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. INTRODUCTION . The razorback sucker, Xyrauchen texanus, is a relatively long-lived member of the family Catostomidae (Minckley 1983; McCarthy and Minckley 1987; Bestgen 1990; Minckley et al. 1991). Endemic razorback sucker was formerly widespread throughout warmwater reaches of the Colorado River Basin but its current range is much reduced due to physical habitat alteration and proliferation of nonnative fishes (Minckley et al. 1991). Declines in distribution and abundance prompted federal listing of razorback sucker as endangered in 1991 (U.S. Fish and Wildlife Service 1991). Substantial populations (e.g., > 200) of razorback sucker in the lower Colorado River Basin are presently restricted to Lake Mohave Reservoir and Lake Mead Reservoir (Minckley et al. 1991; Marsh et al. 2003; Holden and Abate 2000). Substantial populations in the Green and Colorado rivers in the Upper Colorado River Basin also exist, mostly a result of stocked hatchery fish (Bestgen et al. 2002; Zelasko 2008). Flood plain wetlands are presumed important habitat for early life stages of razorback sucker in the middle Green River (Modde et al. 1996; Wydoski and Wick 1998; Muth et al. 1998; Modde et al. 2001; Bestgen 2008). Reproduction by razorback suckers in the middle Green River occurs before or during the ascending limb of the spring hydrograph when water temperatures are 10-l80C (Tyus 1987; Muth et al. 2000; Bestgen et al. 2002) and produces razorback sucker larvae when flows are high and flood plain wetlands may be accessible. These habitats can be 3-80C warmer than the river, are food-rich, and may promote higher survival of . larvae and recruitment to juvenile and adult life stages (Modde 1996; Muth et al. 1998; Modde et al. 2001; Bestgen et al. 2002; Bestgen 2008). Flood plain habitats are also thought to benefit two . . . . . . of the other Colorado River Basin endangered fishes: Colorado pikeminnowPtychocheilus . 13 .