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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52

Conclusion
The results of our calculations indicate that flows similar to the peak of
the 1993 runoff produce a shear stress approximately equal to two times the
value for initial motion ('t* = 0.06). The results of Andrews (1984) and Parker
(1979) indicate that two times the value for initial motion should be near the
threshold for bank erosion, and that this commonly occurs near bankfull flow.
Our observations of the effects of the 1993 flows are consistent with this since
the peak flows were near bankfull and we noted freshly deposited gravel bars
throughout the study reach, but only isolated areas of bank erosion. Our
calculations also predict that the flows of 1994 should have caused little or no
movement of bed material. Again, our field observations are consistent with
this result. We did not observe any large changes in gravel bars, and a study of
tagged gravels near the Redlands Parkway showed no movement of particles
during the 1994 runoff.
The results of our calculations for above the Gunnison River
confluence (the 15 mile reach) indicate that a flow of approximately 300 cms
(10,600 cfs) is needed for initial motion of the bed material, and a flow of 625
cms (22,000 cfs) will cause significant motion (Table 11). Results for below the
Gunnison River confluence (the 18 mile reach and Ruby-Horsethief Canyon)
indicate that a flow of approximately 450 cms (15,900 cfs) will initiate
movement of the bed material and a discharge of 1000 cms (35,300 cfs) causes
significant movement (Table 11). This flow range is similar to results for a
single site in the 18 mile reach reported in our previous study (Van Steeter
and Pitlick, 1994).