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

r

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

37

In 1994, flood flow barely inundated the highest elevations of the bank-attached bar. Consequently, while

localized areas of scour and fill occurred on the bar platform, the most dramatic response to flood passage occurred in

the deepest portions of the channel. Immediately prior to flood passage, the thalweg was located on river left at cross-

sections 12 and 11. crossed to the bar margin on river right above cross-section 10, then crossed back to river left

between cross-sections 8 and 7. During the ascending limb of the hydrograph, the thalweg moved up to 50 m with the

resultant thalweg following the river left channel margin for most of the length of the bar and crossed to river right near

cross-section 7. One to 2 m of scour and fill occurred during the lateral migration of the thalweg in this subreach

(Appendix C).

Changes observed across the bar platform in 1994 were much less dramatic. About 0.25 m of fill occurred on

the upper platform near cross-section 9, with scour of about 0.25 m occurring on the upstream end of the bar (Fig. 16,

Appendix C). The higher portions of the secondary channel, measured by cross-sections 8, 9, and 10, aggraded on the

ascending limb, but scoured during flood recession.

1994 - Emerllent bar form

The large-scale topography of the bar changed little from that of 1993, but the topographic complexity

substantially increased. The net effect of the reworking of bar platform sediments was to increase the area of the bar

above 95.0 m from 3540 to 11730 m2, but to decrease the area of the bar between 94.0 and 95.0 m by about 50 percent,

from 61620 to 30270 m2. Upon bar emergence, three low-amplitude bars were superimposed on the shoulder of the

compound bar. These superimposed bars made the topographic form of the bar, especially at base flow elevation (about

93.5 m), more complex than it had been in 1993 (Fig. 16). No vegetation on the bar was removed by scouring. The

secondary channel accumulated about 0.25 m of sediment in the depression between cross-sections 8 and 9 (although

net scour occurred at the measured cross sections) and accumulated more than 1.0 m at the downstream end of the

channel. About 0.25 m was eroded from the upstream portion of the bar, which is the area that blocks through flow into

the secondary channel. This overall flattening of the secondary channel topography greatly reduced the range of

discharges at which water, but not throughflow, occupied portions of the secondary channel. In 1993, this range was

from less than 45 m3fs to a discharge greater than 136 but less than 170 m3fs; in 1994, the range of these flows was more

narrow and was between 76 m3fs and 136 m3fs. It is important to note that the minimum flow necessary to inundate the

secondary channel in 1994 is greater than normal base flows~ Thus, the channel was not a backwater habitat at normal