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Effect of Flow and Ice on Movement and Habitat Use Movement and habitat use by radiotagged adult Colorado squawfish and razorback suckers in both the Green River and Yampa River were substantially influenced by river flows and ice conditions. Flow was higher in both rivers in Winter 1 than in Winter 2, but the main difference in winter conditions between the two rivers was daily fluctuations and unstable ice on the Green River, and relatively steady flows and stable ice on the Yampa River. The effect of flow magnitude and flow fluctuation, combined with ice conditions, is the focus of the ensuing discussion. Average activity rate of nine Colorado squawfish in the Green River under nonfluctuating (<5 cm/hr stage change), ice-free conditions, was 23.0 m/hr (n=15, SD=22.3), while average rate of seven fish exposed to fluctuations (>5 cm/hr) was significantly greater (student's t=1.84, p>0.90) at 66.9 m/hr.(n=7, SD=63.1). This suggests that daily flow fluctuations of the Green River increased activity rates of adult Colorado squawfish by 190 percent. Valdez and Masslich (1989) interpreted these results to mean that changes in flow stage violated microhabitat needs, by forcing fish to find deeper water during low flow, or lower velocity during high flow. This study also showed that adult Colorado squawfish were more active during highest rates in flow stage change, and more sedentary during relatively stable flows, at high (4,000 cfs, 113 cros) and low (1,500 cfs, 42 cros) flow magnitude. Ice also affected movement and habitat use of adult Colorado squawfish in the Green River. Solid surface ice, frazil ice, and jam ice, each had variable effects. Fish observed under solid surface ice in 1987-88 behaved similar to fish in ice-free, nonfluctuating flows, with respect to local movement and activity rate. Local movement of fish under solid ice was typically within a 50 m diameter area, between two to eight closely-spaced favorite spots. Although unconsolidated frazil ice did not appear to alter normal activity, frazil ice trapped beneath solid surface ice, or mixed with jam ice, appeared to force 6sh to leave iced habitats, and restricted movement to ice-free regions. Frazil ice between Chew Bridge and Brush Creek on the Green River, in January and February, 1988, reached a thickness of 3 m beneath a surface ice layer of about 0.5 m, and filled most shorelines and backwaters. The frazil also filled the main channel, through which the river formed ice-free tunnels of flowing water. Radiotagged fish located beneath the ice were found exclusively in these open- water channels, and did not appear to swim through the frazil mass. Jam ice had an immediate effect on fish movement and habitat use by physically displacing fish from its path. Daily fluctuations on the Green River in Winter 2 prevented surface ice from thickening and stabilizing, and resulted in frequent ice jams between Echo Park and the Bonanza Bridge. Valdez and Masslich (1989) 30