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, Migration to spawning areas roughly coincides with the decrease of spring runoff and an increase in water temperatures. Tyus (1990) reported migration beginning approximately 28 days (d) after peak spring flows when mean water temperatures were approximately 140C. Wick et al. (1983) observed migration in radio-tagged pikeminnow after temperatures rose to 160C. Tyus (1990) noted the midpoint of migration coincided with the summer solstice and suggested photoperiod may be important in initiating migration. Tyus (1990) reported spawning migrations occurring earlier in low water years and later in high water years. Wick et al. (1983) suggested other environmental factors besides flow may be important in initiating migrations. Spawning migrations may be an important adaptation to a highly variable environment (Smith 1981; Tyus 1986). Migration to suitable spawning habitat may have some advantages in widely fluctuating environmental conditions (Tyus 1986). Although adult Colorado pikeminnow may migrate long distances, movement within their home range is relatively limited. In the Yampa River, Wick and Hawkins (1989) found that pikeminnow were often active within a particular habitat but rarely moved outside the reach (average of 0.3 miles) selected for overwintering. Valdez and Masslich (1991) also found that Colorado pikeminnow generally overwinter in localized Green River regions (3-5 km or 1.9-3.1 mi long). Tyus and McAda (1984) reported that adult Colorado pikeminnow 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. Fish occupied significantly different water depths and velocities between rivers. It is not known whether Colorado pikeminnow select different conditions between river systems, or simply tolerate the wide range of conditions existing between the Green River mainstream and its two upstream tributaries. In the' 15 mile reach' near Grand Junction, Colorado, Colorado pikeminnow used run habitats most during the summer (Osmundson and Kaeding 1989, Osmundson et al. 1995). They noted that Colorado pikem innow seek deep water when water clarity is high. Tyus and Karp (1989) reported that adult Colorado pikeminnow 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 YampaRiver. During the winter, Wick and Hawkins (1989) and Valdez and Masslich (1991) observed Colorado pikeminnow using embayments, backwaters, and runs in the Yampa and Green rivers. Valdez and Masslich 30