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268 Y.K. CONVERSE ET AL. In this study, the physical environment of subadult humpback chub and how habitat was related to discharge during base-flow conditions is examined. This study was developed to address three objectives: (l) identify geomorphic differences among reaches and among shoreline types within reaches; (2) determine if subadult humpback chub were associated with these geomorphic differences; and (3) determine how discharge, during base flow conditions, affects sub adult humpback chub habitat conditions. Biology and status of the humpback chub Historically, humpback chub were thought to have ranged throughout the Colorado River system in swift canyon reaches that are now inundated or decimated by water development projects. Presently, six known populations occur in canyon reaches. The largest and most stable population resides in the Colorado River in Grand Canyon in and near the Little Colorado River (LCR). Successful spawning occurs only in the LCR (Valdez and Hugentobler, 1993). This is the only population remaining below Glen Canyon Dam (US Fish and Wildlife Service, 1990). In 1978 the USFWS determined that the operation of Glen Canyon Dam was likely to jeopardize the continued existence of the humpback chub in Grand Canyon. Consequently, the USFWS required an investigation of the effects of dam operations on the resources of Grand Canyon, including the endangered humpback chub. One result of these studies was the implementation of interim flow operating criteria (interim flows) in August 1991 to reduce the effects of fluctuating flows. A second result of the GCES studies was implementation of an ecological assessment of the humpback chub in Grand Canyon (Valdez et al., 1992). These studies have yielded valuable information on the ecology of this species, and the USFWS has assimilated findings from ongoing humpback chub studies into the Glen Canyon Dam Environmental Impact Statement (US Department of Interior, Bureau of Reclamation, 1995). However critical aspects of the ecology of humpback chub remain unclear. The endangered status of humpback chub requires that operations of Glen Canyon Dam (GCD) are conducive to the recovery and sustain ability of this population. Dam operations directly affect the Colorado River in Grand Canyon through changes in flow, temperature, sediment transport and vegetation dynamics (Stanford and Ward, 1991). To date, most of the scientific and political concern regarding flow regime has centered on the magnitude and timing of peak flows, as exemplified by the experimental flood in Grand Canyon in spring of 1996 (Collier et al., 1997). We contend that although peak flows are important for sediment transport, habitat maintenance, and perhaps environmental spawning cues, base flows, which occur in the Colorado River through the Grand Canyon 7-10 months of the year, are also important to the maintenance of resident fish populations. Yet, this aspect of the flow regime has been largely ignored. The relationship between habitat condition and base flows should be examined and better understood before operation of GCD is decided. In this study, we examine the relationship between base flows, habitat condition and habitat use by early life-history stages of humpback chub. STUDY AREA Hydrology and management This study took place in a 24-km reach of the Colorado River that flows through Grand Canyon beginning at the confluence with the LCR (Figure 1). Hydrology of the Colorado River through Grand Canyon is regulated by Glen Canyon Dam, approximately 25 km upstream of Lee's Ferry, Arizona. For consistency with published maps, river distances refer to river miles (RM) below Lee's Ferry (Belknap and Evans, 1989). RM is the standard distance metric used in Grand Canyon to describe location (Valdez and Ryel, 1995). @ \998 John Wiley & Sons, Ltd. Regul. Rivers: Res. Mgmt. 14: 267-284 (\998) " ~ ..