9534

• UCREFRP Catalog Number: 9534
• Author: Mussetter Engineering Inc.
• Title: Findings of the Sediment Monitoring Program Peer Review Panel and Recommended Work Plan.
• USFW Year: 1997.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

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therefore, is probably the best river in which to evaluate the specific habitat requirements of the
endangered fish species (Tyus, 1992).

Closure of Flaming Gorge Dam in 1962 has not appreciably affected the mean annual water
discharge of the Green River (Andrews, 1986), but has significantly reduced the magnitude of the
flood peaks (e.g., the 100-year peak flow has been reduced by about 21,000 cfs) (Harvey and
Mussetter, 1994). The 90 percent exceedance flow for the pre- and post- Flaming Gorge periods
are 1,500 and 2,300 cfs, respectively, which indicates that there has been a significant increase
in the flow magnitude during the low-flow periods of the year. Because of the changes in the flow
duration curve, the mean annual sediment load has decreased by about 54 percent at the Jensen
,gage and by about 48 percent a~ the Green River gage (Andrews, 1986).

Andrews (1986) evaluated the effects of the dams on the stability of the Green River,and
conduded that it was degradational upstream of the Yampa River, in approximate equilibrium from
the Yampa River confluence to the Duchesne River, and aggradational in the reach downstream
of the Duchesne River. He further concluded that, in the alluvial reaches downstream of Jensen,
channel width between 1964 and 1978 decreased by about 13 percent (700 to 610 feet) and
would have to decrease by a total of about 25 percent (700 to 524 feet) to reach equilibrium. Most
bank width reduction was due to bank attachment of mid-channel bars, but the number of mid-
channel bars through the reach increased significantly during the period. In contrast, Lyons (1991)
concluded that an equilibrium channel width that was about 13 percent less than the pre-dam
width had been re-established in the Jensen reach by 197 4. Downstream of Green River, Utah,
channel width had reduced by about 5 percent (474 to 454 feet).by 1981.

In addition to those specifically mentioned above, many other studies have been perfonned during
the last few decades to characterize the flow, sediment transport, and sediment storage dynamics
of the Yampa and Green River systems. A bibliography of studies that are potentially relevant to '
. the proposed sediment monitoring program is included in the references (Chapter 4).

The two primary factors that have affected, and are likely to continue to affect, the habitat
characteristics of the Yampa and Green Rivers are flow modification and sediment trapping by
Flaming Gorge Dam on the Green River, and landuse changes and water development in the
Yampa and Little Snake River basins. In addition to the effects of Flaming Gorge Dam, the peer
review panel identified several land-use and water development activities that may have
implications for the recovery of the endangered fish. These issues included:

. Ongoing efforts by the Natural Resources Conservation Service (NRCS) and local entities "
to reduce sediment yield in the Little Snake River Basin in Wyoming,

. Potential flow modifications in the Yampa River associated withthe operation of Elkhead
and/or Stagecoach Reservoirs,

. Flow reduction in the Little Snake River due to transbasin diversions by the Cheyenne
project, and

. Potential water quality problems associated with nutrientloading to the Yampa River in the
Craig area.

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Mussetter Engineering. Inc.