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INTRODUCTION ~ The Recovery Implementation Program (RIl') has administered research on the reproduction and habitat requirements of Colorado pikeminnow (Ptychocheilus lucius) since 1987. A specific element, the Interagency Standardized Monitoring Program (ISMP), includes yearly monitoring of young - of -year (YOY) pikeminnow by seining backwaters in the fall. Research leading up to the establishment of the ISMP suggested that young pikeminnow ~ preferred large, deep backwater habitats with temperatures higher than the main channel (Tyus et al. 1982; Valdez et al. 1982; Archer et al. 1985). During the first five years of the ISMP and during other studies, high variation in catch rates was observed within these habitat limitations, suggesting a need to further refine the definition of quality nursery habitat (McAda et al 1992). In addition, studies were needed to define the physical characteristics of these habitats and the ~ hydrological processes by which they are created and maintained. This Nursery Habitat (NH) study on the Colorado River is part of the ongoing Aspinall Unit Five Year Research Program administered by the Recovery Implementation Program. The study began in 1992, and field investigations continued through 1996. The objectives of this study were: 1) To determine current status and characteristics of reproductive success by / Colorado pikeminnow (Ptychocheilus lucius) in the upper mainstem Colorado River; 2) to determine the effect of experimental flows from the Gunnison River created within the Aspinall Study Plan on Colorado pikeminnow; and 3) determine the relationship between habitat availability on the Colorado River at recommended releases from the Aspinall unit to: a) the habitat actually utilized by young-of--year (YOY) Colorado pikeminnow, b) the number of young ~ Colorado pikeminnow present in the Colorado River during the autumn nursery period, and c) the effect upon growing season and size of Colorado pikeminnow YOY as it relates to potential overwinter survival. METHODS Two 16 kilometer reaches on the Colorado River in Utah were sampled. The reaches were selected on the basis of high catch rates for YOY pikeminnow in the preceding five years of the Interagency Standardized Monitoring Program study (McAda et at. 1994). The upper reach ~ was located near Moab between River Kilometer (RK) 104.5 and 88.5 (River Mile [RM] 65-55). The lower reach was below the Potash Mine from RK 48.3 to 32.2 (RM 30-20) (Figure 1). Within each reach every backwater and many other low-velocity habitats were measured and seined consistent with the ISMP handbook guidelines (LTSFWS 1985). Three sampling trips were executed each year, in spring, mid-summer, and fall, for a total of 15 sampling occasions on each ~ reach. The sampling schedule followed each cohort of pikeminnow through their first year of life, beginning post-spawn, and ending the following spring before the spring runoff. Sampling times were not chosen to target particular flows, but rather a specific life stage of the Colorado pikeminnow. On each trip all backwaters, and other low-velocity areas with YOY habitat potential, ~ were sampled. All sampled sites will be hereafter referred to as habitats. Physical measurements were taken on each habitat including length, and width, which was measured at three transects; at the mouth, 1/3 and 2/3 of the length. Across each transect, water depth was measured at the