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-jlow Requirements for the Duchesne River

identify portions of coverages in need of correction. Any polygon indicating a change from an
active channel to a terrace surface is clearly false. A local displacement error equal to or greater
than the local width ofthe false polygon must exist at each point along the channel adjacent to
the false polygon. Significant false polygon errors were initially detected at between one and
five locations on each change coverage. For each sequentially paired set of coverages, one
coverage was rectified relative to the other using a spatially-limited rubber-sheeting procedure.
Near channel features on the coverage being rubber-sheeted were adjusted to a position chosen to
reduce or eliminate any false polygons upon subsequent spatial overlay with the other coverage
in the pair. All adjustments were limited to the immediate vicinity of false polygons by using the
"limitadjust" feature of the software.

Development of a Gravel Budget
Sediment budgets were developed for subreaches 5-19 by multiplying areas of erosion
and deposition for each subreach by the estimated average thickness of gravel deposits
associated with particular map units. This approach required estimation of the elevations of each
geomorphic surface above a datum tied to the channel bed and estimation of the elevation of the
top of the gravel deposits in each surface. Reach-scale budgets for gravel were developed by
establishing a downstream boundary condition where gravel transport rates were assumed to be
zero, then sequentially accumulating storage changes in the upstream subreaches. The
downstream zero-transport boundary was assumed to exist near t~e point where the channel
gradient abruptly flattens to less than one-third of its upstream value and the river bed begins a
rapid transition from gravel to sand.
Gravel-thickness attributes were assigned to particular map units based on field
measurements. The longitudinally variable deposit heights used in calculating gravel volumes in
this study are reported in Table 6. Terraces within subreaches 5-19 are composed of gravel
deposits overlain by a layer of sand or silt of variable thickness. The elevations of terrace and
the tops of gravel deposits underlying the terraces above a base-flow water surface level of 500
ft3 Is were observed and mapped at all cutbanks through subreaches 5-19 in October 1999. An
average stage-discharge relationship was estimated using several surveyed cross sections and
used to adjust subsequent observations of deposit heights to a similar water surface datum.

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