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.. Rhithron - headwater reaches of a river continuum characterized by cold, clear water. bedded gravel and cobble substrata on the river bottom and alternating canyons (constrained) and intermontane floodplains (less constrained). Potamon - the downstream zone of a river continuum characterized by warm, often turbid waters, sandy. unstable bottoms and complex channels that may be constrained in canyon segments but more often meander through broad valley or coastal floodplains (after lilies and Botosaneanu 1963 and Stanford and Ward 1993). Ecosystem - the totality of ecological, social and economic processes (function) that interconnect organisms (structure), including humans. with their environment in a given place and time period. Ecosystem boundaries are permeable with respect to energy and materials flux and often are best determined by the nature of the ecological issue or question of concern. I. Introduction Endan~ered Fishes of the Upper Colorado River Basin Four endemic fishes, Colorado squawfish (Ptychocheilus Lucius), bonytail chub (Gila eLegans), humpback chub (Gila cypha) and razorback sucker (Xyrauchen texanus), ofthe Colorado River are protected under the Federal Endangered Species Act, and a recovery program for these fishes has been established by the U.S. Fish and Wildlife Service (Wydoski and Hamill 1991). These endemic, big-river fishes were abundant throughout the potamon reaches of the Upper Colorado River Basin (Figure 1) during settlement and initial development of the basin (circa 1870s-1950s) (Minckley 1973, Quartarone 1993). However, current population size and recruitment of these fishes are reduced substantially, underscoring the rationale for their listing under the Endangered Species Act. Bony tail chub and razorback sucker are virtually extirpated in the Upper Colorado River Basin. Reproducing populations of humpback chub are known only in five canyon segments (Colorado River: Black Rocks and Westwater Canyons; Green River: Gray and Desolation Canyons; Yampa: Yampa Canyon). Squawfish remain comparatively abundant, 1