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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

6.0 GEOMORPHIC ANALYSIS AND RESPONSE

6.1 Channel Mor,phol~

In common with the other major rivers that drain the Colorado Plateau physiographic province, the
Green and Yampa Rivers are composed of both alluvial and canyon-bounded reaches. As stated by
Andrews (1986), alluvial channels adjust over a period of years so that the sediment supplied to the channel
is transported with the available discharge such that the dimensions of the channel can be considered to
represent a condition of quasi equilibrium. Such a relationship is not necessarily apparent in the canyon
reaches of the rivers where bedrock exerts a major influence on the channel dynamics (Ashley et al., 1988;
Webb et aI., 1988). Therefore, the response of the rivers draining the Colorado Plateau to dam
construction (e.,g., Flaming Gorge) needs to be considered on a reach basis and general statements about
cause and effect are unlikely to be of much value in predicting the response of specific channel reaches to
changes in discharge and sediment supply. The variability of the reaches along these rivers is especially
important because it appears that the spawning requirements for the endangered fish species are found
within the canyon reaches, whereas the rearing requirements are located within the alluvial reaches (see
Section 13). Furthermore, within each of these macro-scale reaches it is apparent that meso-scale features
(pools, riffles, bars) are also of significance with respect to sediment transport processes and both spawning
and rearing.

For the Yampa-Green River system a substantial body of knowledge has been assembled regarding
channel response to both natural processes and water resources development, particularly the construction
of Flaming Gorge Dam. The following sections discuss geomorphic response mechanisms at both the macro
and meso-scale as a baseline for evaluating channel response to water resources development alternatives.
In addition, several unique characteristics of the Yampa-Green River system are introduced and evaluated
in relation to channel response mechanisms.

6.1.1. Macro-Scale Mor,phology and Response. Andrews (1986) and Lyons (1989) have concluded
that the Green River has responded to the installation of Flaming Gorge Dam and that the response has in
general been one of channel adjustment to the reduced sediment supply and altered hydrologic regime.
However, the principal morphologic adjustment has been that of channel narrowing (approximately 10
percent) in the alluvial reaches (e.g. Brown's Park). Morphologic changes have not been documented in the
canyon reaches. The extent of the adjustment in the alluvial reach~s and the equilibrium status of these
reaches is equivocal. Andrews (1986) concluded that there was degradation and channel narrowing between
Flaming Gorge Dam and the Yampa River confluence as a result of reduced sediment supply to the reach.
He concluded that the reach between the Yampa River confluence and the confluence with the Duchesne
River was in a state of quasi-equilibrium, and that the river was aggrading downstream from the confluence.
Lyons (1991) concurred with Andrews with respect to the Green River being in a state of quasi-equilibrium
prior to the emplacement of Flaming Gorge dam, but he differed in his interpretation of the changes
attributable to the dam in the post dam period. Lyons (1991) divided the Green River into two reaches; the
lower from Green River, Utah (rm 94) to river mile 121, and the upper from the confluence with the
Duchesne River (rm 237) to Jensen, Utah (rm 310). Utilizing time sequential aerial photography he
determined reach average channel widths In the upper reach (rm 237-310) average width decreased from
1964 to about 1974 and then appeared to stabilize or increase slightly. In contrast Andrews (1986) indicates
that channel width continued to decrease until 1978. In the lower reach (rm 94 to 121) average channel
width showed no change until 1974, at which time a reducing trend commenced that appears to have lasted
until about 1981. In contrast Andrews suggests that channel width decreased continuously between 1952 and
1981. Lyons' data on channel width change suggest that the upper reach of the Green River responded
rapidly to the reduction in sediment supply, but the lower reach response was lagged for a period of about
10 years. Lyons (1991) suggests that the responses that he identified were consistent with the changes in
channel morphology postulated by Schumm (1977) with respect to changes in bed material discharge. From
a fish rearing perspective the morphological adjustments in the alluvial reaches of the Green River may have
led to a reduction in the extent of juvenile-rearing backwater areas (Andrews and Nelson, 1988; PuchereUi,

6-1