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1 1 ii BIG-RIVER FISHES The "big-river" fishes are characterized by endemism and unique adaptations to this harsh environment and display remarkable physiological, morphological, and behavioral traits. Today the remaining populations of these species are small and isolated. The disappearance of native fishes in the lower basin has attracted attention since early days of river modification (Dill 1944; Miller 1946, 1961). Two inter-related actions- are primarily responsible for the elimination of the "big-river" fish. One is the alteration of habitat due to dam construction and diversions and the other is the introduction of nonnative fish. Dams and permanent diversion structures interrupted the seasonal flow patterns, replacing them with managed flows, and also altered thermal regimes in reservoir impoundments and the remaining river channel. Sediment transport was interrupted, causing changes to channel morphology and dynamics. Dams also blocked migration routes and isolated groups offish. The meandering of the river within its historic floodplain, which had created backwaters and marshes., was halted, and the floodplain was isolated behind levees or stabilized banks. Some habitats suitable for adult fish persistence still remain throughout the system but the low variance in flows and thus seasonal conditions critical for key life stages of native fish in the presence of nonnative predators and competitors, are greatly reduced. Concurrently, the introduction and spread of nonnative fishes may have been even more detrimental than loss of habitat. The new physical conditions in the lower basin provided suitable habitat for these introduced species and they spread rapidly. Most nonnatives are ecological generalists and are very competitive.with other fish species. Many are predators, or if omnivorous, are still a threat to native fish eggs and young. Decades of monitoring and research in the lower basin have shown that nonnative predators or competitors prevent native fish populations from completing their life cycle in the few suitable habitat patches remaining (Blinn et al. 1993, Minckley and Deacon 1968, 1991; Minckley 1973, 1985, 1991; Pacey and Marsh 2000). These nonnative species now dominate the aquatic habitats available in the lower basin. Bon ail 3