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 2003
High-flow Requirements for the Duchesne River

Measurements of gravel thicknesses in the cottonwood terrace unit were made upstream
from subreach 14 and downstream from subreach 12. No measurements were made in
subreaches 12, 13, and 14 because all cutbank exposures in this reach of river are in the high
terrace unit, in which no exposed gravel is present. Gravel deposit thicknesses in the cottonwood
terrace for subreaches 8 through 10 were estimated using a least -squares linear fit through the
data point upstream and downstream (Figure 8). These data indicate that gravel thickness
declines slightly in the downstream direction between the upstream study area boundary and
subreach 4. The standard deviation of measurements made upstream from subreach 10 is 0.33
m, as is the standard deviation of measurements made downstream from subreach 8. In subreach
4, gravel thickness in the cottonwood terrace d~clines rapidly as the cottonwood terrace unit is
replaced by the tamarisk terrace. unit.
High bars in the study area typically consist of a gravel platform capped by sand. The
mean height of high bar surfaces above the reference water surface level at detailed survey sites
was determined using surveyed cross sections. Chute channels are common on point bars in the
study area, and often scour away sand overburden to reveal the coarse basal platform underlying
the point bar. These exposures, plus a small number of pits dug in bar and floodplain surfaces,
were used to estimate the mean elevation of the gravel base at the same locations. Floodplain
surface elevations were also determined from surveyed cross sections. Pit, cutbank, and side
channel exposures indicated the gravel depth of floodplain surfaces is similar to that of high bars.
The elevation of the top of gravel deposits in high bar and floodplain units maintains a fairly
consistent relationship with the elevation of the top of gravel deposits in the cottonwood terrace
upstream from subreach 4. Gravel thickness in the floodplain and high bar units also decrease in
the downstream direction, maintaining an average top elevation about 0.4 m below the top of the
cottonwood terrace gravels.

Uncertainty in Gravel Erosion and Deposition Volumes
Uncertainty in estimated volumes of erosion or deposition was calculated for each
erosion or deposition polygon as the product of polygon area and the uncertainty in the thickness
of gravel in the alluvial deposits (t5H). The value of &I is estimated as the standard deviation of
field measurements of gravel elevations, as described above. Volumetric uncertainty pertaining
to gross erosion and deposition estimates therefore incorporates any uncompensated positional

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