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squawfish traveled the furthest downstream during periods of high discharge, and returned to the tailrace as discharge decreased. At high discharge, fish that remained in tailwaters were usually in backwaters and sheltered shorelines, while at low discharge, they remained primarily in the main channel near the dam. Rogatnykh and Morozov (1988) found some species of Pacific salmon actively spawning in ice free sections of rivers flowing into the Okhotsk Sea. These sections were kept open by spring-fed creeks and effluents of relatively warm ground waters. They reported that freezing of spawning beds was the principal cause of egg and larval mortality in Pacific salmon, and that rivers with the largest sections of ice free water were the most favorable for reproduction. Absolute numbers of salmon were directly related to the extent of ice free sections in river basins from which coefficients were derived for identifying importance of individual rivers to reproduction by chum salmon (Onchorynchus keta) and coho salmon (O. kisutch). Arctic char (Salvelinus al inus) are also known to spawn and overwinter in habitat associated with perennial springs (Schmidt et al. 1987), while anadromous Dolly Varden (Salvelinus malma) often overwinter in small spring-fed freshwater streams (Valdez et al. 1977). Fish Response to Physical and Chemical Conditions Habitat for overwintering riverine fish can be greatly reduced by the presence of ice and supercooled water gradients. Schmidt et al. (1987) found water temperatures in overwintering areas consistently at or near the freezing point for ambient salinity, with coldest temperatures occurring in areas of highest salinity. Low numbers of fish were found in marginal habitats, where insufficient water depth, ice-blocked passage, and overcrowding led to low-oxygen stress and mortality of some fish. Fish maintained a position in the salinity gradient, where dissolved oxygen was 8-9 mgll, minimising energy required for osmoregulation. Dissolved oxygen levels were greater than 5 mg/1 at all stations sampled, except for one, where dissolved oxygen was 1.1 mg/l, because of the large number of fish found in a relatively small pool. In relating hydrologic regime to fish abundance and density in the Danube River in Czechoslovakia, Holcik and Hruska (1981) identified velocities critical to fish. Velocities greater than 0.2 m/sec caused many fishes to be carried downstream, and explained low density of resident fishes in the foothill sections of the Danube, where current velocity averages. more than 2 m/sec. Winter Food Sources and Feeding Habits Fish in temperate zones typically feed less in winter because colder water reduces their metabolism and feeding activity. Most invertebrate food sources are concurrently at low numbers or 38