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INTRODUCTION The Gunnison River joins the Colorado River at Grand Junction, CO, contributing an average of 60% to the Colorado River peak discharge. The discharge of the Gunnison River is controlled by three large dams collectively called the Aspinall Unit, which were completed in the late 1960's. The construction and operation of the Aspinall Unit dams has had a major effect on the Gunnison and Colorado rivers by regulating flows to control floods, store water, and provide power. All of these factors attenuate the spring peak flows, increase late summer/winter flows, and otherwise alter the natural flow regime (McAda and Kaeding, 1991). The alteration of natural flows in the Colorado River may affect the reproductive timing and successful recruitment of fishes, including the endangered Colorado pikeminnow (Ptychocheilus lucius) (USFWS, 1987; Osmundson and Kaeding, 1991). Flows in the Colorado River in Utah are also influenced by several tributaries which join the Colorado River downstream of the Gunnison River, the largest of which is the Dolores River, which contributes an average of 10% of the flow during peak discharge. Colorado pikeminnow spawn on the descending limb of the hydrograph in June and July as water temperatures rise (Haynes et al. 1984; Osmundson et al. 1995). Following hatch, larvae emerge from the substrate, become entrained in the current below spawning areas and are transported to nursery habitat areas which may be located substantial distances downstream. Drift by larval Colorado pikeminnow has been documented in the Yampa, Green and Colorado Rivers (Kesler et al. 1988; Valdez et al. 1985; Bestgen et al. 1998). Drifting larval fish can be collected using passive drift nets. The goal of this study, conducted from 1992 to 1996, was to determine the current status and characteristics of reproductive success of Colorado pikeminnow and other fish in the upper mainstem Colorado River and the effect of flows on reproduction. This study was conducted on the Colorado River in Utah, under the Aspinall Unit Study Plan of the Colorado River Recovery Implementation Program. The specific objectives were to use collections of drifting larvae to evaluate the effect of flows on: 1) timing and duration of Colorado pikeminnow drift and spawning, 2) relative abundance of drifting Colorado pikeminnow larvae, and 3) to determine if relative abundance of larvae varies similarly with the relative abundance of juveniles in the nursery habitats in the fall. This report covers two study sites in Utah administered by the Utah Division of Wildlife Resources (UDWR), with additional information supplied by Colorado Division of Wildlife (CDOW) on a concurrent larval drift study on several sites on the Colorado and Gunnison rivers within Colorado (Anderson 1999). METHODS Two sites on the Colorado River within Utah were sampled. Site selection was dependent on previous collection sites of YOY pikeminnow, and the location of suspected spawning areas upstream. The upstream, or Westwater site, first located slightly above River Kilometer (RK) 212.4 near the Colorado-Utah border, was relocated in the second year, to RK 205.1 at the Westwater Ranger Station, below a suspected spawning site near the Colorado-Utah border (Doug Osmundson, personal communication). The downstream, or Moab site was also moved after the first year for improved access. It was relocated at RK 105.2, approximately 1.0 1