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while most frequently used effective depth was 1.1 to 2.7 m. The most frequently used depth in pools was 3.0 m, while the most frequently used effective depth was 0.9 m, showing the effect of packed frazil ice on this deep, low velocity habitat. The Yampa River fish were predominately in backwaters, embayments, and eddies with mean column velocity of 0.0 m/sec, in runs and shorelines with velocities of 0.0 to 0.1 m/sec, and in pools with velocities of 0.2 m/sec. Habitat use shifted with time, flow, and ice conditions. Typically, the fish used off-channel habitats, such as backwaters and embayments, when available. Main channel habitats, such as runs and eddies, were used during lowest flows and maximum ice formation. The fish also used low velocity habitats adjacent to ice jams and frazil ice masses. During one cold period, several Colorado squawfish were located beneath stationary frazil ice (0.03-1.4 m thick), packed beneath solid ice cover. Wick and Hawkins (1989) believe that Colorado squawfish were attracted to these areas by small prey fish feeding on entrapped aquatic invertebrates (mainly stoneflies). Razorback Suckers Eight razorback suckers monitored in Winter 1 (ice-free conditions) were observed in runs, slackwaters, and eddies for 52, 27, and 21 percent, respectively, of the time observed with radiotelemetry. In Winter 2, nine Green River fish in ice-free conditions were observed in runs, slackwaters, and eddies for 71, 15, and 14 percent, respectively, of the time. The same nine fish in ice-covered conditions occupied runs and eddies for 81 and 19 percent, respectively, of the time observed. Like Colorado squawfish, greater use of runs by razorback suckers in Winter 2 was attributed to lower main channel velocities from lower river flows. Unlike adult Colorado squawfish, overwintering adult razorback suckers were not observed in backwaters or other off-channel habitats. Adult razorback suckers in the Green River occupied habitats with low average channel velocity of 0.03 to 0.4 m/sec, and a depth of 0.6 to 1.4 m. Like Colorado squawfish, razorback suckers in the Green River were commonly associated with instream cover. Fish in midchannel runs were monitored in deep sand troughs, while fish in slackwaters were immediately downstream of sand shoals, at the base of steep sand banks. Razorback suckers were also found in eddies near the tip of cobble jetties. One fish was located between masses of jam and frazil ice, reflecting use of these as cover, as a source of food (invertebrates entrapped in ice), or the only available tnicrohabitat with free-flowing water. 29