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other potential pollutants, and numerous studies have demonstrated a tolerance to these water quality parameters in the endangered fishes (e.g. Bulkley et al. 1982, Beleau and Bartosz 1982, Bulkley and Pimental 1983, Pimental and Bulkley 1.983, Black and Bulkley 1985, Marsh 1985, Nelson and Fickinger 1992). In addition to temporal changes in the UCRB that have historically occurred, spatial differences also are important. Three different stream zones are recognized in the basin (Hinckley et al. 1986), and each contains a characteristic native fish fauna, albeit with overlap. At high elevation, the Headwater Zone is a productive region of cold water, high gradient streams that have rocky substrate and support cold water fishes (predominantly salmonids). The Intermediate Zone, which may receive input from the cold water streams, has streams of lower gradient and finer substrate. The water is warmer and more turbid, and productivity remains substantial, but benthic fauna are limited to rocky outcrops. Streams of the intermediate Zone are dominated by cyprinids and catostomids, but some cool water salmonids (e.g., whitefish) also occur. Streams of the Lower Zone, also called the large-river zone, are characterized by even lower gradients and warmer, more turbid water. In the Colorado River, this Lower Zone is composed of two major habitats: canyons and alluvial reaches. Native fishes in this region were exclusively minnows and suckers. The inhabitants of the main channels comprised the Big River fish community. Evolutionary forces have produced a fish community adapted to a riverine system, but flexible enough to make use of conditions ranging from lacustrine to riverine. "~fhe fishes are extreme generalists that exploited every available natural habitat and evolved some complex life histories to facilitate survival in the Colorado River (e.g., see Hinckley and Deacon 1991, Smith 1981, Hinckley et al. 1986). For example, the ancient Colorado River watershed was a much wetter environment than now exists (Smith 1981). The evolution of native fishes was strongly influenced by an ecological history of long pluvial episodes, each lasting about 100,000 years, that were separated by short interpluvial episodes of desert climates lasting only 10-20,000 years. During pluvial episodes, portions of the river system included extensive lacustrine habitat (Stanford and Ward 1986a, Hinckley et al. 1986) used by ancestral Colorado River fishes. In recent times, the climate of the basin has been extremely arid. Nevertheless, the native fishes persisted and thrived even during such dry periods. The key to survival of these fishes no doubt includes physiological and behavioral adaptations which are not completely understood. However, a great tolerance to changes in water temperature, suspended and dissolved solids, complex behavioral patterns such as highly adaptive selection of spawning locations over a long time period (Wick et al. 1983, Tyus 1990), and high mobility to exploit diverse habitats (e.g., Tyus 1986) provides some explanation for documented adaptations of Colorado pikeminnow, the species for which we have the .most information. 9