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t c 8 upstream reach. These observations in the downstream reach or that the reach and drifted downstream to the recorded. may indicate that most larvae were deposited area where most of the fish were spawned in the upstream captures were Postlarval Young-of-Year Occurrence No postlarval young-of-year Colorado squawfish greater than 25 mm total length were collected from above the Gunnison River confluence in a total of ~ 57 samples collected in the fall of 1982-1986. However, a total of 4 62 Colorado squawfish were collected in an 18-mile reach below the confluence of the Gunnison River (54 samples). The 1982-1984 catch rate of young-of-year Colorado squawfish in the 10-mile reach immediately downstream of the confluence of the Gunnison River (river miles 160-170) warranted classification of this reach as a "Young-of-Year Nursery Area" by the Basin Biology Subcommittee (U.S. Fish and Wildlife Service 1984). Nonspawning Adult Occurrence Osmundson and Kaeding (1989) reported that adult Colorado squawfish catch rates in the upstream 15-mile reach were twice as high as those in the adjacent downstream river reach. During 1986-1989 adults were most abundant in a 1.3-mile segment (river miles 174.4-175.7) of the 15-mile reach during high water, particularly in two gravel-pit ponds that were accessible during high flows. These fish may have moved into these ponds to feed and rest, or they may have been attracted to the warm, productive environments that the ponds provided (pond temperatures were as much as 10.5° C warmer than the adjacent river). Some of the squawfish captured from one pond were well tuberculated by June 3, when nearby river temperatures were only 10° C-13° C (Kaeding, pers. comm.). It has been hypothesized by some investigators that thermal energy units above those provided in the mainstream are important to gonadal maturation. If this is true, then access to these sheltered off-channel pools may be very important to successful spawning in the upper reaches of the Colorado River. Historically, bottomlands that routinely flooded during the spring runoff period would have provided these warm productive habitats; in recent years, flooded gravel pits may have provided the only comparable habitat. Razorback Sucker The razorback sucker, an endemic species unique to the Colorado River Basin, was historically abundant and widely distributed within warmwater reaches throughout the Colorado River Basin. Historically, razorback suckers were found in the main stem Colorado River and major tributaries in Arizona, California, Colorado, Nevada, New Mexico, Utah, Wyoming, and in Mexico (Ellis 1914; Minckley 1983). Bestgen (1990) reported that this species was once so numerous that it was commonly used as food by early settlers and, further,. that commercially marketable quantities were caught in Arizona as recently as 1949. In the Upper Basin, razorback suckers were reported in the Green River to be very abundant near Green River, Utah, in the late 1800's (Jordan 1891). An account in Osmundson and Kaeding (1989) reported that residents living along the Colorado River near Clifton, Colorado, observed several thousand