9455

• UCREFRP Catalog Number: 9455
• Author: Lyons, J. K., M. J. Pucherelli and R. C. Clark.
• Title: Sediment Transport and Channel Characteristics of a Sand-Bed portion of the Green River Below Flaming Gorge Dam, Utah, U.S.A.
• USFW Year: 1992.
• USFW - Doc Type: R-92-08,
• Copyright Material: NO

no appreciable changes in average channel width or depth were apparent for the 2-kilometer
reach.
The Green River channel is considerably narrower in reach 2 (table 4); however, the magnitude
of reduction in channel width, 5 percent, resembled that of reach 1. Channel width reduction in
reach 2 occurred between 1974 and 1981, at least 10 to 17 years later than in reach 1.
Andrews (1986) reported a 10-percent decrease in channel width, from 157 to 142 meters, for a
24-kilometer reach downstream from gauge 09315000 between 1952 and 1981. His estimate of
channel width for the lower reach in 1952 was about 9 percent greater than that of the present
study. His measurements were based on only 14 cross sections taken in a relatively short
section of reach 2. Other studies have reported similar discrepancies; cross sectional
measurements were shown to overestimate channel area on the Platte River in central
Nebraska, U.S.A. by Sidle et al., 1989.
Andrews (1986) predicted that adjustment of the Green River channel width downstream of
gauge 09315000 was nearly complete and would reach a quasi-equilibrium width of 137 meters.
Our data indicate that adjustment of the river channel was complete by 1981, when average
channel width was 138 meters in this reach.
Effective Discharge
As reviewed by Ashmore and Day (1988), the magnitude of the effective discharge for sediment
transport may not always equal that of the dominant discharge for channel morphology. The
dominant discharge for channel morphology depends on environmental conditions, the recovery
time for large floods (Wolman and Gerson, 1978), the sequence of large floods, and the presence
and interactions of thresholds and feedback mechanisms that are likely unique to the river
studied (Carling, 1988).
Andrews (1986) equated effective discharge and channel-forming discharge for his prereservoir
and postreservoir periods for the Green River. He based this comparison upon his findings of
similarity between effective discharge (total sediment load) and measured bankfull discharge in
the Yampa River Basin (Andrews, 1980). Andrews and Nelson (1988) estimated bankfull flow to
be about 475 cubic meters per second for the channel reach they studied near site B. This
channel-morphology discharge is about 45 percent greater than the sediment transport flow
reported by Andrews for this reach and confirmed in our study. Our studies of open channel
flow near site B indicate that overbank flow would commence at about 500 cubic meters per
second. Thus, the available field evidence for the Green River suggests that the computed
effective discharge in this reach is less than the bankfull channel discharge.
Several factors in the Green River Basin could cause the dissimilarity in magnitude between
the computed effective discharge for sediment transport and the bankfull discharge. The
amount of sediment stored in the channel is much greater than the average annual
13