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When water levels receded in the overbank areas, the larvae returned to the main channel. ~ Humpback Chub In contrast to the Colorado pikeminnow and razorback sucker, humpback chub are relatively sedentary and occupy restricted river reaches for most or all of the year (Valdez and Clemmer 1982; Valdez and Ryel 1995, 1997). The fish may remain in or near specific eddies for extended periods of time, and may return to the same eddy for ~ spawning (Karp and Tyus 1990b). Radio-tagged fish from the UCR at Black Rocks stayed almost entirely within a 1.8 mile reach (Archer et al. 1985, Kaeding et al. 1990). Behavior is similar in the Grand Canyon, where 60 tagged fish were recaptured only about one mile from their original capture location (Valdez and Ryel 1995). Although remaining in one reach for most of the year, the fish tends to make use of microhabitats ~ where there is a natural flow regime. Humpback chub in the Yampa River were forced to move into deeper pools as water levels dropped in summer (Karp and Tyus 1990b) and similar behavior was noted in the Little Colorado River (Gorman 1994). Ontogenic shifts in humpback chub habitat use have been reported by Valdez and Ryel ~ (1995,1997) in the Grand Canyon, with subadult fish (50- 200 mm TL) using primarily shallow shoreline habitats and adults using deeper offshore habitats. Microhabitats preferred by adult humpback chub during warmer months are large recirculating eddies and slow runs (Valdez and Clemmer 1982, Karp and Tyus 1990b). ~ Humpback chub reproduction occurs shortly after peak runoff when water temperatures exceed 16°C (Valdez et al. 1982). In the UCR, spawning has been recorded between mid-June and late July (Archer et al. 1985), although the two years of study were both characterized by unusually high flows that may have delayed spawning. Ripe fish are captured mainly in deep shoreline eddies, but spawning presumably occurs in mid- channel and lateral cobble bars. Virtually nothing is known about habitat preferences of ~ the larvae. Postlarval chubs are most often captured in shoreline habitats, including backwaters, small eddies, side channels, and embayments (Valdez et al. 1990), however as the fish grow larger than about 40mm TL they begin to move into deeper and swifter habitats. ~ This ontogenic shift in habitat use was dramatic in the Grand Canyon, where younger fish (larvae through subadults (less than 200mm) occupied in shallow shorelines, but adults used deeper offshore habitats (Gorman 1994; Valdez and Ryel 1995,1997). Habitats used by humpback chub are greatly affected by flows. Higher flows tend to ~ maintain the recirculating eddies used primarily as habitat for adults. An insectivore, the chub benefit from higher flows that provide allochthonous inputs, including terrestrial insects. 22