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physical barriers to dispersal and seasonal migration. The importance of barriers to fish movement will be discussed in connection with species-specific limiting factors. ~ The operation of reservoirs and other components of the water storage and distribution system affects fish habitat by altering water depth, water velocity, and sediment load, properties that are critical for the creation and maintenance of fish habitat. The quantitative hydrodynamic connection between flow alterations and loss of fish habitat in the main channel is poorly understood, with the notable exception of work on ~ Colorado pikeminnow spawning habitat (Harvey et al. 1993). The relationships are complex and probably better to explore in the context of needs for speck life history stages (e.g., nursery backwaters for larval pikeminnow). The operation of reservoirs also has had some effect on temperatures in the rivers. Reservoirs store cold meltwater in spring and, even though the surface layer of each ~ reservoir will warm during the summer, the release of water from near the bottom of each reservoir will yield cold water through much of the summer. The result is a depression of water temperatures below reservoirs during the months when the native fishes have spawned historically. Colder temperatures could affect spawning as well as the growth and survival of young larvae in the drift (Berry 1988). The association ~ between water temperature and initiation of spawning is relatively well known for the Colorado pikeminnow (Tyus and Karp 1989, Tyus 1990), but less so for the other species. Lower temperatures may have implications for other life history stages, but less is ~ known, and some of the research results seem contradictory. Early lab studies provided information about preferred temperatures of young life history stages of Colorado pikeminnow, from hatching success to optimum temperatures for growth of young of the year (Hamman 1981, Black and Bulkley 1985, Marsh 1985, Bozek et al. 1984). Studies also were done to determine how temperature changes might affect ~ survival and behavior of young fishes (e.g., Berry 1988, Childs and Clarkson 1996). Findings of these laboratory studies have been applied to the river system with varying results. For example, Kaeding and Osmundson (1988) used main channel temperatures to evaluate habitat suitability for Colorado pikeminnow. More recent studies have shown that endangered fishes in riverine habitat may not select the temperatures conditions predicted as optimal by laboratory studies (e.g., spawning ~ temperatures for razorback sucker: Tyus and Karp 1989, 1990). One factor that may mitigate the effect of lower temperatures is the capacity of all life history stages to move toward suitable temperatures in the river. Young pikeminnow can and do move between habitats such as backwaters, eddies, and main channel ~ shorelines in response to differences in temperature regime (e.g., Valdez et al. 1982, Tyus 1991 b). Adult pikeminnow and razorback suckers use a wide range of off-channel habitats such as semi-isolated backwaters, gravel pits, and cut-off side channels, and they may move into shallow, flooded habitats in spring (Wick et al. 1983, Tyus 1987 and 1990, Tyus and Karp 1990). It is thought that active selection of a preferred 25