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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47

Old Gauge
This site shows a general increase in reach averaged velocity, depth,
slope, and shear stress with discharge (Table 9). Figure 22 shows that at low
discharge theflow accelerates over a riffle at the downstream end of the site.
As the discharge increases, the discrepancy of velocity between cross-sections
evens out as the bed topography becomes drowned out and the water surface
slope becomes more uniform. The discharge for initial motion of the bed
material occurs near 525 cms (18,500 cfs) and significant motion occurs near
1,050 cms (37,000 cfs) (Fig. 22). These are approximately the 1.42 and 5.5 year
flood respectively.
New Gauge
At this site the reach averaged slope is steeper at low flow than at
moderate to high flows due to a riffle at the downstream end of the site.
Overall, however, the shear stress increases with discharge due to an increase
in depth (Table 10). Figure 23 illustrates how the water surface slope mimics
the bed topography at low flows, but again becomes more uniform as
discharge increases. 1?e average slope is steeper at this site than at the old
gauge site, so there is a higher shear stress for a given flow. Initial motion of
bed material occurs at a discharge near 275 cms (9,700 cfs), and significant
motion occurs at 875 cms (30,900 cfs) (Fig. 23). These flows have return periods
of 1.02 and 2.9 years respectively.