9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

Table 2.1 Water and sediment yield, Little Snake and
Yampa Rivers (after Andrews, 1978).

Area Mean Annual Mean Annual Sediment Load (tons)
Discharge
(sa mi) (cfs) Sus>>ended Bedload Total
Little Snake R. 3,730 575 1,300,000 70,000 1,400,000
at Lily
Yampa R. at 3,410 1,550 420,000 120,000 540,000
Maybell

Andrews (1980) evaluated effective and bankfull discharges at 15 locations in the Yampa River
basin. The effective discharge was defmed as the increment of discharge that transports the largest fraction
of the annual load over a period of years, while the bankfull discharge was defined as the discharge that
filled the channel to the level of the floodplain. Effective discharges were found to range from the 1.18 to
3.26 year retwn period, and were equaled or exceeded from 1.5 to 11 days per year. The bankfull discharge
was found to nearly equal the effective discharge, suggesting that the stream channels were adjusted to
their effective discharge. For the Little Snake River at Lily the effective discharge was 4485 ds and the
bankfull discharge, 4697 cfs. Similarly, for the Yampa River at Maybell the values were 9111 ds and 9005,
respectively.

O'Brien (1984) completed detailed field sampling of water and sediment discharge in the Yampa
River Canyon at Mathers Hole during 1982 and 1983. Standard USGS sampling techniques were used and
the data base is considered quite valuable and useful. Analytical studies were also completed which
suggested a minimum streamflow hydrograph; however, these analyses were not utilized nor adopted in
this report (Resource Consultants, Inc., Yampa River Task 1 Report, 1989).

23 Green River Literature Review

Andrews (1986) evaluated the effects of Flaming Gorge Reservoir on the Green River. Prior to
construction of Flaming Gorge Dam and Reservoir, the drainage area upstream of the damsite supplied 27
percent of the total basin discharge, but only 2.2 percent of the mean annual sediment loading. The
conclusion was that the large water-contributing portions of the basin lie around the rim, particularly the
northeast divide, while the sediment contributing areas are located in the central and southern parts of the
basin. After completion of the reservoir, Andrews concluded that the reach downstream to the Yampa
River (68 river miles) was degradational, but limited somewhat by armoring. From the Yampa to the
Duchesne River the channel was in equilibrium, and downstream of the Duchesne the channel was
aggradational. Andrews concluded that the greatest impact of Flaming Gorge is not immediately
downstream, but instead several hundred miles downstream. He also found that the magnitude of the
effective discharge had decreased significantly in all reaches and that channel width had narrowed,
however, he concluded that a new equilibrium had not been achieved and significantly more narrowing
could occur.

Andrews and Nelson (1988) found that the most significant process narrowing the channel was
aggradation of channel bars resulting in attachment to the bank and incorporation into the floodplain.
Based on detailed photo interpretation by Pucherelli et aI. (1988), Lyons (1989) generally concurred with
this assessment, indicating 40 percent of the observed channel narrowing was attributed to island
attachment.

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