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INTRODUCTION Determining key limiting factors for populations of endangered fish is a prerequisite to designing and implementing appropriate management activities for species recovery. Factors that limit population size and distributions are often poorly understood. Within the Colorado River system, identification of these factors has often been hampered by insufficient knowledge of changes in the riverine ecosystem that were triggered by changes in flow. Identification of limiting factors has been further hindered by the lack of knowledge about the ecosystem prior to the extensive management of flows (Carothers and Brown 1991). Many aspects of the riverine ecosystem are strongly influenced by the timing, duration and magnitude of river flows. Flow regimes shape the gross physical structure of the river, such as channel morphology and substrate distribution (Leopold et al. 1964). In turn, this structure determines the quantity and quality of a habitat available for use by fish (Huet 1959; Dolan et al. 1977; Platts 1979; Schlosser 1985), and influences the properties (e.g., velocity, depth, temperature, turbidity, chemistry) of the medium in which fish live (Moeller et al. 1979; Stanford and Ward 1986; Liebermann et al. 1989). The biotic environment of fish is also greatly affected by flows with densities of food items, predators, and competitors affected by the flow-influenced physical environment (Liebfried and Blinn 1986; Peterson 1987; Rader and Ward 1988, 1989; Blinn et al. 1989). Recent flow alterations in the Colorado River basin have been hypothesized to be a major factor in reductions of native fishes (e.g., Vanicek and Kramer 1970; Holden et al. 1974; Kaeding and Osmundson 1988; Osmundson and Kaeding 1991; Minckley 1991), with four species now listed as endangered. While impoundments alter fish distribution and movements, effects due to changes in flow are usually not so obvious. Much of this lack of knowledge can be attributed to a paucity of data, minimal integration of studies, and the difficulty of obtaining suitable data sets (NRC 1987). This study was prompted by the limited understanding of how flow-induced changes in the biotic environment effect populations of Colorado pikeminnow (Ptychocheilus lucius). A more complete understanding of the effects of flow regulation on these endangered fish requires a working knowledge of the ecosystem components and how they are inter-related. Background The endangered Colorado pikeminnow was once widespread throughout the entire Colorado River basin, but the population size and range have been substantially reduced in this century (Seethaler 1978). Since the early 1970s, it has been extinct in the entire lower basin of the Colorado River from Glen Canyon to the Gulf of California (Carothers and Brown 1991) and is now restricted to the upper basin of the Colorado River. Within the upper basin, the pikeminnow inhabits the Colorado River from Lake Powell to Palisade, Colorado, and the Green River from its confluence with the Colorado upstream to Echo Park. It also occurs in portions of associated large tributaries including the White, Yampa, Gunnison and San Juan rivers (Valdez 1990). Presently, the Green River supports the largest population of Colorado 1