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INTRODUCTION Ice processes in rivers are well documented because of the great economic damage of icing and ice jams on bridges, buildings, roadways, and other manmade structures (Ashton 1979, 1980, 1983, 1988; Calkins 1979, 1984; Donchenko 1978; Osterkamp 1978; Wilson et aI. 1987). However, there are few studies that describe effects of ice on habitat of temperate fishes and associated biological resources, particularly as a result of dams and river regulation (Shumway and Springer 1992). Understanding the effects of ice processes on habitat, movement, feeding, growth, and survival of northern fish species provides valuable insight into relationships between dam regulation and ice processes in southern high elevation desert rivers, such as the Green River of the upper Colorado River basin, Utah (Valdez 1995). Unregulated rivers can be relatively stable, sheltered environments in winter because oflow stable flows and ice cover that insulates the water from subfreezing air temperatures (Wick and Hawkins 1989). Low stable flows minimize unnecessary movement and presumed expenditure of energy. and allow fish to remain in sheltered, consistent habitats. Low water temperature reduces community metabolism, lowers autotrophic food production, and reduces metabolism of fishes (Schmidt et at. 1987). Survival of individual fish and the integrity of the population often depend on the quantity and quality of winter habitat available, especially in rivers with severe icing, such as those in arctic regions. Ice cover can lower atmospheric oxygen exchange and dissolved oxygen; shoreline ice and ice in shallows can block fish movement and transport of food; and ice encroachment can displace or kill fish (Schmidt et al. 1987; Holcik and Hruska 1981). Some fish relocate or use alternative habitats to avoid stresses from ice processes. Overwintering northern pikeminnow (Ptychocheilus oregonensis) below a hydroelectric facility in the Columbia River moved to deep, low velocity habitats during low and high discharge periods (Fraler et al. 1988). Rogatnykh and Morozov (1988) found some species of Pacific salmon actively spawning in ice-free sect ions of rivers kept open by spring-fed creeks and effiuents of warm ground water in rivers where ti-eezing was otherwise identified' as the principal cause of mortality of eggs and fry. As cold-blooded or poikilothermic animals, fish exposed to subfreezing conditions can succumb to frozen body fluids and tissues that can lead to death. The first effect of subfreezing temperatures is inhibited water diffusion pressure, or freezing point depression, that prevents free exchange of fluids across cell membranes (Lagler et al. 1962). Most freshwater teleosts (modem bony fishes) have a freezing point depression for blood and plasma of about -0.570C, which means that surrounding water temperature below about -0.5 oC may cease water diffusion in the circulatory system and lead to death of the fish. Riverine fish are particularly at risk because water in motion otten has to be supercooled to -0.001 to -0.1 oC before freezing (Ashton 1983). Once an ice cap forms. water is insulated from supercooling. However, frequent breakup of ice cover allows supercooling of water and development of frazil ice, which can be driven deep into the water column to establish a supercooled gradient (Calkins 1979). Valdez and Masslich (1989) observed large accumulations of frazil ice in the Green River that became transported downstream for several kilometers beneath the ice cap. 1