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INTRODUCTION Use of the 15-Mile Reach The 15-mile reach of the upper Colorado River between Palisade, Colorado and the confluence of the Colorado and Gunnison rivers at Grand Junction is habitat for the endangered Colorado squawfish (Ptychocheilus lucius) and the very rare razorback sucker (Xyrauchen texanus), a species proposed for listing as endangered (Fed. Reg. Vol. 55 No. 99). A summary of rare- fish use of the 15-mile reach was recently provided by Osmundson and Kaeding (1989). Adult squawfish use the reach year-round; razorback sucker may also use the reach year-round, but are found there predominate- ly during their spawning season (April-June). An aggregation of ripe Colorado squawfish within the reach in 1982 and the subsequent capture that year of larval squawfish there indicates that the reach can provide spawning habitat for the species. Main-channel temperatures of 20-22 C, apparently necessary for Colorado squawfish spawning and important in egg- hatching success (Hamman 1981, Tyus and McAda 1984, Haynes et al. 1984, Marsh 1985) consistently occur in the 15-mile reach (Appendix Fig. I and Table IX). No young razorback sucker have been found in the reach, nor anywhere within the upper Colorado River in the past 27 years. Such appar- ent lack of recruitment portends the imminent extirpation of this popula- tion (Valdez et al. 1982). However, two running-ripe adult razorback sucker were captured in a flooded gravel pit in the 15-mile reach during 1986 and the movement of others to and from the reach during and after the presumed spawning period indicate that razorback suckers routinely spawn or attempt to spawn there. The Grand Valley in general, of which the 15- mile reach is a part, has been a concentration area for razorback suckers