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<br />f <br /> <br />,.....,. <br /> <br />Response of a Sand-Bedded River to High Discharge and <br />Resulting Changes in Availability of Colorado Squawfish <br />Nursery Habitat <br /> <br />C L Olsson* (Watershed Science Unit, Utah State <br />University, Logan, UT 84322-5240; e-mail: <br />cindy@rsgis.nr.usu.edu) <br />J C Schmidt (Watershed Science Unit and Dept. of <br />Geography and Earth Resources, Utah State University, <br />Logan, UT 84322-5240) <br />T A Crowl (Ecology Center and Dept. of Fisheries and <br />Wildlife, Utah State University, Logan, UT 84322-5210) <br /> <br />The geomorphic effect of high discharge on the number and characteristics of nursery <br />habitat for endangered Colorado squawfish on the Green River near Ouray, UT, was <br />measured. Cross-sections in a 1.3 km sand-bedded reach previously studied by <br />Andrews and Nelson (1986) were re-surveyed throughout passage of the spring 1993 <br />flood and compared with annual monitoring data (collected since 1986) at these cross <br />sections. The 1993 flood of 570 m3/s, has an estimated recurrence of 3-4 years, based <br />on the hydrologic record for the period since closure of Flaming Gorge Dam (1963). <br />This discharge would have been approximately bankfull (1.5 year RI) prior to <br />completion of Flaming Gorge Dam. Data show that high discharges caused net <br />transfer of sand from the thalweg to alternate bar crests. Although high scouring floods <br />are often prescribed as a mechanism for the restoration of aquatic habitats, the 1993 <br />flood resulted in a net decrease in the number and area of available nursery habitat. <br />Flow modeling, sedimentologic analysis, and analysis of bar characteristics in other <br />years indicates that flood magnitude, the elevation of the newly deposited bar crests in <br />relation to subsequent low water stage, flood duration, and the rate of flood recession <br />affect the resulting distribution of critical habitats. Lower floods in subsequent years <br />may also scour bar platforms, resulting in increased bar complexity and nursery habitat <br />availability in the future. <br /> <br />* CL Olsson changed name to CL Rakowski and will be known as such for all future <br />work. <br />