8188

• UCREFRP Catalog Number: 8188
• Author: Muth, R. T., et al.
• Title: Flow and Temperature Recommendations for Endangered Fishes in the Green River Downstream of Flaming Forge Dam.
• USFW Year: 2000.
• USFW - Doc Type: \
• Copyright Material: NO

Final Report

3-45

September 2000

from upstream due to deposition in the upstream pool. A clean cobble substrate, at incipient motion
and suitable for egg adhesion, is found within the chute channels.

The downstream hydraulic control for this bar is formed by two coarse-grained and
horizontally opposed alluvial fans that have prograded into the channel to form a constriction in the
flow path. Hydraulic analysis of the reach indicated that two of the three mid-channel bars located
in the middle and left branch channels consist of gravels and cobbles and that a condition of incipient
motion is attained at a range of discharge between 79 and 227 m3/s at these bars.

3.6.4 Vertical Accretion of Banks and Channel Narrowing

Flow regulation reduces the dynamics of sediment deposition and erosion patterns. Each
year, sediment deposits exposed during base flows are colonized by vegetation, and if these areas
are not scoured by subsequent floods, a process of channel narrowing and increasing bank elevation
can occur. At some point, this process becomes difficult to reverse because older, deeper-rooted
vegetation is difficult to remove by all but the most extreme flood events.

Andrews (1986) and Lyons et al. (1992) presented sediment budgets and channel-width data
for portions of the Green River during pre-dam and post-dam periods. Flaming Gorge Dam has
affected the quantity of sediment transported by a given flow as a result of altering the channel
morphology and/or the availability of sediment within the channel. Historic wet and dry periods also
influence these factors.

Andrews (1986) described a sequence of degradation, equilibrium, and aggradation
downstream from Flaming Gorge Dam that has developed in response to flow and sediment
regulation by the dam. The degrading portion of the Green River channel, where sediment outflow
exceeds sediment inflow, occurs just below Flaming Gorge Dam in Reach 1. Equilibrium conditions,
where sediment inflow equals sediment outflow, occur in Reach 2. Aggradation (where sediment
inflow is greater than sediment outflow) occurs in Reach 3, especially just downstream of the White
River and Duchesne River confluences.

Andrews (1986) described channel narrowing in Reach 2 as a response to changes in
sediment load and flooding caused by Flaming Gorge Dam operations. He determined that, on
average, the channel had narrowed by 13% from 213 to 186 m since dam closure and that further
narrowing would continue for another 30 years. Lyons et al. (1992) conducted additional analyses
and arrived at somewhat different conclusions. Their results indicated that, in Reach 2, channel
narrowing in response to construction of the dam had been completed by 1974 and produced a
reduction from 217 to 204 m (6% reduction). The large floods from 1983 to 1986 reversed some of
this narrowing and produced an average channel width of208 m (4% reduction from pre-dam width).

Merritt and Cooper (1998) provided additional information on channel changes in Browns
Park in Reach 1 following regulation by Flaming Gorge Dam. Three stages of channel change were