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INTRODUCTION Native fishes of the upper Colorado River (UCR) basin have declined in distribution and abundance. Alteration of the natural riverine environment during the last 100 years is the most likely cause of their decline, principally by human actions that resulted in physical habitat loss and the introduction of nonnative species. Construction of water development projects began in the UCR basin in the early 1900s (Fradkin 1984, Carlson and Muth 1989), and by the 1960s, more than 50 dams and major diversions had been constructed on mainstream rivers (Figure 1). Impoundment by these structures converted many river reaches into lacustrine habitat, and operation of dams has altered the natural timing, duration, and magnitude of annual flood flows. Flow regulation and the presence of structures also have caused changes in water temperature, sediment load, nutrient transport, and other facets of water quality (Carlson and Muth 1989). In some reaches, sediment load has been reduced 90% (Fradkin 1984). Most existing mainstream habitats are now different than the historic habitats in which the native fishes evolved, and some have been modified so extensively that native fish can no longer survive in them. The historic riverine habitat also has been changed by the introduction and proliferation of nonnative fish species, including many that are predaceous, highly competitive, and harmful to the native fish fauna (reviewed by Hawkins and Nesler 1991, Lentsch et al. 1996, Tyus and Saunders 1996). Although the native fishes were well adapted to their natural environment, alterations to the physical habitat may have created corrditions that are now more favorable to many of the introduced species. Even where physics! habitat has been altered relatively little, nonnative fish abundance has increased, and the abundance of native fishes has been reduced. As a result, most of the native fish habitat is occupied now by introduced species (Minckley 1982, Tyus et al. 1982x, Carlson and Muth 1989). Physical and biological changes to the river system have resulted in endangerment of four native fish species: Colorado pikeminnow Ptychocheilus Lucius (formerly Colorado squawfish), humpback chub Gila. cypha, bonytail G. elegans, and razorback sucker Xyrauchen texanus. These and other fishes native to the main channels of the Colorado River system ("Big River Fish Community") were once widespread and abundant (e.g., Jordan 1891, Jordan and Evermann 1896, Quartarone 1993), but they have disappeared from most of their original habitat. Their endangerment is attributable to a suite of environmental factors that is essentially the same for all four species.. The problem exists at the ecosystem level because an entire fish community is threatened, and threats include biotic and abiotic factors. Successful recovery of all four of the endangered "big river' fishes in the upper Colorado River basin will depend on the maintenance and expansion of present endangered fish populations, and in some cases the establishment of new populations. In both cases, expansion of the occupied range of the existing populations will be a requirement for recovery to aless-endangered status (U.S. Fish and Wildlife Service