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exchange of fluids across cell membranes (Lagler et al. 1962). Freezing point depression ('d') is a measure of the solute content of a solution, and hence its water diffusion pressure through tissue membranes. Most freshwater teleosts (modern bony fishes) have a 'd' for blood and plasma of about -0.57°C, which means that body temperature--and therefore, water temperature--below minus 0.5°C may cease water diffusion in the circulatory system, and lead to death of the fish. Standing freshwater typically freezes at 0 C, while flowing &eshwater may become supercooled to below 0°C before freezing, possibly jeopardizing fish health. These supercooled conditions commonly occur in rivers and streams, and explain large aggregations of fish in warm springs and tributary inflows. Effects on Fish Habitat Winter conditions in rivers, particularly in the presence of ice, can have dramatic physical and chemical effects on habitat. Ice cover can lower atmospheric oxygen exchange and reduce dissolved oxygen available for fish respiration. Frozen shorelines and ice jams can physically restrict access to shallow, sheltered resting and feeding areas. Lowered water temperature reduces community metabolism and lowers food production and availability (Schmidt et al. 1987). Studies of anadromous fish in the Beaufort Sea of Alaska revealed that both anadromous and freshwater fish species overwinter for 8-9 months in isolated pools of coastal rivers, where free water remains under the ice. Survival for many of these Arctic fishes depends on the amount and quality of overwintering habitat available (Schmidt et al. 1987). This becomes a critical limiting factor for many Arctic fishes, where only about 3 percent of total habitat is ice-free during winter; many arctic rivers nearly freeze solid in winter months. Overwintering fish seek areas of low velocity to conserve energy during periods of lowered metabolism. Availability of these low velocity areas may be limited by frozen shorelines, ice jams, and frazil ice encroachment. Also, midchannel water velocity may increase from effective channel narrowing from shoreline freezing and ice jams. Increased water velocities frequently impact species dispersal, often resulting in increased numbers of drifting fish and increased energy expenditure. Holcik and Hruska (1981) found denser and more permanent groupings of fishes in areas of low current velocity, such as deep pools, behind large obstacles, and at tributary junctures. Fish found in large rivers with high IIows, such as the Danube, concentrated in areas that serve as refuges from adverse conditions of high current velocities. Distribution of overwintering northern squawfish (Ptychocheilus oregonensis) below a hydroelectric facility in the Columbia River appeared to be related to water discharge and associated velocity distributions (Fraley et al. 1988). Of several species identified in tailwaters, northern 37