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ABSTRACT OF DISSERTATION <br />PHYSICAL PROCESSES AND HABITAT CRITICAL TO THE <br />ENDANGERED RAZORBACK SUCKER ON THE GREEN RIVER, UTAH <br />The last self sustaining, riverine population of razorback sucker occurs <br />on the middle Green River. Since 1962, operations at Flaming Gorge Dam have <br />reduced flow variability, reduced duration of flood peak, and altered timing of <br />spring flows. These flow changes modified fluvial processes related to sediment <br />movement and deposition at the primary spawning site and connectivity to <br />downstream floodplain nursery habitat which are critical to razorback sucker <br />` reproductive success. <br />Physical and biological studies related to the early life history of the <br />razorback sucker were conducted from 1992 to 1996. The primary spawning <br />site of the middle Green River population is located along the right side of an <br />alluvial channel around a large island at river kilometer 500. Physical <br />evaluation of the primary spawning bar was conducted using repeated cross <br />sectional surveys and one dimensional HEC-2 and HEC-6 modeling. This <br />analysis found this site to be influenced by the backwater effect of a constriction <br />at the downstream end of the island complex. This backwater condition causes <br />reduced velocity and reduced water surface-slope in the spawning channel as <br />flows rise during spring runoff. <br />HEC-6 analysis was conducted using averaged post-dam USGS <br />t suspended load data, 1996 USGS bedload data, and 1993 bed material samples. <br />The magnitude and pattern of model-simulated deposition and scour were very <br />similar to empirical measurements in 1993 and 1996. Major sedimentation at <br />the site (HEC-6 output) began at 325 m's"' and ultimately resulted in an average <br />of 0.6 meters deposition of sand at peak flows approaching 650 m's''. Model <br />simulation and physical measurements at the site showed that sediment was <br />iii '