7920

• UCREFRP Catalog Number: 7920
• Author: Van Steeter, M. M., J. Pitlick and B. Cress.
• Title: Aerial Photograph/GIS Analysis and Field Studies of the Grand Valley and Ruby-Horsethief Canyon of the Colorado River.
• USFW Year: 1995.
• USFW - Doc Type: \
• Copyright Material: NO

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55

begins when the material at the base of the bank starts to move, and this
occurs at a dimensionless shear stress of 0.06 (Parker, 1979).
The method used for this analysis is similar to that described previously
for the evaluation of coarse sediment transport, but the water surface slope
was used instead of the friction slope. The assumption that the water surface
slope is a reasonable estimate of the friction slope was tested at the flow
modeling sites, and results showed that they were similar.
Results
Cross-sections in the Grand Valley are generally wider and shallower
than in Ruby-Horsethief Canyon (Figs. 24 and 25, Tables 12 and 13). This is
reasonable since the Canyon reach is confined by bedrock walls in most areas
which results in a narrower and deeper channel. The bankfull dimensionless
shear stress ranges between 0.039 and 0.106 for the Grand Valley and between
0.0370 and 0.094 for Ruby-Horsethief Canyon. This range shows that there are
areas of relatively high and low shear stress at bankfull discharge which
would cause local scour and fill. This is expected since at high flows pools are
generally scoured and riffles accumulate coarse particles (Leopold et al., 1964).
The mean value for both reaches is near 0.06 which indicates that bankfull
flow causes significant transport of bed material, and that it produces a shear
stress near the threshold for bank erosion. More importantly, these values are
similar to bankfull results at the flow modeling sites where the dimensionless
shear stress is also near 0.06. This indicates that prescribed flows based upon
the flow modeling sites will yield similar results throughout the entire reach.