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Minckley 1992). The recruitment bottleneck for razorback sucker in the Upper Colorado River Basin very likely relates to the current paucity of low velocity, warm, food-rich and non-predator dominated habitats during spring and summer. Instream flow recommendations predominantly are based on ecological knowledge of Colorado River squawfish, which are the most abundant and best known of the endangered big river endemics. Squawfish occur most abundantly in the potamon reaches of the Yampa, Green, White, Gunnison (Le., downstream from the Redlands diversion dam; a few are isolated upstream) and mainstem Colorado Rivers (downstream from the Grand Valley diversion dams) (Figure 1). Colorado squawfish are long-lived piscivores that grow to more than a meter in length and exhibit long migrations (e.g., between White and Yampa Rivers) (fyus 1990) associated with 15- 200C isotherms (my interpretation based on data in Tyus 1984, 1990). The fish spawn on chute channels (Harvey et al. in press) that form on specific alluvial bars in the Yampa and Green Rivers (Figure 2) in association with the decline of spring runoff and spates (Nesler et al. 1988, Tyus 1990). Eggs of squawfish hatch within about 5 days after spawning at 20-220 C, which is the critical temperature for successful reproduction (Hamman 1981, Haynes et al. 1984, Tyus and McAda 1984, Marsh 1985). Upon hatching, larvae drift downstream (Figure 3) where they are entrained in backwater nursery habitats in alluvial reaches (shown generally in Figure 1). In lab experiments, young of the year (YOY) prefer and grow best at 250C (Black and Bulkley 1985). The YOY and juveniles are most often found in specific low velocity environments, created by the complex relationship of flow and channel geomorphology (Figure 2). These nursery and rearing sites also are inhabited by native and nonnative fishes, particularly flannelmouth sucker (Catostomus latipinnis), roundtail chub (Gila robusta), green sunfish (Lepomis cyanellus), red shiners (Cyprinella lutrensis), sand shiners (Notropis stramineus) and channel catfish (lctalurus punctatus), that compete with the endangered fishes for available food resources or prey upon them directly (Valdez and Wick 1983, Karp and Tyus 1990). Adult squawfish also prefer areas of the channel that are braided and complex, where low velocity habitats (e.g., eddies, pools and slow runs) are abundant. Like razorback sucker, adult squawfish tend to move in and out of large backwaters 12