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I I I I I I I I I I I I I I I I I I I summer/fall/winter flows. High spring flows maintain channel and habitat diversity, flush s from spawning areas, rejuvenate food production, and form flooded bottomlands used ry backwaters. High spring flows also disadvantage nonnative fishes (Hoffnagle et al. 1999), reducing predation and competition. Low base flows also increase shoreline food production. 3.1.2 Habitat Alteration It is believed that the four endangered fishes once inhabited the larger streams ofthe Colorado River Ba . m Wyoming and Colorado to northwestern Mexico. The historic abundance and dist . n of the species were never documented, but a precipitous decline occurred in known popu ations concurrent with extensive habitat alterations. These alterations led to a loss of the contiguous complement of habitats used by the various life history phases of the species. Since 1905, numerous man-made dams have been constructed throughout the Colorado River Basin, fragmenting habitat and blocking migration corridors. These dams have also reduced river flow, altered temperature and flow regimes, trapped sediments and nutrients, changed water quality, and created rese s as a source of nonnative fishes (Maddux et al. 1993). In the lower basin, 15 major d_ s e restricted fish movement through the Colorado, Gila, Salt, and Verde rivers since completion of Hoover Dam in 1935; other dams on the Colorado River include Davis, Parker, Palo Verde Diversion, Imperial, and Laguna. Glen Canyon Dam approximately divides the lower from the upper basin and also is a barrier to fish movement. Seven barriers are identified in the upper basin abov Kaeding 1990) and five on the San Juan River (Masslich and of the upper basin structures are classified as medium or high- seasonal barriers to fish movement. The Price-Stubbs Diversion presently prevents upstream movement of fish in the upper Colorado River; a second structure, the Government Highline Diversion, is immediately upstream. Passage by these diversions could reestablish contiguous habitat, expand the range of several native fish species, and allow greater opportunities for recovery. The Redlands Fishway on the lower Gunnison River has already been mo e for fish passage, and the Gunnison River provides additional habitat opportunities. Canyon Dam (Burdick and en 1996, Holden 1999). Five 13