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

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spacing and the degree of pool-riffle development are related to the bed material size relative to the
frequency of occurrence of the dominant flow condition that is capable of transporting the bed material.

Unlike alluvial reaches, the morphology of canyon reaches of rivers does not appear to be related to
the 1- to 2-year recurrence interval flow (Wolman and Miller, 1960). Pool-riftle spacings in canyon rivers
appear to be related to infrequent high magnitude discharges (Graf, 1979; O'Connor et al., 1986). Coarse
sediment deposition that forms the riflles occurs where stream power drops below critical-power thresholds
necessary for cobblefboulder transport during the infrequent high magnitude events. The stream power
minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of
canyon expansions, and therefore, canyon morphology plays a large role in the distribution of the pool-riftle
sequences in canyon reaches (O'Connor et al., 1986). Elimination of the high magnitude flows as a result of
dam construction is likely to cause freezing of existing pool-riffle morphology and spatial distribution
because the lower more controlled flows will not be able to mobilize the riflle-forming sediments.

lisle (1986) demonstrated that bedrock bends have significant effects on the location of bars even
in channels where the bed materials are mobilized by frequent flows. Bars tend to be located 3 to 4 bed-
widths downstream and 1 bed-width upstream of bedrock controlled bends. Changes in hydrology as a result
of dam emplacement under these conditions are therefore unlikely to lead to changes in the locations of
bars, or to lead to a reduction in the frequency of bars, since bar location and frequency are controlled by
the spatial distribution of resistant materials and the frequency of mobilization of the bar-forming materials.

6.1.3. Potential Long-term Sediment Supply. As stated by Andrews (1986) the majority of the
sediment transported by the Green River prior to the emplacement of Flaming Gorge Dam was derived
from the lower elevation portions of the basin whereas a higher proportion of the runoff was derived from
the upper elevations of the basin. Andrews' analysis of the morphologic changes along the Green River that
he attributed to the emplacement of the Flaming Gorge Dam (i.e., degradation and channel narrowing) was
based on the assumption that the river was in a state of quasi-equilibrium before dam construction, and that
the recorded changes in channel morphology were due to the reduction in sediment supply to the reach
downstream of the dam. Lyons (1991) concluded that the Green River downstream of the confluence with
the Yampa River had reached a new condition of quasi-equilibrium since dam closure in 1963. He
considered that the dimensions of the channel are adjusted to the supply of sand-sized sediment.

However, recent work by Schumm et al. (1987) and Schumm and Gellis (1989) brings into
question the validity of the assumption that the Green River was in a state of quasi-equilibrium prior to the
closure of Flaming Gorge Dam. Their work suggests that the sediment load of the Colorado River had been
decreasing since the mid-1920s and that an abrupt decrease occurred in the period 1940-45. The declining
sediment load has been attributed to: 1) drought in the sediment producing areas of the drainage basin
(Thomas et al., 1963), 2) changes in sediment sampling procedures by the USGS (Schumm et al., 1987), 3)
major reductions in the number of livestock and erosion control efforts (Hadley, 1974) and 4) sediment
storage within entrenched channels and arroyos following the widespread occurrence of channel trenching in
the latter part of the 19th century (Hereford, 1984, 1985; Graf et ai., 1987; Schumm and Gellis, 1989).
However, when plotted as a time series the sediment load data indicate that the reductions in sediment load
have been progressive rather than abrupt (Schumm and Gellis, 1989). This interpretation of the decline in
sediment loads tends to suggest that the reasons are related to geomorphic evolution of the incised channels
(Schumm et ai., 1984), but the evolution of the channels is affected by climatic cycles as wen (Hereford,
1984, 1985).

Gellis et al. (in press) examined the historical trends in sediment loads and dissolved-solid loads in
the Colorado, Green and San Juan Rivers. All of the rivers showed a decline in sediment loads in the 1940s,
but the Green River was the only one that also had a change in discharge in the same time period. A
characteristic of the Green River was the very large variability of the sediment loads in contrast to the other
rivers that were evaluated. However, notwithstanding the high degree of variability, they concluded that
there was a substantial decrease in the average annual sediment load in the early 19408 and they estimated
that the decrease was on the order of 38 percent at the gaging station located near Green River, Utah. It is

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