend: Delta builds Seaward. If Floodwaters happen to reach

the sea via a new shortcut, they flow by the shorter (steeper) path cutting a

new channel, abandon old

A fan-delta in a lake

Part 2 - GroundwaterPart 2 - Groundwater

• water found in sediment, plus narrow fractures in bedrock

• Groundwater is the largest usable reservoir of fresh water available to humans, but much more in glacier ice

Distribution of Groundwater

Groundwater tablegenerally follows topography

•Water table is the upper limit of saturation•Variations in the water table

Depth is highly variable

•Varies seasonally and from year to year

•Shape is usually similar to surface topography

Factors influencing movement of Factors influencing movement of groundwater - 1groundwater - 1

• Porosity – Volume of Pore Space

• Permeability – the ability of a material to transmit a fluid through pore space

High Permeability Low Permeability

Factors influencing movement of Factors influencing movement of groundwater - 2groundwater - 2

• Aquifer – a permeable rock layer or a sediment that transmits groundwater freely (such as sands and gravels)

• Aquitard – an impermeable layer that hinders water movement (such as mudstone - clay)

Springs may result from a perched water tableSprings may result from a perched water table

Rollie’s Well Aquifer

GEYSERS

•Intermittent hot springs•Water erupts with great force•Occur where extensive underground chambers exist near hot igneous rock•Groundwater heats, expands, changes to steam, and erupts

Yellowstone Geyser erupting

WellsWellsTo obtain a continuous supply of To obtain a continuous supply of

water, we must cut down below the water, we must cut down below the water tablewater table

Sometimes a pump is not needed

Demo: Artesian

Discussion:Quicksand

Hydraulic head and urban water supplies

Just like an artesian, saves moneyTowns pump the water up to their water tank at night, whenelectricity is cheap. Artificial artesian.

Excessive Pumping of wells can causeExcessive Pumping of wells can causeDrawdown (lowering) of the water tableDrawdown (lowering) of the water table

Cone of depressionCone of depression in the water table in the water tableMay cause dry wellMay cause dry well

Discharge exceeds recharge

Saltwater contamination due Saltwater contamination due to excessive well pumpingto excessive well pumping

Wildwood Crest

Well design requires knowledge of local geology

Don’t fire the town engineer!

Aquatard

Geologic work of groundwaterGeologic work of groundwater

• Acidic groundwater dissolves limestone• Groundwater is often mildly acidic

– Contains weak carbonic acid, H2CO3 which forms when carbon dioxide from the air dissolves in cloud water droplets

– Groundwater becomes more acidic from decaying plants "Humic Acid"

• Acid dissolves calcite in limestone, releasing CO2 gas

CaCOCaCO33 + 2H + 2H++ ↔ Ca ↔ Ca++++ + H + H22O + COO + CO22 (g) (g)

The reaction reverses, and calcite is deposited, if COThe reaction reverses, and calcite is deposited, if CO22 gas concentration gets high gas concentration gets high

Geologic work of groundwaterGeologic work of groundwater• Caverns

• Features found within caverns– Composed of dripstone (travertine)Composed of dripstone (travertine)

• Calcite deposited as dripping water Calcite deposited as dripping water evaporatesevaporates

• Includes Includes stalactitesstalactites (hanging from the (hanging from the ceiling) and ceiling) and stalagmitesstalagmites (form on the floor of (form on the floor of a cavern)a cavern)

CaCO3 + 2H+ ↔ Ca++ + H2O + CO2 (g)

Reaction reverses, and calcite is deposited, if COReaction reverses, and calcite is deposited, if CO22 gas concentration gets high gas concentration gets high

Karst Topography

– Sinkhole or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse)

– Disappearing (aka sinking) streams

Stream incision, cavedevelopment and karsttopography

In Xanadu did Kubla KhanA stately pleasure-dome decree:Where Alph, the sacred river, ranThrough caverns measureless to man Down to a sunless sea.

Xanadu, by Samuel Taylor Coleridge  

Solution and Collapse SinkholesWinter Park, Florida. 1981 Porsche Dealership and PoolDon’t fire the town’s engineer!

Water weighs 8.34 pounds per gallon,Water weighs 8.34 pounds per gallon,and is incompressibleand is incompressibleLimestone weighs 5744 pounds/ mLimestone weighs 5744 pounds/ m33

Pool 50x25x2 meters