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SUMMARY Radiotracking of adult Colorado squawfish (PtXchocheilus lucius) was conducted on the upper Yampa River, Colorado, over two winters (December 1986- March 1988) under coaxiitions historically found in the upper portions of the Colorado River System. During Winter 1, 10 squawfish were radiotracked for 590.8 total observation hours, from which 118 hours were used to develop habitat utilization criteria. During the fall and winter of the second year, 74 observation hours were accumulated on 10 squawfish. Of these, 34 hours were used to develop winter habitat utilization criteria on depth, velocity ' , and substrate. Squawfish were often active within a particular habitat but they did not move outside the river reach they selected for ing. Squawfish showed fidelity to very specific habitat areas by remaining in ' neither one or a few favorite habitats trout the winter. During the ice covered period, total range of movement of all fish averaged only 0.3 miles -each year. Several squawfish demonstrated fidelity to specific fall and Vinter habitats and river reaches over one or more years. During Winter 1, ' embayment, backwater, and run habitats were most frequently used. Pool and run habitat were used most often in Winter 2. Habitat use also differed between the three study areas. This appeared related to habitat availability and diversity. A backwater habitat was used almost exclusively by three fish in the Government Bridge study, area, river mile (RMI) 95-100, during Winter I. Run habitat was used most frequently the next year prdably because channel bed changes prevented access to the area used most frequently in Winter 1. arbayment and run habitat appeared to be preferred over pool habitat in the Maybell study area (RU 70-82) which had diverse habitat availability. Pool 'habitat was used almost exclusively in the Lily Park study area (RMI 51-54), in Winter 2, where pool habitat was dominant. Effective depth (ice free water wider packed frazil and/or solid ice cover) and velocity utilization for each trip were averaged over the winter period for three habitat categories. ' Shallowest mean effective depths (2 feet) and velocities (0.1 feet/se=-Id (ft/s) ] were from backwater and embayment habitats. Eddys and pools had the deepest average effective depth utilization (3.3 feet) with an average mean velocity of 0.2 ft/s. The run and shoreline habitat category had the highest average velocity (0.5 ft/s) with an average effective depth of 2.4 feet. Naturally stable flow conditions allowed ice cover to exist thr ut the majority of the winter period both years. Flows and air temperatures were above normal in Winter 1 and below normal in Winter 2. Formation of different types of ice on the Yampa River changed hydraulic conditions. During early winter, water surface elevations were maintained in ' spite of reductions in discharge. During Winter 2, changes in effective depths used by squawfish were examined at the Maybell study site by carpar'ing elevation changes of water surface, ice surface, and ice thickness in response to changes in discharge. Discharge increased thrnucpout most of Winter 2, a ' relatively law water year, ranging from 142 cfs on December 15, 1987 to 340 cfs on March 3, 1988. Ice thickness increased from 0.85 feet December 15, 1987 to 2.12 feet March 15, 1988. Increasing water surface elevations ' compensated for increasing ice thickness resulting in relatively stable effective depth. Effective depths measured biweekly in embayment habitat- between December 15 and February 17 varied only 0.1 feet. Regression analysis of discharge on water surface elevation was used to predict the effect of ' hypothetical reductions in flaw below the lowest measured discharge of 142 cfs.. For each 20 cfs loss in discharge, water surface elevation was predicted viii '