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I razorback sucker survival (30-60 organisms per fish per day; Papoulias and Minckley 1990) occurred in floodplain habitats along the Green River but rarely reached that density in backwaters, and never reached it in the main channel of Upper Basin rivers (Tables 1 and 2; Cooper and Severn 1994 a,b,c,d,e; Grabowski and Hiebert 1989; Mabey and Shiozawa 1993). Only two large backwaters in the Upper Basin contained zooplankton densities that were adequate for razorback sucker larvae during their critical period -- Intersection Wash in the middle Green River (Mabey and Shiozawa 1993) and Millard Canyon in the lower Green River (Nance 1997). I I J It is important to point out that only a portion of the zooplankton or benthic invertebrate biomass is available to razorback larvae since their mouths are gape-limited but they select the largest organisms that will fit into their mouths (Marsh and Langhorst 1988). Another important point is that aquatic organisms normally found in the water column such as zooplankton also occur in benthic samples and benthic organisms that are either free-swimming or emerging pupae also occur in the water column. Larval fishes, including razorback suckers, feed on both benthic and planktonic food organisms that are available and of the right size. I I The year-class strength of fish species is often determined by environmental conditions such as suitable water temperature as well as the quality and quantity of food organisms available to larval fish within the first few weeks of hatching (i.e., critical period; Hjort 1914, 1926; Houde 1987). During years with optimum environmental conditions, high survival of larval and juvenile fish produces strong year classes. The timing, extent, and duration of flooding greatly influences fish species that use floodplain habitats and these factors may exert a moderate to strong control in year-class strength of some fishes (Lambou 1963; Baker and Killgore 1994). I I I The endangered Colorado River fishes are long-lived and are known or believed to produce strong year classes (Henrickson and Brothers 1993; Kaeding and Zimmerman 1983; McCarthy and Minckley 1987, Miller et al. 1982; Minckley et al. 1991; Osmundson and Burnham 1996; Vanicek and Kramer 1969). The ultimate year-class strength of many riverine fishes depends on the magnitude and duration of overbank flooding (Bayley 1991), suggesting that reconnect ion of floodplains with Upper Basin rivers (i.e., re-establishing the flood pulse) may be a key strategy in recovery of the razorback sucker. I I E. Use of Floodolain Habitats bv Adult Razorback Suckers. During high spring runoff, adult razorback suckers in the Upper Colorado River Basin were captured in large eddies at the mouths of rivers, off-channel ponds that have a connection to the rivers, and wetlands in floodplain areas (Modde 1997; Tyus 1987; Tyus and Karp 1990; Valdez and Wick 1983). These investigators believed that adult razorback suckers used these low water velocity habitats to escape the high water velocities that occur in the main channel during high spring streamflows. Since adult razorback suckers were also found in wetlands in floodplain areas and off-channel ponds after the spawning season on the descending limb of thehydrograph, some investigators suspected that adult fish also used these productive habitats to regain body condition after spawning. Although adult razorback suckers feed on benthic and drifting invertebrates, algae, and detritus in rivers (Bestgen 1990), their diet in reservoirs (and probably floodplain habitats) consists largely of zooplankton filtered from the water column (Marsh 1987). I I I I I Floodplain habitats may also be used as spawning sites by adult razorback suckers in the Upper Colorado River Basin (Kennedy..1979;. Osmundson and Kaeding 1989; Tyus and Karp 1990). Ripe female razorback suckers were collected from a large embayment on the Walter Walker Wildlife Area along I 20 I I