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Introduction <br />The Gunnison River is historic habitat for Colorado squawfish Ptychocheilus <br />lucius and razorback sucker Xyrauchen texanus. Anecdotal accounts from long-time <br />residents suggest that both species were common in the river during the early and <br />middle parts of this century (Quartarone 1995). However, a variety of human- <br />induced impacts to the Gunnison River have substantially reduced these two <br />species' abundance in the river. In addition to other impacts, Redlands Diversion <br />Dam -constructed in 1917 at river mile (RM) 3 -prevented upstream movement of <br />fish from the Colorado River and isolated fish in the Gunnison River from the rest of <br />the river basin. Relatively few Colorado squawfish or razorback sucker remained in <br />the river above Redlands Diversion when organized sampling efforts began in the <br />1970's. A total of six Colorado squawfish were collected from the Gunnison River <br />between 1979 and 1983 (Valdez et al 1982; Wick et al. 1985). Three razorback <br />suckers were collected from the river near Delta, CO at about the same time (Holden <br />et al. 1981). Intensive sampling by Burdick (1995) between 1992 and 1994 yielded <br />five Colorado squawfish captured in the river above Redlands Dam. Four others <br />were positively sighted but could not be captured. No razorback sucker were <br />captured by Burdick (1995). However, the Gunnison River from its mouth to its <br />confluence with the Uncompahgre River was declared Critical Habitat in 1994 <br />(USFWS 1994) based on historic distribution of the two species. <br />Three reservoirs (Blue Mesa, Morrow Point, and Crystal), composing the <br />Aspinall Unit of the Colorado River Storage Project, were completed by the Bureau <br />of Reclamation (Reclamation) in the upper Gunnison River between 1966 and 1976. <br />These reservoirs were built upstream of habitat occupied by the endangered fishes <br />and have a substantial impact on critical habitat of these species. The largest <br />impact was changes in timing and magnitude of river flows within critical habitat <br />(McAda and Kaeding 1991). <br />Although the Aspinall Unit substantially altered habitat occupied by the <br />endangered fish, authorization and construction of the project occurred before <br />passage of the Endangered Species Act of 1973, as amended. However, the Fish <br />and Wildlife Service (Service) and Reclamation recently agreed to initiate <br />consultation on effects of operation of the Unit on the endangered fishes. This effort <br />was one of several coordinated studies initiated to evaluate those impacts and make <br />recommendations to improve habitat for the endangered fishes in the Gunnison <br />River. This study was done to document changes in macro habitat available to fish <br />with changes in river flows. This information along with biological and physical data <br />collected during previous and concurrent studies will be used to assess the impacts <br />of the Aspinall Unit on the endangered fishes. <br />1 <br />