www.elsevier.com/locate/geomorph

Geomorphology 75 (

Hydrologic modeling of flood conveyance and impacts of historic

overbank sedimentation on West Fork Black’s Fork, Uinta

Mountains, northeastern Utah, USA

Eric C. Carson *

University of Wisconsin-Madison, Department of Geology and Geophysics, Lewis G. Weeks Hall, 1215 W. Dayton St., Madison, WI 53706, USA

University of Wisconsin-Madison, Department of Geography, Science Hall, 550 N. Park St., Madison, WI 53706, USA

Received 15 October 2004; accepted 27 July 2005

Available online 11 November 2005

Abstract

This study assesses historic overbank alluvial sedimentation along a low-gradient reach of West Fork Black’s Fork in the northern

Uinta Mountains, Utah. In this previously glaciated setting, an alluvial floodplain that is approximately 400 m wide by 1500 m long has

been modified by the combined effects of valley morphometry and the recent history of clear-cut logging during the late 19th and early

20th Centuries. To quantify the effects on sedimentation and flow conveyance, three natural streambank exposures were sampled and

analyzed for nuclear bomb fallout 137Cs. The distribution of 137Cswithin the three profiles suggests that a remnant outwash terrace exerts

a first-order control over the deposition of overbank alluvium. Upstream from a constriction in the floodplain caused by the terrace

remnant, as much as 40 cm of overbank alluvium has been deposited since the beginning of clear-cut logging. Immediately downstream

of that constriction, no evidence exists for any overbank sedimentation during that same period. Vibracore samples and Oakfield soil

probe sampling throughout the study reach quantified the geographic extent and thicknesses of the historic alluvial package. Flood

conveyance through the study area was modeled using the U.S. Army Corps of Engineers HEC-RAS modeling program. Model

simulations were run for modern conditions (using surveyed topography) and for prehistoric conditions (using the modern topography

less the historic alluvial package determined by 137Cs analyses). Model results indicate that the floodplain constriction caused a

significant impediment to flood conveyance at even modest discharges during prehistoric conditions. This promoted ponding of

floodwaters upstream of the constriction and deposition of alluvium. This has increased bank heights upstream of the constriction, to

the point that under modern conditions 1- to 5-year recurrence interval floods are largely confined within the channel. These results

confirm the validity of this new approach of combining 137Cs dating of alluvial sediments with HEC-RAS flow modeling to compare

flood conveyance along a single stream reach prior to and since an abrupt change in alluvial sedimentation patterns.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Uinta Mountains; Fluvial; Overbank sedimentation; HEC-RAS

1. Introduction

Floodplains of alluvial rivers have been recognized

as important components of the drainage basin, for

* Current address: San Jacinto College, Geology Department, 5800

valde Road, Houston, TX 77049, USA. Tel.: +1 281 458 4050x7398.

E-mail address: eric.carson@sjcd.edu.

U

0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2005.07.022

conveyance and storage of floodwaters and also as

significant sinks for suspended sediment deposited

during flood-induced inundation of the floodplain

(e.g. Macklin et al., 1994; Pinay et al., 1995; Walling

et al., 1996). The deposition of suspended fine-grained

sediment during periods of floodplain inundation con-

stitutes an important component of the development

and evolution of alluvial floodplains (Anderson et al.,

2006) 368–383

E.C. Carson / Geomorphology 75 (2006) 368–383 369

1996; Lewin, 1978; Wolman and Leopold, 1957).

Because of the inherently dynamic nature, floodplain

sediments are certainly susceptible to future reworking

and re-incorporation into the downstream alluvial sys-

tem. In light of this, significant recent research has

investigated the spatial patterns and rates of overbank

alluvial sedimentation, involving methods ranging

from the use of sedimentation traps (e.g. Asselman

and Middlekoop, 1995; Lambert and Walling, 1987)

to surveys immediately following individual deposi-

tional events (e.g. Marriott, 1992; Walling et al., 1997)

to the application of distinct, datable surfaces within

the floodplain sediments (e.g. Costa, 1975; Knox,

1987; Lewin and Macklin, 1987; Trimble, 1983).

Recently, refinements in the methodologies associated

with using airborne radionuclides have provided a

powerful tool for studying average rates of alluvial

sedimentation over medium-term timescales of dec-

ades to ca. 100 years (e.g. He and Walling, 1998;

Walling and He, 1998). Whereas many studies have

applied the basic approach of using fallout Cesium-

137 (137Cs) as an end to quantifying rates of overbank

alluviation, this project presents the use of 137Cs data

as a basis for evaluating changes in flood hydrology

related to recent overbank deposition.

The general objective of this research is to evaluate

one site in the northern Uinta Mountains for evidence

of increased rates of historic overbank alluvial sedimen-

tation, and the resultant impacts on flood conveyance.

Coring transects across the floodplain have identified

spatial heterogeneity in the distribution of a surficial

layer of alluvial sediment; this horizon is tentatively

interpreted to represent a period of accelerated historic

alluviation. Concentrations of 137Cs provide an ideal

dmarkerT in the alluvial sediment column for testing this

hypothesis, recording the depth of sediment deposited

and preserved on the floodplain surface since the onset

of nuclear bomb testing. These model data are used in

conjunction with the U.S. Army Corps of Engineers

HEC-RAS flow-modeling program. The results com-

pare parameters of flood flows in the modern hydro-

logic system with parameters at the same modeled

discharges prior to historic overbank alluviation.

2. Setting

2.1. Geography and geology

The main area of study is on the north flank of the

Uinta Mountains in northeastern Utah. The Uinta

Mountains are an east–west trending mountain range

that extends approximately 200 km eastward from the

Wasatch Front at Kamas, UT, into northwestern Color-

ado (Fig. 1). The crest of the Uinta Mountains contains

the highest peaks in the state of Utah, with Kings Peak

at 4124 m as the highest. The streams in the Uinta

Mountains occupy valleys scoured by multiple late

Wisconsin glaciations (Atwood, 1909; Bradley, 1936;

Richmond, 1965; Munroe, 2001). Channel beds rest on

bedrock in headwater reaches, on boulders in confined-

valley reaches, and on glacial outwash on mainstem

reaches of the streams. The site selected for this study is

on a reach of the West Fork of Black’s Fork (Fig. 1) in a

broad sub-alpine meadow underlain by glacial outwash.

Despite the high elevations of the study reaches (be-

tween 2900 and 2950 m asl elevation), channel gradi-

ents are much more gentle than typical of many streams

in mountainous settings.

The study reach lies to the south (upstream) of late

Pleistocene terminal moraines, and to the north (down-

stream) of high-gradient, confined-valley portions of

the stream system. The study reach occupies an alluvial

meadow up to 400 m wide and 1500 m in length. The

stream in this reach has a gradient of approximately

0.004 m/m. Because of the width of the meadow and

the relatively low gradient, the stream is largely uncon-

fined by valley wall topography and displays a mean-

dering planform atypical of sub-alpine settings (Fig. 1).

Stream sinuosity commonly ranges between 1.7 and

2.0. Floodplain surfaces are well developed by alluvial

processes; numerous abandoned, relict (cutoff) chan-

nels are preserved on the surface of the valley floor.

Bed-load sediments that move through the reach at or

near bankfull discharge consist of sand, granules, peb-

bles, cobbles, and boulders derived primarily from

resistant quartzite of the Uinta Mountain Group. The

suspended sediment loads transported at or near bank-

full discharge largely consist of silt and clay derived

from shale within the Uinta Mountain Group.

2.2. Recent research

Much of the recent research on fluvial geomorphol-

ogy of the Uinta Mountains has focused on recent

processes and landforms occurring in basin marginal

settings and on the major streams—particularly the

Green River and Duchesne River—that flank the Uin-

tas. Ringen (1984) and Lenfest and Ringen (1985)

quantified suspended sediment–discharge relationships

for a number of stream gages on tributaries of the Green

River, including multiple gages on Henry’s Fork and

Black’s Fork. Brink and Schmidt (1996) discussed bed-

load transport and channel stability on the south slope

of the Uintas, particularly the assessment and method-

Fig. 1. Location of study area on West Fork of Black’s Fork in northeastern Utah. (a.) Location of the Uinta Mountains in northeastern Utah. (b.)

Study reach on the West Fork Black’s Fork river. (c.) Topographic map of study area showing locations of 137Cs sampling sites, and the valley

constriction caused by the remnant outwash terrace. Map interpolated from U.S.G.S. 7.5V Elizabeth Mountain quadrangle and surveying conducted

in August 2001. (d.) Aerial photograph of study area showing locations of HEC-RAS step-backwater modeling cross-sections. Cross-sections are

numbered 1 to 28 from northeast (right of photo) to southwest (left of photo). The shaded area represents the approximate limits of the alluvial

floodplain.

E.C. Carson / Geomorphology 75 (2006) 368–383370

ology of quantifying channel migration in mountain

streams. Counts and Pederson (2003) assessed the geo-

morphic record of a large paleflood on the Green River;

Pederson (2004) evaluated the record of drainage evo-

lution in the Green River basin; and Larsen et al. (2004)

evaluated reworking of debris fan deposits in the Green

River canyon downstream of Flaming Gorge Dam.

Gaeuman et al. (2003, 2005) studied the effects of

historic changes in discharge and sediment transporta-

tion along a reach of the lower Duchesne River near its

confluence with the Green River in the Uinta Basin.

Additionally, several unpublished theses have been

completed (Smelser, 1998; Stamp, 2000; Paepke,

2001; Gaeuman, 2003; Larsen, 2003). Most of these

previous studies have been conducted downstream of

the maximum glacial limit in the Uintas. In contrast, the

research presented here quantitatively evaluates historic

overbank sedimentation in a previously glaciated sub-

E.C. Carson / Geomorphology 75 (2006) 368–383 371

alpine setting and models flood conveyance in detail at

a single, relatively small site.

3. Methodology

3.1. Description of alluvial sedimentary packages

Coring transects were completed across the width of

the alluvial sediments in the West Fork Black’s Fork

meadow using a modified vibracore system. Aluminum

7.5 cm diameter cores were inserted and removed from

the alluvial sediments with the assistance of a hydraulic

ram on the University of Wisconsin-Madison Depart-

ment of Geology and Geophysics Mobile B-50 drilling

rig. The cores penetrated to a maximum depth of 2.4 m,

in all cases but two encountering a coarse layer of

cobbles and boulders underlying the alluvial sediments

(Fig. 2). Cores were spaced 10 to 15 m apart in two

transects perpendicular to the valley slope (see Fig. 1),

Fig. 2. Cross-sections of floodplain sediments in West Fork Black’s Fork v

section # 1 (see Fig. 1 for exact location) is located upstream of valley constr

boulders interpreted to be late Pleistocene outwash. The elevation of this su

fluvial incision has occurred. Coring across the width of the alluvial valley,

overbank alluvium (approximately 40 cm thick near channel and gradually

Fig. 1 for exact location) is located downstream of valley constriction. Coring

occurrence of historic overbank alluvium. All cores penetrated the layer o

Pleistocene outwash; the uninterupted presence of this surface demonstrates

spanning the entire valley width. The two transects

were located on either side (upstream and downstream)

of a prominent constriction in the alluvial meadow

formed by a remnant outwash surface. Transect 1 is

located upstream of the constriction and contains 23

cores spanning 350 m across the meadow. Transect 2 is

located on a narrower portion of the meadow and

incorporates nine cores spanning 150 m.

The cores were returned to the University of Wis-

consin-Madison Department of Geology and Geophys-

ics for measurement and description. The cores were

split lengthwise, and core descriptions distinguished

between valley-bottom cobbles, channel lag gravel,

lateral accretion (point bar) packages, channel-fill sedi-

ments, vertical accretion (overbank sediments), and

sandy diamicton (colluvium) (Fig. 2). Valley-bottom

cobbles were encountered at the base of each core.

Auger drilling indicates that this deposit consists of

cobbles and boulders, with boulders estimated to be

alley, with locations of vibracores shown by grey boxes. (a.) Cross-

iction. All cores except two penetrated the layer of coarse cobbles and

rface as recorded in the vibracores indicates that little or no Holocene

including two relict channels, shown wide-spread presence of historic

tapering in thickness toward valley walls). (b.) Cross-section # 2 (see

across two relict channels and associated alluvial deposits showed no

f coarse cobbles and boulders that have been interpreted to be late

minimal fluvial incision during the Holocene.

ig. 3. Collection of sediment samples for 137Cs analysis at site WFB-

s-1. View of the sampling site looking downstream (a.) shows the

istinct pale horizon at the base of the interpreted historic sediment.

he base of that horizon can be seen extending in the cutbank toward

e sheep in the midground. Close-up view of the sampling (b.) shows

ins located at 4 cm intervals, with the lowest pin at the elevation of

e pale horizon. In both views, the lowest pin in the profile is located

2 cm below the surface of the floodplain. Site WFB-Cs-1 is inter-

reted to over-estimate the average amount of historic overbank

lluviation because it is located within an abandoned, relict stream

hannel which has been partially in-filled by the historic alluvium.

E.C. Carson / Geomorphology 75 (2006) 368–383372

20 cm in diameter and larger; it is interpreted to repre-

sent late Pleistocene outwash. Channel lag gravel con-

sists of coarse sand, granules, and pebbles that are

typically red or grey in color. The lag was found

prominently at the bases of abandoned, relict channels

where it discontinuously overlies valley-bottom gravel.

Lateral accretion packages are very fine sand to coarse

sand; the color is predominantly either light to medium

brown or the red-grey typical of the Uinta Mountain

Group Quartzite. Cross-bedding is apparent in some

core samples, where in places it is emphasized by

dispersed organic material and distinct wood fragments

that are oriented parallel to sediment bedding.

Channel-fill sediments are dominated by dark grey

thinly bedded to laminated silt and clay, and fibrous

peat. A few layers of fine to medium sand occur, as do a

few organic-rich layers, including leaves, wood frag-

ments, pine cones, and bark. Pieces of wood, as much

as 15 cm thick, were found. Vertical accretion packages

consist of medium to dark brown silt and clay, in places

showing horizontal bedding. This facies is found over-

lying lateral accretion packages and channel-fill sedi-

ments. Sandy diamicton consists of fine to medium

sand with various proportions of granules to pebbles.

These sediments were only identified within cores in

valley marginal locations, and are interpreted to repre-

sent colluvium.

Cores in Transect 1 also contained a widespread

facies located on the surface of the valley floor,

overlying the vertical accretion overbank sediments.

Similar to the vertical accretion sediments, this surface

deposit is silt and clay, although it is distinctly reddish

brown in color (Munsell 2.5 YR 3/2). This sediment

package is visible in unvegetated stream banks (Fig.

3). It is tentatively interpreted to represent an addi-

tional, distinct layer of overbank alluvium that has

been recently deposited. Logging in the West Fork

Black’s Fork valley was conducted in the late 19th

and early 20th Centuries to provide railroad ties for

the westward expansion of the Union Pacific Railroad

through the Green River Basin ~40 km to the north

(i.e. Baker and Hauge, 1913). In logged areas, hill-

slopes were cleared of all trees large enough to pro-

duce 20�20 cm ties. It has been well documented

that clear-cut logging leads to increased rates of slope

erosion, mobilizing sediment into the fluvial system,

and ultimately increasing storage of sediment on the

floodplain (e.g. Helvey, 1980; Van Lear et al., 1985;

Marston and Haire, 1990). To test the validity of this

interpretation, sections of the streambank have been

sampled and analyzed for concentrations of 137Cs

through the alluvial sediment profile.

F

C

d

T

th

p

th

5

p

a

c

3.2. Analysis of historic alluviation

Fallout 137Cs is an artificial radionuclide with a half-

life of 30.17 years. Accelerated fallout occurred during

atmospheric testing of nuclear weapons in the later

1950s and early 1960s (Walling and He, 1997). The

earliest introduction of the radionuclide into the atmo-

sphere dates to the first high-yield thermonuclear test in

1952, with subsequent tests lasting until the signing of

the Nuclear Test Ban Treaty in 1963 (Perkins and

E.C. Carson / Geomorphology 75 (2006) 368–383 373

Thomas, 1980). Maximum fallout of 137Cs from global

atmospheric circulation occurred between 1956 and

1967. Rates of 137Cs fallout have steadily decreased

since that time, although some portions of central Eur-

ope have experienced an additional short-term flux

related to the 1986 accident at the Chernobyl reactor

(Rimmer et al., 1991). In most settings, fallout 137Cs is

Fig. 4. Profiles of 137Cs activities from sampling sites alongWest Fork

of Black’s Fork; error bars in all cases representF1 standard deviation

Thepeak in 137Cs activity, seen in (a.) at 16–20cm, is interpreted to over

estimate the average depth of the ca. 1963 surface because this site was

located within a relict channel which previously preserved remnan

topography on the floodplain surface. The maximum value of 137Cs

activity, seen in (b.) at 8–12 cm depth, is interpreted to represent the

averagedepthof the ca. 1963 surfaceupstreamof thevalley constriction

The downstream profile (c.) shows activities uniformly less than 0.25

pCi/g through the depth of the profile; this is interpreted to represen

minimal historic overbank sedimentation downstream from the valley

constriction. The dashed lines in (a.) and (b.) represent the base of the

prominent 2.5 YR 3/2 silt and clay layer interpreted to be historic.

rapidly fixed to clay particles on the surface sediments

(Frissel and Pennders, 1983; Livens and Rimmer, 1988;

Ritchie and McHenry, 1990). During erosion and trans-

port of sediments that have been laden with 137Cs, the

radionuclide is transported with the sediment particles

as part of the suspended sediment load rather than in the

dissolved load (McHenry and Ritchie, 1977).

Therefore, as sediments are deposited and redistrib-

uted in an alluvial environment, 137Cs accumulates

within a sediment column from two sources: initial in

situ fixing of fallout 137Cs to fines, and redeposition of

sediments which have been eroded from the upstream

basin since the onset of 137Cs fallout. As a result of this

dual source, sediment profiles in depositional environ-

ments often display an initial peak in 137Cs concentra-

tions at some depth in the soil column followed by a

gradual decline in 137Cs concentrations nearing the

surface. This has commonly been interpreted to repre-

sent the initial pulse associated with direct air fallout

followed by a slow decline representing subsequent

alluvial/colluvial sedimentation (Ritchie and McHenry,

1990; Ely et al., 1992; Walling and He, 1997).

Three profiles in the West Fork of Black’s Fork

meadow were sampled for 137Cs concentration analysis

(Fig. 1a). Sampling sites WFB-Cs-1 and WFB-Cs-2 are

located upstream from the constriction caused by a

remnant outwash surface; WFB-Cs-3 is located down-

stream from the constriction. All three locations are

stream bank profiles (see Fig. 3). At each location,

the surface vegetation was removed from a 50 cm2

area at the stream bank, and the bank was trimmed to

a vertical profile. Soil pins were inserted into the ver-

tical profile at 4 cm intervals. Samples were removed in

the 4 cm increments as marked by the pins, producing

~1 kg samples for each 4 cm horizon. The samples were

analyzed for 137Cs activity at the Radiochemistry Lab-

oratory of the Wisconsin State Laboratory of Hygiene.

For the two profiles located upstream of the constric-

tion, the profiles extended below the base of the con-

spicuous layer of reddish silt and clay (Fig. 4).

.

-

t

.

t

ig. 5. Comparison of described sediment section and 137Cs analyses

t site WFB-Cs-2 (as shown in Figs. 3 and 4a). The peak in 137Cs

ctivity occurs at the 8–12 cm sampling depth and is interpreted to

present the A.D. 1963 atmospheric peak in 137Cs concentrations.

he entire package of 2.5 YR 3/2 silt and clay is, therefore, interpreted

have been deposited since the onset of logging ca. A.D. 1900 in

is basin.

E.C. Carson / Geomorphology 75 (2006) 368–383374

The resultant profiles (Fig. 4) show 137Cs activities with

increasing depth below the modern floodplain surface.

[See Carson (2003) Appendix C for complete 137Cs

analyses.] Several methods have been proposed for

interpreting an age associated with the maximum137Cs activity. Ritchie et al. (1975) and Ely et al.

(1992) proposed interpreting the peak value of 137Cs

activity as representative of ca. 1950. Walling and He

(1997) alternatively assign this peak to the 1963 peak of137Cs emission into the atmosphere; this is the value

that will be used here. The three profiles show two

distinct patterns. The one site located downstream of

the valley constriction (WFB-Cs-3) shows low 137Cs

activities through the entire profile, with the peak value

located in the uppermost sampling interval (0–4 cm

depth). The lack of any significant peak at any depth

below the surface indicates that no substantial net over-

bank sedimentation has occurred since 1963. The two

sites upstream from the constriction (WFB-Cs-1 and

WFB-Cs-2) show a different history. Both show peaks

in 137Cs activities in the upper portions of the profile,

followed by a decline in activities at shallow depths in

one profile (Fig. 4a) and a plateau in activities in the

other profile (Fig. 4b). The highest 137Cs activity values

in each profile are interpreted to represent the 1963

surface. Because the periods of logging predate intro-

duction of 137Cs into the atmosphere by ca. 50–95

years, it is reasonable to assume that sedimentation

rates on the floodplain surface would have increased

prior to 1963. Therefore, I interpret the entire package

of 2.5 YR 3/2 silt and clay as historic overbank alluvi-

um related to historic activities and impacts in the

watershed. This package exhibits significant lateral var-

iation in thickness as it is exposed in the stream bank.

Particularly, relict channels that are preserved in the

floodplain stratigraphy are bisected by the modern

channel at several locations. Site WFB-Cs-1 is located

within the margins of one such abandoned, relict chan-

nel. The form of the relict channel is visible in the

stream bank (Fig. 2a), and apparently retained some

remnant topography that has been in-filled by the his-

toric overbank alluvium. Site WFB-Cs-2 is located on a

relict point bar that closely approximates the elevation

of the floodplain. Therefore, the precise locations of the

two sites make WFB-Cs-2 more representative of the

average depth of post-1963 overbank sedimentation on

the floodplain; further discussions and interpretations of

historic sediment deposition will focus on site WFB-

Cs-2. Based on the interpretation that the peak in 137Cs

activity represents 1963, average rate of overbank sed-

imentation at this site has been approximately 0.25 cm/

yr for the past 40 years. Based on the interpretation that

the base of the 2.5 YR 3/2 silt and clay is ca. 1900,

average rate of overbank sedimentation at this site was

approximately 0.48 cm/yr for the period 1900 to 1963.

While these two rates of sedimentation are only crude

estimates of actual conditions, they do represent a de-

cline in rate of floodplain sedimentation through time.

These values are both in excess of the estimated long-

term (Holocene) average rate of overbank sedimentation

of approximately 0.12 cm/yr on alluvial streams in the

northern Uintas (Carson, 2003). These historic estimates

are consistent with a setting where accelerated over-

bank sedimentation from a point–source disturbance

increases floodplain elevation, and eventually results

in a general slowing of floodplain sedimentation as the

banks’ heights progressively increase.

Using site WFB-Cs-2 as a reference, I estimate that

upstream from the valley constriction about 40 cm of

historic alluvium has been deposited on the floodplain

near the modern stream (Fig. 5). The coring transect,

located closest to site WFB-Cs-2 (Fig. 2a), shows that

the thickness of the historic alluvium decreases with

increasing distance from the modern channel. Addition-

al vibracoring has been conducted at a variety of loca-

tions on the floodplain to constrain the geometry of

relict, abandoned channels (Carson, 2003). The alluvial

stratigraphy associated with the abandoned channels, as

well as reconnaissance Oakfield coring across the val-

ley floor, also indicates that the package of 2.5 YR 3/2

silt and clay thins away from the modern channel and

F

a

a

re

T

to

th

E.C. Carson / Geomorphology 75 (2006) 368–383 375

toward the valley walls. The geographic distribution

and thickness of the historic alluvial package across

the floodplain was constrained by a total of 85 de-

scribed vibracore samples and 222 Oakfield soil

probe samples within the study area (Carson, 2003).

3.3. Flood conveyance modeling

Flood stages and inundation of the floodplain along

the selected reach was modeled using the HEC-RAS

model developed by the U. S. Army Corps of Engineers

Hydrologic Engineering Center. This program is

designed specifically to model steady, gradually varied

flow in one dimension, with the ability to calculate

water surface profiles for subcritical, supercritical, or

mixed flow regimes. The loss of energy from friction is

calculated between successive valley cross-sections and

determines the changes in downstream water levels.

User-supplied discharge values produce a reconstructed

flood stage via an iterative process (U.S. Army Corps

of Engineers, 2001).

A total of 28 cross-sections in the West Fork Black’s

Fork meadow were surveyed with a total station in

August 2001 (Fig. 1d), providing sub-decimeter accura-

cy. On the floodplain surface, survey points were spaced

between 1 and 10 m apart, depending on the amount of

elevation change.Within the channel, survey points were

spaced between 0.25 and 1 m to accurately describe the

cross-section geometries of the channels. For purposes of

discussing results, the portion of the study reach up-

stream of the constriction (cross-sections # 16 to 28)

will be referred to as the dupper meadowT, the portion

of the study reach at the constriction (cross-sections # 12

to 15) will be referred to as the dconstrictionT, and the

portion of the study reach downstream of the constriction

(cross-sections # 1 to 11) will be referred to as the dlowermeadowT (Fig. 1b).

Downstream reach lengths and overbank flow lengths

between successive surveyed cross-sections are required

as input parameters in the flow model. The downstream

reach lengths were calculated by identifying the left

streambank on each surveyed cross-section and measur-

ing along the channel margin between successive cross-

sections. The overbank flow lengths were calculated by

identifying the point on each cross-section that repre-

sented the axis of flow for the left and right overbank

areas. Flow lengths for the left and right overbank areas

were then calculated as the straight-line distance between

successive surveyed cross-sections. The downstream

reach lengths, therefore, represent the distances along

the channel between successive cross-sections, whereas

the overbank flow lengths roughly represent the straight-

line distances between successive surveyed cross-sec-

tions. The HEC-RAS model uses the differences be-

tween downstream reach lengths and associated

overbank flow lengths to account for the actual channel

sinuosity (U.S. Army Corps of Engineers, 2001).

The default contraction/expansion coefficients of 0.1

and 0.3 were used; these values are recommended by

U.S. Army Corps of Engineers (2001) to account for

gradual changes in river cross-sectional area. Man-

ning’s n values for the overbank reaches were estimated

using established tables of the roughness coefficient

(Chow, 1959; U.S. Army Corps of Engineers, 2001),

to account for areal differences in vegetation commu-

nities with significantly different characteristics of

roughness. Roughness coefficients in the study reach

ranged from 0.03 to 0.15. Models were run with rough-

ness coefficients ranging from 0.02 to 0.20 to test

sensitivity of the model to varying coefficients. Refer

to Carson (2003) for further discussion and results of

sensitivity tests.

Stream flow was modeled using an estimate of the

1.58-year discharge, as well as estimated discharges with

recurrence intervals 5, 10, 20, 50, and 100 years. Dis-

charge values associated with these recurrence intervals

were estimated using available hydrologic data. Values

of stream discharge, computed by the U.S.G.S., were

compiled for 19 U.S.G.S. gage stations in the northern

and western Uintas. These gages were selected because

of the length of records, the similar channel slopes,

roughness characteristics, and average elevations of the

basins. These data were used to derive an empirical

equation that estimates the magnitude of the 1.58-year

discharge for any site in the northern Uintas with a

known drainage area. In the study reach, the 1.58-year

discharge is estimated to be 11 m3/s. The estimate for the

1.58-year discharge in the study area was then compared

to the magnitude of the 1.58-year discharge at the nearest

downstream U.S.G.S. stream gage station (gage #

09217900, Black’s Fork near Robertson, WY); the

1.58-year discharge in the study area is 29.7% of the

1.58-year discharge at this gage. It is, therefore, assumed

that the discharge in the study area for any recurrence

interval is 29.7% of the same recurrence discharge at

gage # 09217900 (Table 1). Based on this relationship,

discharges for a range of recurrence intervals were esti-

mated for the study reach.

Leopold et al. (1964) and Dunne and Leopold

(1978) studied flood discharges over a wide range of

recurrence intervals for numerous rivers in the United

States. Their results show that the 50-year discharge at

any particular site is typically four to five times larger

than the 1.58-year discharge at the same site. The

Table 1

Estimation of discharges using HEC-RAS

Gage # 09217300 Study reach

West Fork Black’s

Fork near

Robertson, WY

West Fork Black’s

Fork at Forest

Rd. 063 ford

Drainage area (km2) 337 76

Elevation (m asl) 2686 2900–2950

Discharges (m3/s)

1.58-year 37.0a 11.0b

5-year 54.2a 16.1

10-year 55.5a 16.5

20-year 62.2a 18.5

50-year 66.1a 19.6

100-year 71.4a 21.2

a Determined by Log Pearson Type III method.b Determined using the empirical equations CSA=1.479(DA)0.781

and Qbkf=2.197 (CSA)0.834, which were developed for the northern

Uinta Mountains (Carson, 2003).

E.C. Carson / Geomorphology 75 (2006) 368–383376

various recurrence interval discharges shown in Table 1

differ considerably from that general relationship. At

gage # 09217900, and, therefore, by default at the study

site, the 50-year discharge is approximately 1.7 times

larger than the 1.58-year discharge. While the various

recurrence discharges, estimated in Table 1 and used in

the HEC-RAS model, are conservative values com-

pared to the data used by Leopold et al. (1964) and

Dunne and Leopold (1978), the estimates used herein

are derived directly from the actual discharges on West

Fork Black’s Fork. Furthermore, this discrepancy be-

tween the West Fork Black’s Fork data and the data

from Leopold et al. (1964) and Dunne and Leopold

(1978) is mirrored in the gage data from numerous sites

across the Uinta Mountains. For several gages on

streams in the northern Uintas, the 50-year discharge

ranges between 1.5 and 2.8 times larger the 1.58-year

discharge. This suggests that while the 50-year dis-

charges are abnormally small compared to the associ-

ated 1.58-year discharges, it is a phenomenon that

exists across the northern Uintas. The modal floods

for streams in the Uintas are almost exclusively related

to the spring snowmelt, as is the case with the dis-

charges with higher recurrence intervals. The rate at

which the snowpack can melt and contribute to runoff

controls the magnitude of the nival flood. The abnor-

mally small difference between the 50- and 1.58-year

discharges may, therefore, reflect a fundamental limit-

ing threshold on the possible magnitudes of snowmelt-

derived floods in the northern Uintas.

Finally, the interpreted historic alluviation in the

upper half of the meadow has been incorporated into

the flood conveyance modeling to create model runs that

will be designated as dmodernT and dpre-disturbanceT.

Fig. 2a shows the estimated distribution of the historic

alluvial sediments through the cored cross-section on

the floodplain upstream from the valley constriction.

Surveyed cross-sections from the upper meadow were

adjusted to lower the surface of the floodplain consistent

with the pattern inferred in Fig. 2a to estimate the dpre-disturbanceT floodplain topography. Surveyed cross-

sections in the lower meadow were left unchanged for

dmodernT and dpre-disturbanceT flood conveyance mod-

els; this is consistent with the results of the valley coring

transects (Fig. 2b) and 137Cs analyses that indicate

minimal historic sedimentation below the floodplain

constriction (Fig. 4). Surveyed cross-sections located

near the valley constriction (Cross-sections # 13–16)

were adjusted to lower the surface topography with

amounts ranging from 10 to 35 cm of historic alluvium

to create a transition between the upstream and down-

stream reaches (Carson, 2003, Appendix D). Creating a

dmodernT topography from surveys and a dpre-dis-turbanceT topography with the addition of knowledge

gained from 137Cs sampling and coring represents a

novel approach to evaluating flood conveyance with

HEC-RAS. The occurrence of the distinct reddish silt

and clay layer that is easily observable in Oakfield and

vibracore samples has allowed for an accurate estima-

tion of the distribution of the historic alluvial sediments.

This has allowed for the creation of two sets of topo-

graphic profiles, which in turn allowed for flood mod-

eling for two distinct time periods.

While topography has been altered between

dmodernT and dpre-disturbanceT model conditions, the

discharges associated with each recurrence interval

flood has not been changed between the two model

conditions. I have attributed the increase in historic

overbank sedimentation to logging that occurred in the

drainage basin in the late 19th and early 20th Centuries.

Recent investigations throughout the western United

States have documented variations in magnitudes of

specific recurrence interval floods following logging

(e.g. Bowling et al., 2000; Harr and McCorison, 1979;

Hicks et al., 1991; Jones and Grant, 1996; Thomas and

Megahan, 1998). Discharges with 1- to 2-year recur-

rence intervals have been documented to increase by as

much as 90% to 100% following logging (Jones and

Grant, 1996), although such drastic changes in the mag-

nitudes of peak discharges have been directly disputed

(Thomas and Megahan, 1998). The change in high

recurrence floods diminishes with time following log-

ging as vegetation within the affected basins recovers,

although Thomas and Megahan (1998) still detected

residual effects as long as 20 years after logging.

While I do not change discharges between dmodernT

E.C. Carson / Geomorphology 75 (2006) 368–383 377

and dpre-disturbanceT conditions for HEC-RAS model

development, I do recognize that the logging in this

basin may have altered flood magnitudes. This is a

conservative treatment of the data, but discharges in

the dpre-disturbanceT model condition predate logging

by definition. Discharges in the dmodernT model condi-

tion would only reflect any residual impacts from log-

ging that remain after eight decades of recovery.

4. Results

One flood conveyance model was developed for the

actual (modern) and the estimate (pre-disturbance)

topographies at the discharges corresponding to the

1.58-, 5-, 10-, 20-, 50-, and 100-year recurrence inter-

vals. This produced a total of 12 model runs. In all

cases, the model was developed with flows in the lower

flow regime. Results for each model run include water

surface elevation, critical water surface, critical depth,

energy gradient elevation, energy gradient slope, in-

channel water velocity, left and right overbank veloci-

ties, flow area, top width, and unit stream power (Car-

son, 2003, Appendix E).

Estimation of pre-disturbance topography altered

surveyed cross-sections # 12 to 28, lowering the actual

surveyed surfaces by 0 to 40 cm to reflect the thick-

nesses of the historic alluvium. In the lower meadow,

the topography was identical for dmodernT and dpre-disturbanceT model runs. The HEC-RAS model strictly

evaluates flood conveyance and resultant elevations of

the water surface as energy loss due to friction between

successive valley cross-sections. With flow conditions

specified as being in the lower flow regime, the model

calculates friction losses starting at the downstream end

of the study reach and moving upstream. As a result,

the dmodernT and dpre-disturbanceT model runs are

identical to one another downstream of cross-section

# 12. Therefore, model results will primarily be dis-

cussed in terms of differences between the dmodernTand dpre-disturbanceT runs in the upper meadow, those

being the cross-sections that the 137Cs data indicate

have been significantly altered in the past ~100 years.

4.1. Modeled water surface elevations

dModernT and dpre-disturbanceT elevations of the

water surfaces are presented for the 1.58-, 5-, and 50-

year recurrence interval discharges (Fig. 6). Modeled

elevations of the water surfaces show an average in-

crease of 0.16 m in stage between the 1.58- and 5-year

discharges, and an average increase of 0.11 m in stage

between the 5- and 50-year discharges. This is inter-

preted to reflect the fundamental significance of the

1.58-year discharge as the bankfull flood. The bankfull

flood represents the discharge at which the channel

capacity is fundamentally full, and any increase in dis-

charge, and, therefore, stage will cause the flow to begin

to inundate the floodplain surface. Accordingly, once

the stage exceeds the elevation of the channel bank, the

rate of increase in stage for a unit increase in discharge

will be significantly reduced as the flow area begins to

include the channel and floodplain. Therefore, the

change in stages between the 5- and 50-year discharges

is exceedingly small because the top surface of the flow

area is much larger than the top surface associated with

the 1.58-year discharge (i.e. the channel width).

The relationship between water surfaces associated

with 1.58-year discharge runs of the model and model

runs for the surveyed floodplain surface is consistent

with the observation that the 137Cs data indicate rapid

recent alluviation on the floodplain in the upper portion

of the study reach. In the lower portion on the study

reach, downstream of the floodplain constriction, the

water surface associated with the 1.58-year discharge is

generally at the same elevation as the surveyed flood-

plain surface, as would be expected for the bankfull

discharge (Fig. 6). The bank elevation profile shown in

Fig. 6 represents surveyed elevations at the break in

slope at the top of the streambank; at most locations,

this elevation is approximately equivalent to the bank-

full stage as defined by the alluvial lateral accretion

sedimentology. In the upper portion of the study reach,

the relationship of the water surface to the floodplain

surface varies significantly between dmodernT and dpre-disturbanceT runs of the model. In the dpre-disturbanceTcase, the elevation of the water surface for the 1.58-year

discharge is at or slightly above the bank elevation at

most surveyed cross-sections. This is fundamentally

consistent with the bankfull discharge as being that

which fills the channel to its capacity. In the dmodernTcase, the elevation of the water surface for the 1.58-year

discharge is lower than the bank elevation at most

surveyed cross-sections, in some places as much as

0.2 m below the bank surface (Fig. 6a). This suggests

that the historic alluvium deposited on the floodplain in

this portion of the study reach has raised the floodplain

surface to the point that it is not inundated by anything

less than a 5- to 10-year recurrence interval flood.

4.2. Inundation of floodplain surface and flow areas

Projecting modeled elevations of the water surface

on the topography of the surveyed cross-sections shows

the surface area of the floodplain inundated by a given

Fig. 6. Modeled elevations of the water surface for the 1.58-, 10-, and 50-year discharges for modern survey conditions (top) and estimated pre-

disturbance survey conditions (bottom). In all cases, the 10- and 50-year discharges exceed bank elevations, whereas the 1.58-year discharge in the

modern case only sporadically exceeds bank elevation (i.e. is often contained entirely within the channel).

E.C. Carson / Geomorphology 75 (2006) 368–383378

discharge (Fig. 7). Consistent with the data of modeled

elevations of the water surface, the dmodernT 1.58-yeardischarge model run shows little or no inundation of the

floodplain surface (Fig. 7a), whereas the same dis-

charge with the dpre-disturbanceT topography creates a

much larger area of the floodplain encroached by water

(Fig. 7b). When the discharge is increased to the 5-year

recurrence interval, the floodplain is largely inundated

upstream of the constriction in the modern and pre-

disturbance cases (Fig. 7c and d).

4.3. Modeled flow velocities

Flow velocities have been modeled for in-channel,

left overbank, and right overbank flow areas at all mod-

eled discharges. General patterns throughout the mod-

eled reach show the effects of the floodplain constriction

on flood conveyance, and comparisons of dmodernT anddpre-disturbanceT runs of the model elucidate the impact

of the historic accumulation of sediment on the flood-

plain surface. Under all conditions of the model, flow is

concentrated to the in-channel portion of the flow area.

Estimated average left and right overbank flow velocities

range from ~0.1 m/s for the 1.58-year recurrence interval

discharge (total discharge 11.0 m3/s) to ~0.3 m/s for the

100-year recurrence interval discharge (total discharge

21.2 m3/s). In contrast, estimated average in-channel

flow velocities ranged from ~0.8 to ~1.2 m/s.

As the modeled discharge increases, the effects of

the floodplain constriction become more significant. At

Fig. 7. Modeling results showing inundation of floodplain surface with various model runs. Solid black lines represent survey cross-sections as

shown in Fig. 1d; dashed black lines represent limits of alluvial sediments; shaded areas represent modeled inundation for each model run. (a.) 1.58-

yr discharge with modern topography; (b.) 1.58-yr discharge with estimated pre-disturbance topography; (c.) 5-yr discharge with modern

topography; and (d.) 5-yr discharge with estimated pre-disturbance topography. Note that with 1.58-yr discharge, the modern topography contains

the flow to the channel in most locations above the valley constriction, whereas the pre-disturbance floodplain was more widely inundated by the

same discharge. In the case of the 5-yr discharge, a larger area of the floodplain above the constriction is inundated in the modern model run than the

pre-disturbance. This interpreted to reflect the generally shallower flow depth on the modern floodplain compared to the pre-disturbance floodplain

because of the addition of the recent sediment package deposited on the surface.

E.C. Carson / Geomorphology 75 (2006) 368–383 379

E.C. Carson / Geomorphology 75 (2006) 368–383380

the dmodernT 1.58-year discharge (Fig. 8a), flow veloc-

ities show little sensitivity to the constriction, whereas

at the dmodernT 100-year discharge (Fig. 8c), flow

velocities markedly increase at the upstream end of

the constriction (cross-section 17) and decrease at the

downstream end of the constriction (cross-sections 11

through 9). For all model runs, flow velocities increase

at the upstream end of the constriction, although this is

most apparent at the higher recurrence interval dis-

charges. The lower portion of the meadow experiences

contrasting conditions during model runs. In-channel

velocities in the lower meadow are 24% to 140% higher

than in the upper meadow. The most extreme disparity

occurs with the 100-year discharge under dpre-dis-turbanceT conditions, during which velocities in the

upper meadow are roughly 0.4 m/s and in the lower

meadow are as high as 1.2 m/s.

When comparing dmodernT to dpre-disturbanceTresults, the disparity between the upper meadow and

the cross-sections adjacent to the constriction is most

striking. In all runs of the model, the upper meadow

Fig. 8. HEC-RAS modeled flow velocities for in-channel, left overban

(downstream). The four graphs show dend-memberT conditions (modeling

and 100-year discharge for both dmodernT and dpre-disturbanceT conditions.discharge and the 100-year discharge is interpreted to reflect that the add

accommodated by an increase in flow area rather than flow velocity.

shows higher in-channel flow velocities for the

dmodernT conditions compared to the dpre-disturbanceT(Fig. 8). At each surveyed cross-section in the upper

meadow and at all modeled discharges, flow velocities

increased between dpre-disturbanceT and dmodernT con-ditions by 52% to 107%. I interpret this to reflect that

historic overbank sedimentation in this area has in-

creased streambank heights, thus, confining more of

the modern flow to the channel and increasing veloci-

ties in the channel.

5. Discussion

Little argument exists that changes in land use that

decrease total cover of vegetation can promote acceler-

ated erosion (e.g. Knox, 1977; Helvey, 1980; Van Lear

et al., 1985; Marston and Haire, 1990; Phillips, 1993;

Fitzpatrick and Knox, 2000). This is particularly true

throughout the western United States where clear-cut

logging has occurred in mountainous topography (Mer-

sereau and Dyrness, 1972; Beschta, 1978; Ambers,

k, and right overbank for cross-sections 24 (upstream) through 1

extremely high and low recurrence floods): the 1.58-year discharge

The relatively small change in flow velocities between the 1.58-year

itional water volume passing through any cross-section is primarily

E.C. Carson / Geomorphology 75 (2006) 368–383 381

2001). In the case of the current study area on West

Fork Black’s Fork, logging activities directed by the

Union Pacific Railroad led to nearly complete defores-

tation of the valley side-slopes. Individual loggers were

assigned 1/2-mile-wide swaths of land extending from

valley floor to ridge crest, and were responsible for

clearing all timber of sufficient diameter for hewing

into railroad ties. At the end of each winter, the com-

pleteness of each logger’s work was verified by inves-

tigators appointed by the railroad (Baker and Hauge,

1913).

Within the study area, West Fork Black’s Fork is a

meandering channel (Fig. 1). Despite the local alpine

setting (valley elevation is approximately 2900 m asl,

and local valley-floor-to-ridge-crest relief is in excess of

400 m), alluvial processes are dominated by lateral

channel migration. The efficacy of lateral migration of

point bar and cutbank systems is evident by the pres-

ence of more than 15 relict, cutoff channel segments

preserved on the floodplain within the study reach.

Radiocarbon dating of cutoff channel-fill sediments

indicates that these processes have dominated this

area for at least the past 8000 calendar years (Carson,

2003). Carson (2003) analyzed the alluvial sedimentol-

ogy of this and other similar stream reaches across the

northern Uinta Mountains and found that prior to his-

toric disturbances, deposits of vertical accretion (over-

bank flood) accumulated on top of lateral accretion

point bars at a rate corresponding to approximately

100 cm in 8000 years. This value was determined by

measuring the thickness of vertical accretion sediments

on top of the point bars of numerous abandoned, relict

channels whose age of abandonment was directly de-

termined by radiocarbon analysis. A linear regression

equation, relating age of abandonment to thickness of

vertical accretion sediments, for 11 channels had an R2

value of 0.96 (Carson, 2003, p. 123), suggesting a

relatively constant rate of overbank alluvial sedimenta-

tion on floodplains over Holocene timescales.

The results of 137Cs analyses within the study reach

show tremendous variation in amount of overbank

alluvial sedimentation since the beginning of the 20th

century, ranging from no historic sediment accumula-

tion downstream of the prominent valley constriction

(Fig. 1) to at least 40 cm of historic deposition imme-

diately upstream of the constriction. Whereas many

previous studies of the effects of logging on sediment

flux and storage have focused on the basin-wide effects

(e.g. Kelsey, 1980; Reid et al., 1981; Trimble, 1983),

the results of this study reflect the importance of con-

sidering valley morphometry in the spatial analysis of

transport, deposition, and storage of even large fluxes

of sediment within a drainage basin. The variability in

amount of overbank sedimentation within the study

reach reflects the complex response of sediment trans-

port, deposition, and storage along the length of the

stream channel.

6. Conclusions

Profiles from three streambank locations within West

Fork Black’s Fork meadow were analyzed for 137Cs

concentrations. The results indicate that a prominent

floodplain constriction within the meadow has exerted

significant control over the spatial patterns of overbank

alluvial sedimentation. Apparently little or no overbank

sedimentation occurred during the 20th Century down-

stream of the constriction; in contrast, the 137Cs data

suggest that upstream of the constriction as much as 40

cm of overbank alluvial sedimentation occurred during

the same period. Model results indicate that at all mod-

eled flood discharges, and particularly at the highest

discharges, the floodplain constriction created a signif-

icant impediment to flood conveyance. In the vicinity of

the constriction and downstream of it, flow areas dras-

tically decrease relative to upstream and velocities in-

crease because of the constriction. Flow velocities

increase from the range of 0.5 to 0.7 m/s found in the

upper meadow to the range of 1.0 to 1.5 m/s in the lower

meadow (cross-sections # 1 to 11).

Use of HEC-RAS modeling of flow discharges for a

range of flood magnitudes was undertaken to compare

flood conveyance through the modern and pre-human-

disturbed topography of the channel and floodplain.

Results indicate that sediment deposited on the flood-

plain in the past century has impacted several hydraulic

properties of flood flow through the study reach. Prior

to disturbance, the 1.58-year flood only slightly

exceeded bank heights and modestly encroached on

the floodplain prior to recent sedimentation. Under

modern conditions, the floodplain is not significantly

inundated by the same discharge because bank heights

have increased because of accelerated historic overbank

sedimentation. In the upper meadow (cross-sections #

16 to 28), increased bank heights largely contain the

1.58-year discharge (bankfull flood) within the channel.

Similarly, in-channel flow velocities here have in-

creased by 50% to 100% because they are more con-

fined to the channel.

Acknowledgements

This work represents a portion of Carson’s doctoral

dissertation at the University of Wisconsin-Madison.

E.C. Carson / Geomorphology 75 (2006) 368–383382

To that end, I appreciate the efforts of J. Knox and D.

Mickelson during the course of my dissertation re-

search. I would like to thank M. Devito, B. Hess, J.

Munroe, L. Murray, N. Oprandy, and T. Sweeney for

assistance in collecting field data for this research; I

would also like to thank J. M. Daniels, D. Douglass,

D. Koerner, and J. Munroe for discussion and critical

commentary during the course of this research. Com-

ments by D. Walling and an anonymous reviewer

improved the final manuscript. Partial funding for

this research was provided by National Science Foun-

dation grant BCS-0081896; Geological Society of

America grants 6672-00 and 6858-01; Sigma Xi

grant-in-aid; the Ashley and Wasatch-Cache National

Forests; and the Morgridge Distinguished Graduate

Fellowship.

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