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been divided into three major hydrologic sub-basins -the Green River, upper mainstem Colorado River, and San Juan River (tomes et al. 1965, Carlson and Carlson 1982) -all of which have been altered significantly by human activities. Most of the water in the ~ system is snowmelt that originates in high mountain streams generally above 10,000 ft amsl. Tributary streams at lower elevations add comparatively little water, but can be important for contributions of sediment and for seasonal inputs of water. The natural hydrograph reflects the regular and prominent influence of spring runoff in May and June (Maddux et al. 1993, Stanford 1994), when peak flows produced extensive seasonal inundation of the floodplain. High discharge and erodible substrate produce ~ very turbid water seasonally. Smaller tributaries, generally at lower elevations, are prone to flash flooding after unpredictable summer storms. Storm events contribute to turbidity in the main river during the base flow period. As a consequence, native Colorado River fishes have had a long evolutionary history of adaptations to a river system characterized by extreme seasonal variations in flow and by generally turbid ~ water. 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, Minckley et al. 1986), and the fossil record demonstrates that ancestral Colorado River fishes used this habitat. In recent times, the climate of the basin has been extremely arid. Nevertheless, the native fishes persisted and ~ thrived even during such dry periods. 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. The fishes are extreme generalists that exploited every available natural habitat and evolved some complex life histories that have facilitated survival in the harsh environment of the Colorado River (e.g., see Minckley and Deacon 1991, Smith 1981, Minckley et al. 1986). Three different stream zones are recognized in the basin (Joseph et al. 1977, Minckley 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 coldwater fishes (predominantly ~ salmonids). The Intermediate Zone, which may receive input from the coldwater 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 #o rocky outcrops. Streams of the Intermediate Zone are dominated by cyprinids and catostomids, but some coowater 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. 3