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INTRODUCTION This five year study to determine the effect of Colorado River flows on the humpback chub (Gila cypha) reproduction and recruitment is a component of the Recovery Implementation Program (RIP) Aspinall Studies. The purpose of the Aspinall studies is to provide the United States Fish and Wildlife Service (FWS) with adequate biological and physical data to support flow recommendations which will be included in their Final Biological Opinion on the operation of the Aspinall Unit on the Gunnison River. The biological base for current flow recommendations in the Upper Basin focuses on the relationship between Colorado pikeminnow population dynamics and observed flows. The goal of this project was to develop relationships between observed flows and humpback chub life history. To meet this goal, the following study objectives were identified: determine spawning and nursery requirements, ,- describe the relationship between geomorphic processes of sediment transport and nursery habitat formation, identify and describe reproductive isolating mechanisms, and assess recruitment. It became apparent early in the study that sampling for adults during the suspected spawning period was not feasible. Targeted habitats could not be sampled efficiently when flows exceeded 6000 cfs. Radio telemetry would be a requisite component of determining spawning habitat for humpback chub in Westwater Canyon. Radio telemetry was discussed in the project proposal as an approach to addressing this issue, but was never identified as a task due primarily to funding constraints. Therefore, to meet the goal of providing a flow recommendation for humpback chub, we focused on monitoring early life stage and nursery habitats to develop relationships between Colorado River flow and humpback chub reproductive success. Considerable effort was also directed at monitoring juvenile and adult fishes to determine levels of recruitment and species (native and nonnative) interactions. A similar approach was taken on the Green River in Desolation Canyon as part of the Flaming Gorge Studies. The humpback chub, originally described from specimens collected in Grand Canyon of the Colorado River in 1945 (Miller 1946), has been found in several canyon-bound areas of the upper basin: Black Rocks, Westwater Canyon, and Cataract Canyon of the Colorado River; and, Desolation /Gray Canyon and Yampa Canyon in the Green River sub-basin. The Westwater Canyon population, discovered in 1979 during the FWS's Colorado River Fishery Project and monitored annually since by the Utah Division of Wildlife Resources (i1DWR), is likely the largest population of humpbacks in the upper basin. The present study represents the most intensive sampling effort in this canyon. Greater emphasis has been placed on the earlier life stages than has been in the past. Although the early life history work in this study was patterned after concurrent UDWR studies of Colorado pikeminnow nursery habitat requirements and habitat /flow relationships, it was soon evident that even at the earlier life stages habitat use /behavior of native chubs differed greatly from Colorado pikeminnow. Native chubs complete life cycles in short sections of the river (Valdez and Clemmer 1982). The steep walled, deep canyon habitats of Westwater Canyon that were known to harbor juvenile and adult roundtail and humpback chub also provide the nursery areas for young of year (YOY). Scour channel backwaters formed in runoff deposited sediments, which are preferred by pikeminnow (Trammel and Chart 1998a; 1998b) are used, but not preferred, by YOY humpback chubs. Additionally, Westwater Canyon YOY chubs vacate low velocity habitats for main channel habitats (shorelines, shoreline eddies and riffles) earlier in development than Colorado pikeminnow, similar to the findings of researchers in the Grand Canyon of the Colorado River and the Little Colorado River (Rich Valdez, personal communication). The early life stages of Colorado River Gila spp. are not easily identified to species. Two species, roundtail chub and humpback chub are commonly found in Westwater Canyon. The last documented collection of a bonytail came in 1984 ten miles upstream of Westwater Canyon in the Black Rocks area (Kaeding et al. 1986 }. Based on the findings of this study and others (McAda et al. 1994, Valdez et al. 1982), we know YOY chubs are found in the presence of adult fish -larvae do not drift downstream as far as Colorado pikeminnow. Based on the distribution of adults chubs -humpbacks