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observed a high use of backwaters in the upper Colorado River by adult Colorado squawfish. They recorded temperatures reaching 10.5°C warmer in the backwater areas than in the main channel. In the Green River subbasin, Tyus (1990) reported high usage of backwater and flooded bottomlands in the Green and Yampa rivers. He reported most captures in run and eddy habitats in early spring but, during peak run-off, Colorado squawfish were most often tracked to backwaters and flooded bottomlands. In low-water years, most fish were tracked to backwaters, but in high water years all radio-tagged fish used flooded bottom lands. Wick et al. (1983) noted that in 1982 (an average flow year on the Yampa River), adult squawfish used flooded shoreline areas in the spring high flows but moved to backwater habitats as the river level dropped. Holden and Stalnaker (1975b) found adult squawfish in all habitat types but mainly in slow water (eddies, backwaters, and flooded canyon months). Valdez and Wick (1981) noted that native fish appear extremely sensitive to fluctuating water levels in gravel pits (i.e. artificial backwaters) and will leave them to avoid being stranded. In order to optimize backwater and flooded bottomland habitats, Tyus and Karp (1989) recommended that Yampa River flows remain unimpeded. Tyus and McAda (1984) reported that adult Colorado squawfish predominately utilized shoreline habitats and were mostly associated with sandy substrate. They also used eddy, run, backwater, and pool habitats and silt, boulder, rubble, and gravel substrates. Habitat and substrate use varied between river systems and time of year. There were significant differences between rivers for water depths and velocities selected by the fish. It is not known whether Colorado squawfish select different conditions between river systems or simply tolerate the wider range of conditions in the Green River mainstream compared to its two upstream tributaries. In the '15 mile reach' near Grand Junction, Colorado, run habitats were most often used by Colorado squawfish during the summer (Osmundson and Kaeding 1989, Osmundson et al. 1995). They noted that Colorado squawfish seek deep water when water clarity is high. Tyus and Karp (1989) reported that adult Colorado squawfish occupied a variety of habitats in mid-to-late summer, but were most common in eddies, pools, runs, and shoreline backwaters. In the Yampa River, Wick et al. (1983) noted a high use of runs during the summer. Wick et al. (1983) reported high use of pool habitat during October and November on the Yampa River. During the winter, Wick and Hawkins (1989) and Valdez and Masslich (1991) observed Colorado squawfish utilizing embayment, backwater, and run habitats in the Yampa and Green rivers. Valdez and Masslich (1991) noted wintering squawfish were often associated with an instream cover element (e.g. sand shoals, sand ridges, cobble jetties, or ice jams). They reported that Colorado squawfish preferred areas of low velocity (0.0-0.15 m/sec) and moderate depths (0.6- 1.1 m). Habitat Availability Wick et al. (1986) censured habitat types (pools, eddies, and backwaters) on the Yampa River during spring runoff and base flows. They reported a strong relationship between the quantity and quality of habitat and the number of squawfish captured. Wick et al. (1986) noted that squawfish were more abundant in higher diversity habitats than in lower diversity habitats. At base flows, the strata with the highest amount of class 1 ( > 2m maximum depth), pools and eddies (n=1.7, area = 5900 m2 per mile) contained 51 % (total n = 74) of the squawfish captured. These collections were all made during the daytime. Final Report - Colorado squawfish habitat Page 5 Miller Ecological Consultants, Inc. December 17, 1997