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8 <br />and reduced seasonal variability in flows have reduced the frequency of floodplain <br />inundation in the middle Green River from a 2 or 3 year recurrence interval to a 10 year <br />recurrence interval (Wick 1997) and resulted in increased sediment storage at lower <br />elevations in the channel (Wick 1997; Grams and Schmidt 1999). Mobilization of <br />sediment at lower discharges (Wick 1997) and reduced floodplain connectivity (Stanford <br />1994; Tyus and Karp 1990; Modde et al. 2001) since closure of Flaming Gorge Dam are <br />two physical alterations that are cited as important factors contributing to reduced <br />recruitment of razorback sucker (Xyrauchen texanus) in the middle Green River. <br />Review of razorback sucker studies <br />The razorback sucker (Xyrauchen texanus) is an endemic catostomid of the <br />Colorado River Drainage and one of four native species found in the Green River that are <br />federally listed as endangered (U.S. Fish and Wildlife Service 1991). Razorback sucker <br />were once widely distributed in warm water reaches of larger rivers from Wyoming to <br />Mexico (Minckley 1973; Bestgen 1990) and were abundant enough in the lower basin of <br />the upper Colorado River to support a commercial fishery (Minckley et al. 1991). <br />Currently, razorback sucker are commonly found in only two areas; Lake Mohave and <br />the middle Green River between the confluence of the Yampa and Duchene Rivers <br />(Bestgen 1990). The Lake Mohave (AZ-NV) population has had near zero recruitment <br />during the last 20 years (Mueller et al. 1998) and numbers declined from about 73,500 to <br />23,300 individuals during 1988 to 1992 (Marsh 1994). The middle Green River <br />population is the largest extant population of razorback sucker that uses riverine habitat <br />exclusively (Tyus 1987; Tyus and Karp 1990). Estimates for the middle Green River