7052

• UCREFRP Catalog Number: 7052
• Author: Andrews, E. D. and J. M. Nelson
• Title: Topographic Response of a Bar in the Green River, Utah to Variation in Discharge
• USFW Year: 1989
• USFW - Doc Type: American Geophysical Union
• Copyright Material: YES

r

Andrews and Nelson

467

the Jensen gage. During the pre-reservoir period from 1948 to 1962, the mean
annual suspended sediment load at the Jensen gage was 6.3><106 ton/yr. The Jensen
gage is located in a steep, narrow reach with coarse gravel and cobble bed-material
immediately downstream of the Split Mountain gorge. All sand-size material is
suspended at the sampled cross-section even at relatively common discharges. Thus,
the sampled suspended sediment loads at thiSJage are probably a very good
estimate of the total sediment load in the alluvi reaches downstream. The mean
annual load of sand-sized material (0.062-2.00 mm) at the Jensen gage from 1948 to
1962 was 2.3><106 ton/yr.
During the period from 1964 to 1982, when daily sampling of suspended
sediment ceased at the Jensen gage, the mean annual suspended sediment load
decreased by 54 percent to 2.9><106 ton/yr. The mean annual load of sand-sized
material decreased by 65 percent to 0.84><1()6 ton/yr.
g Although the mean annual suspended sediment load transported past the Jensen
age has decreased substantially as a result of reservoir.regulation, an approximate
uilibrium persisted between quantity of sediment supplied to and transported out
the reach of the Green River from the mouth of the Yampa River to the mouth of
the Duchesne River, a distance of ...160 river km fAndrews, 1986]. The decrease in
,t e mean annual suspended sediment load at the lensen gage is approximately equal
ito the mean annual quantity of sediment deposited in Flaming Gorge Reservoir.
IConsequently, there is no appreciable accumulation or depletion of be<Hnaterial in
t~he Green River downstream from its confluence with the Yampa River to its
Lconfluence with the Duchesne River, including the study reach.
The quantity of sediment transported by a given discharge at the Jensen gage
does not appear to have been affected by Flaming Gorge Reservoir. The variation of
daily suspended sediment transport rate as a function of water discharge was
computed for each size fraction from <0.004 mm up to the 0.25(H).500 mm fraction
by a least-squares regression of the log-transfonn data during the pre- and
post-n!SerVOir periods. No statistically significant change in the relations was
detected br the F-statistic at the 95th-percentile level of confidence.
One 0 the principal downstream effects of Flaming Gorge Reservoir is to
decrease the range of daily mean flows. The mean annual discharges of the Green
River during the pre- and post-reservoir periods are virtually identical at both the
Jensen and Green River, Utah, gages. The percentage of times that various daily
mean discharges are equalled or exceeded, however, is substantially different for
most flows.
r The decrease in the mean annual sediment transport rate at the Jensen gage
l between 1962 and 1982 compared to the pre-reservoir period is due entirely to a
decrease in the magnitude of river flow that are equalled or exceeded less than 30
percent of the time.
Daily mean water discharges with a duration of 5 percent or less have decreased
in magnitude by 25 and 35 percent during the post-reservoir period at both the
Jensen and Green River, Utah, gages. The magnitude of daily mean discharges with
a duration greater than 30 percent, however, have increased to the extent that the
mean annual runoff measured during the pre- and post-reservoir petiod is virtually
nunchanged at both gages. Thus, the decrease in annual sediment transport results.
from a more uniform annual hydrograph rather than a decrease in the annual runoff. .
- Runoff in the Green River during the period from 1983 to 1986 was quite
extraordinary and greatly complicates the analysis of channel adjustment. In spite
of the presence of Flaming Gorge Reservoir, the duration of relatively large
discharges was only slightly less than during the pre-reservoir period. For example,
a discharge of 510 m3/s was equalled or exceeded 4.96 percent of the time between
1947-62, compared to 4.18 percent of the time between 1978 to 1986. Spring
snowmelt runoff during 1983 and 1984 was exceptionally large. Annual runoffs at
the Green River near Green River, Utah gage during the 1983 and 1984 water years
were the largest and second largest in the period of record, 192<H<<). Approximately