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. , Dynamic Relationships Between Flow. ChanneIGeomoq>holo~y and Food Webs The distribution, abundance and life histories of the endangered fishes appear to be strongly influenced by availability of physical habitats that are created and maintained by flow dynamics in time and space (Figure 6). Indeed, squawfish only spawn on clean cobble on specific bars in the sediment-laden river segments of the Upper Colorado system. Hence, a fundamental process- response relationship involves the movement of the fish to the bars in concert with flows that first form the bars and then flush sediment off of cobble substratum so that the fish can spawn successfully (Figure 2) (Tyus 1990, Harvey et ale in press). Humpback chub primarily occur in eddies and other hydraulically complex habitats found in constrained channels in the steeper gradient segments within canyons (Figure 2) (Kaeding and Zimmerman 1983, Kaeding et al. 1990, Karp and Tyus 1990, Valdez et al. 1990). Squawfish and razorback sucker are almost always captured in low or zero velocity habitats (Tyus 1984, Osmundson and Kaeding 1989), which occur within the active channel (e.g., eddies) or exist as backwaters (e.g, back-bar channels) or floodplain wetlands (i.e., flooded bottomIands) that are continually or seasonally attached to the active channel. Squawfish (Tyus 1991a and b), and perhaps razorback sucker as well (Minckley et al. 1991), must have access to low velocity environments to mature. This strongly implies that low velocity habitats are important feeding, or resting areas or both; but, it does not imply that rivers of consistently low velocity or volume are most suited to the endangered fishes of the Upper Colorado River Basin. Low velocity environments are formed and maintained by complex hydrologic processes that involve the frequency and duration of high velocity, peak flows and associated flux of sediment through the stream segment (cf., Andrews and Nelson 1989). Hence, occurrence of low velocity habitats is dynamic in space and time and strongly linked to the flow regime, sediment supply and channel morphology. Numbers and area of low velocity environments used by squawfish larvae, juveniles and sometimes adults in the alluvial Jensen and Ouray areas of the Green River (Tyus and Haines 1991) apparently are maximized at a given time at river discharge of 1381 cfs (numbers) and 1687 cfs (area) (pucherelli et al. 1990). However, it must be kept in mind that a river stage- backwater relationship observed on a particular year is determined by the volume 17