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I I I I I I I I I I I I I I I I I I I INTRODUcnON Declines in populations of fIShes native to the Colorado River System have been attributed to modification or loss of habitat and interactions with nonnative fish species (Behnke and Benson 1983; Stanford and Ward 1986; Hawkins and Nesler 1991; Minckley 1991). Flow and temperature patterns and channel morphology of mainstem and tnDutary rivers have been greatly altered by water-development projects, and introduced nonnative fishes are now numerically predominant in most fish communities. Successful protection, management, and recovery of natural elements of the Colorado River System require an understanding of relationships among present physical and biological conditions. Studies conducted by the Colorado Division of Wildlife during 1980-1984 on habitat requirements and limiting factors for rare native fishes of the Upper Colorado River System concentrated on reproduction and early life history of the endangered humpback chub Gila cypha and Colorado squawfish Ptychocheilus lucius (Haynes et al. 1985). These studies included extensive sampling of small fishes (primarily age-O) in low-velocity nursery habitats of the lower Yampa River. Data on distribution and numeric abundance of native and nonnative fIShes were produced from this sampling effort. Our primary objective was to funher analyze these data and associate resulting information on annual spawning periods and relative abundance of young of selected fISh species with annual spring-summer flow and temperature regimes of the lower Yampa River. We compared our findings with results of other studies conducted in the Upper Colorado River System. STUDY AREA The Yampa River, located in northwestern Colorado, is the principal tributary of the Green River and the only large river in the Colorado River Basin not greatly affected by water-development projects (Carlson and Muth 1989; Tyus and Karp 1989). Along its approximately 320-km course, the Yampa River drops in elevation about 2,255 m and enlarges to a sixth-order stream before joining the Green River (Joseph et al. 1977; Behnke et al. 1982). Major aquatic-habitat zones of the Yampa River include a mountainous headwater zone with cold and clear water, an intermediate zone with cool to tepid and occasionally turbid water flowing through cottonwood bottomlands and agricultural valleys, and a lower zone with warm and usually turbid water flowing through deep lime- and sandstone canyons. The present flow regime of the Yampa River appears to approximate historic annual and seasonal patterns and is characterized by variable low-, average-, or high-flow years; high flows in spring and early summer; and low Oows during late summer through winter (Haynes and Bennett 1986; Tyus and Karp 1989). Tyus and Karp (1989) summarized U.S. Geological Survey flow records for the Yampa River from 1922 through 1987 and reponed that daily discharge averaged about 61 m3/s annually, 153 m3/s during spring runoff in April through July, and 14 m3/s per month for August through March.