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

IV

bank-attached bars typically aggrade. during floods. Higher magnitude floods cause more extensive aggradation at

compound bars throughout the reach; the thalweg deeply scours at the same time. Thus. the in-channel relief between

the average thalweg elevation and the average bar elevation increases during large magnitude floods. IT lower

magnitude floods barely overtop the compound bars. then chute channels and the superimposed bars tend to increase

topographic complexity. but decrease average elevation. of these bar forms.

During the 4.S-year recurrence flood (post-dam series) that occurred in 1993. the detailed study site was

inundated about 1.5 m during peak flow and aggraded between 0.5 and 1.0 m. The topographic complexity of the.

subsequent emergent bar was much simpler than had existed prior to the flood. In contrast, the much smaller flood of

1994 barely inundated this bar and did not significantly change its average elevation. However. the topographic

complexity of the bar greatly increased because low-amplitude bars. emergent at base flows. were superimposed along

the bar margin during high flow. The ~verage volume of sediment in the l.5-km reach increased between 1992 and

1994 primarily due to net aggradation at the detailed study site. . Comparison of changes in the topographic

characteristics of emergent bars throughout the 10-km study reach showed that measurements made in the l.5-km reach

reflect changes that occurred in longer reaches of the river.

Modifications were made to the flow and sediment transport model of Andrews and Nelson (1986) to simulate

unsteady flows. Modeling of channel hydraulics. sediment transport. and bed elevation which simulated a 2-dy period of

steady flow in the l.S-km reach yielded results consistent with field measurements and with the simulations of Andrews

and Nelson (1986). Longer duration simulations were run for three flow scenarios using an initial topography that was

measured on May 14. 1994. Therefore. model runs simulated passage of a flood that occurred one year after passage of

the unusually large 1993 flood and where the antecedent conditions included high-elevation bar tops. Modeling results

showed that higher flood peaks scour the thalweg more deeply and aggrade bank-attached bars more extensively than do

lower magnitude floods. Thalweg scour and bar aggradation occur quickly. suggesting that extending the duration of

floods does not significantly change the channel topography that is established in the first few days of peak flow. Field

measurements. however. indicate that continued rearrangement of in-channel elements does occur. Additional model

simulations should be conducted to evaluate the role of flood duration in shaping in-channel elements. 1be topographic

data used for model simulations were not of sufficient detail to permit evaluation of the small spatial scale geomorphic

features that create nursery habitat after bar emergence. However. it was possible to examine the longitudinal