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SYNOPSIS Backwaters and physical factors that create them are believed to be vital to successful recruitment of early life stages of Colorado pikeminnow. This study showed that large and relatively infrequent spring flows are needed to transport sediment and maintain channel width and complexity. During such high-discharge events, the elevation of sand bars increases. Subsequent lower flows sculpt and erode sandbars and create complex backwater habitat at base flows that is critical for early life stages of all native fishes, particularly Colorado pikeminnow. High flow years result in fewer backwaters and less total habitat area in both the Ouray and Mineral Bottom reaches, but deep, scour-channel backwater habitat increases. The more frequent lower-flow years dissect and erode sand-bar complexes and may maintain or reduce deep habitats and create more shallow ones as bar elevation decreases. The deep, stable backwaters were shown to be preferred by age-0 Colorado pikeminnow in both reaches. Establishing a single discharge that is intended to maximize habitat availability is inappropriate. The discharge required changes annually and is dependent on antecedent conditions. Flood magnitudes are divided into three categories: less than bar top, less than bankfull, and greater than bankfull. Antecedent bar top elevation, resulting from prior floods, can be high or low. The combined effects on nursery habitat availability of the antecedent condition of bar top elevation and flood magnitude are described in a 2 by 3 matrix (Table 1). Based on interpretation of a 1963 photographic record at base flow, the total nursery habitat availability does not seem to have decreased since the pre-dam condition of the Green River. The habitat area estimated from the digitized photos in 1963 was lower than that estimated from digitized videography at base flows in 1992-1994. Similar to the 1993-1994 water years, the 1962 peak was moderately high, and the 1963 peak was low, however the 1963 mapping flows were substantially lower than mapping flows in 1993 or 1994. Although, in this comparison of only one pre-dam year to three post-dam years, the amount of pre-dam nursery habitat was less on each occasion, it was only a snapshot of the habitat availability in an annually variable system, and may not be consistent over the long term. Despite the generally accepted importance of backwater habitat to successful recruitment of Colorado pikeminnow, neither flows nor habitat availability could be closely tied to year-class strength of young Colorado pikeminnow in this study. Factors affecting year-class strength may be indirectly flow related. Both peak and subsequent flows affect river temperatures, which influence spawning of adults, and growth of larvae. Abundance of the competitive and predacious nonnative fish species was also related to flows. High catch rates of nonnative cyprinids in the summer were negatively associated with fall catch rates of Colorado pikeminnow in both the Ouray and Mineral Bottom reaches. High spring peak flows have been suggested as a method for controlling nonnatives. Control of nonnative cyprinids could be affected by the implementation of peak flows at slightly Less than bankfull, without compromising the availability of habitat at base flow. However, this would be detrimental to the j recovery of razorback sucker (Xyrauchen texanus), which requires overbank flooding of bottomlands. Care must be taken to not benefit the Colorado pikeminnow to the exclusion of other native species. The post-dam hydrograph seems to have favored nonnative species over the native species. Returning to a more natural cycle of high, moderate and low flow years could tip the balance back toward the native fishes. iii