9493

• UCREFRP Catalog Number: 9493
• Author: Gaeuman, D., P. R. Wilcock and J. C. Schmidt.
• Title: High Flow Requirements for Channel and Habitat Maintenance of the Lower Duchesne River between Randlett and Ouray, Utah.
• USFW Year: 2003.
• USFW - Doc Type: \
• Copyright Material: NO

FINAL REPORT, November 2()03
High-flow Requirements for the Duchesne River

matched the total energy head in the main channel at the chute's upstream junction. Downstream
boundary conditions for modeling higher discharge were derived by fitting a curve to stages
measured at the downstream cross section over the observed range of discharges.
Input data for the Green River backwater model consisted of an estimate for Manning's n
for the Duchesne River and a downstream boundary condition comprising a stage-discharge
relationship for the Green River at the mouth of the Duchesne River. Cross sections of the
Duchesne River were assigned Manning's n values of 0.025. The lack of rigorous calibration of
this model is justified on the grounds that it is intended to predict only the spatial extent of
backwater effects extending up the Duchesne River from the Green River. River stage within
this backwater zone is primarily a function of stage at the downstream boundary such that local
channel roughness has little or no effect on local hydraulic conditions. This model is not
intended to accurately predict flow velocity or depth, and has no validity beyond assessment of
the influence of the Green River backwater. A stage-discharge relationship was previously
developed for Old Charlie Wash I, a site on the Green River approximately 500 m upstream
from the mouth of the Duchesne, as part of the US Fish and Wildlife Recovery Program for
Endangered Fishes of the Upper Colorado River (FLO Engineering 1997). We adjusted the
datum used at Old Charlie Wash to better match the elevation at the mouth of the Duchesne
River, and this relationship was applied as a downstream boundary condition to our model. As
geographic data in our model were derived from the work of LC Headwaters, Inc., the water
surface elevation at the mouth of the Duchesne River reported by LC Headwaters was used for
datum adjustment. LC Headwaters reported a water surface elevation of 1,425.2 m at a time
when the Green River discharge was approximately 18,000 ft3/S.

Sediment Sampling
Two standard pebble counts were conducted on each of two riffles at each detail study
site. Surface layer size distributions produced from these data allowed determination of mean
gravel size for use in critical shear stress analysis at each site. Bulk subsurface bed material
samples were also collected from riffles exposed at low flow at each site according to the 2-
percent criterion of Church et al. (1987). These samples were sieved through 8 mm screens in
the field, and the remainder was dry sieved through finer screens in the laboratory. Samples of
fine sediment were collected from several locations in pools and backwaters on occasions before

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