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Lower Gunnison River, targeting summer low-flow months when conditions are not suitable for Colorado squawfish passage. One approach to determine the minimum flow would be to identify which habitat types were preferred by sub-adult and adult Colorado squawfish and then determine the flow level those habitat types are maximized. To determine Colorado squawfish habitat preference, frequency of use by habitat type would be compared to their relative availability. The best means of obtaining habitat-use information is from radiotagged fish. This approach was used to identify and recommend year-round flows for Colorado squawfish in the 15-mile reach of the Upper Colorado River (Osmundson et al. 1995). However, this study did not collect or have past data on habitat use by sub-adult and adult Colorado squawfish and habitat availability to determine habitat preference during low summer flows in the 2.3-mile reach. Without such data, stage and stream channel water depths were used to determine the flow at which insufficient depth might become limiting to sub-adult and adult Colorado squawfish. Peak runoff usually occurs earlier in the Gunnison than Colorado River. In the Gunnison River, runoff usually occurs in mid- to late-May compared to the Upper Colorado River which peaks usually in early- to mid-June. In the Upper Colorado River, July has always provided a transition from the high spring flows to the base flow of late summer and winter. In the Gunnison River, June is the transition month. The Gunnison River is not entirely regulated by upstream dams and reservoirs. Although its contribution is small during summer months, the North Fork influences the magnitude and duration of peak runoff in addition to the shape and timing of when flows decrease in summer. Natural weather patterns determine the rate of snow melt and reservoir levels determine the amount of water delivered on individual days. The winter snow pack determines the magnitude and duration of the runoff. The level of precipitation in the drainage for the water year also influences the duration and abruptness of the descent of the hydrograph following runoff. For most rivers in the Upper Colorado River Basin, during high- and moderate-water years, flows decrease more gradually starting in early-June and taper off in mid-July. During low-water years, flows decrease more abruptly starting in early-June and usually decline to base flows by early-July. Maintaining the shape of the 'natural' hydrograph is critical to benefiting endangered fishes, the associated native fish community, and the maintenance of the channel morphology and habitat characteristics important to native fishes (Stanford 1994; Tyus and Karp 1989; 1990). Flow recommendations that attempt to maintain the pattern of the 'natural' hydrograph benefit Colorado squawfish and razorback sucker recovery. Vanicek and Kramer (1969) first suggested that discharge and temperature influenced spawning in Colorado squawfish. Reproductive activities of Colorado squawfish and razorback sucker in the Upper Colorado River Basin are closely associated with spring runoff (Tyus and Karp 1989). High spring flows are necessary for the initiation of the spring migration. The greatest longitudinal stream movement by Colorado squawfish usually occurs with declining flows (following spring runoff) during spawning season. Spawning is cued by increasing water temperatures concomitant with declining flows in early to mid-summer. 29