8251

• UCREFRP Catalog Number: 8251
• Author: Rakowski, C. L. and J. C. Schmidt.
• Title: The Geomorphic Basis of Colorado Squawfish Nursery Habitat in the Green River Near Ouray, Utah.
• USFW Year: 1996.
• USFW - Doc Type: #93-1070,
• Copyright Material: NO

Draft Final Completion Report to UDWR for Contract #93-1070, Amendment 3

35

Other features of the bank -attached bar during 1993 and 1994 included isolated pools formed by horseshoe vortices at

the head of the stable vegetated island between cross-sections 10 and 11, a deep pool within the secondary channel

between cross-sections 8 and 9, and superimposed bars along the margin of the bank -attached compound bar (Fig. 16).

The elevation. shape, and topographic organization of the bank-attached compound bar within the l.5-1an study

reach changed from year to year. Cross-section surveys and data inferred from the digitized detailed bar topography and

video prints were used to measure small- and large-scale bar changes.

~

The bar was surveyed in November 1992 and video images were obtained in 'uly and September 1992. These

data depict the general setting of the bar prior to onset of the spring 1993 snowmelt flood. The September 1992 video

image shows that the bar was more intricately dissected by chute channels, and there were three emergent features at a

low discharge of 35 m3fs (Figure 8). The largest emergent area included a vegetated island, was attached to the bank.

and occurred at the upstream end of the compound bar. White sand and vegetation. shown to indicate deposits greater

than 94.0 m in elevation determined by subsequent cross-section surveys, also occurred on the downstream emergent

feature. Chute channel flow entered the secondary channel and the secondary channel was not an area of stagnant flow

this discharge. In November 1992, all surveyed cross-section points on the bar were lower than 94.5 In. and the overall

bar platform was lower in elevation than in subsequent years.

1993 - n"lnnpJ. cross-section measurements

Substantial aggradation of the bank-attached compound bar occurred during the 1993 flood. Maximum flood
stage was 1.5 m higher than the highest parts of the bar as surveyed in November 1992. A smooth-edged,
high-elevation compound bar became emergent after flood recession (Fig. 16). By June 10, immediately after passage of
the flood peak but before bar emergence, the bank-attached bar and its secondary channel had aggraded 0.6 m in most
locations and the thalweg had scoured about 1 m.. The chute channels on the bar accumulated up to 1.7 m of sediment
Between June 10 and 22, rain in the Yampa Basin caused a short-term rise in discharge during the overall
descend of the flood. The discharge increased 85 m3fs over a 6-dy period, and then the descending limb resumed its
systematic decrease. During this period, the upstream end of the bank-attached bar eroded 0.5 m (Appendix C). The
middle and downstream end of the bank-attached bar remained at about the same elevation. but a large dune was