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1 Table 1. The effects of floods on Colorado pikeminnow nursery habitat availability on a 10 km reach of the Green River near Ouray, UT. Italics: effects hypothesized, but not tested during this study. 1 1 1 1 Flood magnitude, increasing from left to right A d ntece ent conditions less than bar top less than bankfull greater than bankfull low-elevation Rearrangement of Maintain the Increase availability bar tops habitats. increased availability of deep habitats. of deep habitats Net change in habitat Maintain or decrease Decrease availability availability unknown availability of of shallow habitats. shallow habitats high-elevation Increase shallow Rearrangement of Maintain the bar tops habitat availability. habitats. increased availability of deep habitats Decrease deep habitat Net change in habitat Maintain or decrease availability. availability unknown availability of shallow habitats INTRODUCTION The assessment of Colorado pikeminnow nursery habitat on the Green River was initiated in 1991 on the Green River as part of the Flaming Gorge Flow Recommendations Investigation, a five-year research program administered by the Recovery Implementation Program (RIP). This program, conducted through 1996, included many other studies designed to produce appropriate flow recommendations for the operation of Flaming Gorge dam to benefit the four endangered fish species in the Green River; Colorado pikeminnow, razorback sucker, humpback chub (Gila cypha), and bonytail (Gila elegans). The primary causes of species declines are the construction of mainstem and off-channel dams which greatly alter natural flow patterns and habitat, and the introduction and proliferation of nonnative species. These factors have severely impacted the upper Colorado River and its tributaries. The majority of the Colorado River Basin population of Colorado pikeminnow resides in the Green River sub-basin, which is likely related to the abundance of quality nursery habitat for young pikeminnow (Archer et al. 1986). Tyus and Karp (1991) considered availability of nursery habitats important to successful recruitment. Backwaters have been recognized as important nursery habitats for Young-of--Yea:` (YOY) Colorado squawfish [pikeminnow] (Tyus and Haines 1991). Since construction of Flaming Gorge Dam, however, the Green River channel has narrowed and become less complex, which was thought to have reduced the amount of available nursery habitat. In addition, most habitat is now overrun with nonnative species. Nonnative fishes may compete for limited resources and/or prey directly on native species. iv