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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: [email protected].
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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