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13 adult female was captured near river mile 175 that weighed nearly 1 pound more than when previously captured a month earlier, suggesting the development of spawning (gravid) condition. Two Colorado pikeminnow larvae were subsequently collected within the 15-mile reach. During 4 years (1982-1985) of larval sampling throughout the Grand Valley, 100 larval pikeminnow were collected with fine-mesh hand nets from the two Colorado River reaches immediately upstream and downstream of its confluence with the Gunnison River (McAda and Kaeding 1991). Although the sampling effort was similar in the two river reaches, 98 percent of the larval captures occurred downstream of the Gunnison River confluence. Only two (2 percent) of the larvae were collected from the upstream reach. These observations may indicate that most fish were spawned in the downstream reach or that the larvae were deposited in the upstream reach and drifted downstream to the area where most of the captures were recorded. In 1995, drift nets set in the lower portion of the 15-mile reach captured 3 Colorado pikeminnow larvae (Anderson 1998). No postlarval young-of-year Colorado pikeminnow greater than 25 mm total length were collected from above the Gunnison River confluence in fall collections from 1986-1994; however, one yearling-sized individual was captured there in 1986 (Osmundson and Burnham 1998). A total of 122 Colorado pikeminnow were collected in the 31-mile reach downstream of the confluence of the Gunnison River during 1982-1996 (McAda and Ryel 1999). The 1982-1984 catch rate of young-of-year Colorado pikeminnow 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 (USFWS 1984). Catch rates of adult (> 500 mm long) Colorado pikeminnow in the 15- and 18-mile reaches of the Grand Valley are significantly higher than in any other portion of the Colorado River (Figure 1). In the 15-mile reach, adults are most abundant during spring in a 1.3-mile segment between river miles 174.4 and 175.7, particularly in two gravel-pit ponds that were accessible during high flows. Some of the pikeminnow captured from one pond in 1986 were well tuberculated by June 3, when nearby river temperatures were only 10-13° C (L. Kaeding pers. com.). It has been hypothesized by some investigators that additional thermal units, above those provided in the mainstream, are important in increasing annual growth rates and perhaps in gonadal maturation. If this is true, then access to these sheltered off-channel pools may be very important in increasing rates of survival and 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 provide the only comparable habitat.