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In addition to evaluating the direct benefits that may accrue to endangered fish populations, we also evaluated the relative contribution of various tributary streams to endangered fish habitat downstream. We are especially sensitive to the need for evaluating whether water flow regimes, water quality, and sediment supply provide critical inputs for the endangered fish populations in receiving waters. In determining relative tributary inputs of water, sediment, and other materials, we have relied on information obtained from state and federal agencies, and especially data maintained by the U.S. Geological Survey (e.g., water flows: http://waterdata.usgs.gov/nwisw/us/ ; sediment: http://webserver.cr.usgs.gov/sediment), and the U.S. Environmental Protection Agency (EPA: http://www.epa.gov/surf/ ). In pursuing our goals and objectives, we focus primarily on Colorado pikeminnow, humpback chub and razorback sucker. Sufficient information does not exist for a substantive evaluation regarding the importance of tributaries to the bonytail. THE UPPER COLORADO RIVER BASIN Environmental Setting The Upper Colorado River Basin (UCRB) consists of about 98,000 mil, including parts of Colorado, Wyoming, Utah, New Mexico, and Arizona (logs et al. 1965). The UCR basin has been divided into three major hydrologic subbasins -the Green River, upper mainstream Colorado River, and San Juan River (lours et al. 1965, Carlson and Carlson 1982) -- all of which have been altered significantly by human activities. The Colorado River system is characterized by a wide range in temperature and flow conditions. Most of the water in the system originates as melting snow that fills high mountain streams generally above 10,000 ft msl, and travels downstream to arid regions and hot deserts. 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 produce extensive seasonal inundation of the floodplain. Smaller tributaries, generally at lower elevations, are prone to flash flooding after unpredictable summer storms. Storm events contribute sediment that creates turbidity in the main river during the base flow period. Historic flows ranging from 759 to 300,000 cfs have been documented at Lee's Ferry, Arizona, which averaged about 0.6% sediment by volume and carried 100,000 AF of soil to the Gulf of California (White and Garrett 1988, USFWS 1991). As a consequence, native Colorado River fishes have had a long evolutionary history of adaptations to a river system that has been characterized by turbid water and extreme seasonal variations in river flow and water temperature. Water in the warmwater reaches of the historic Colorado River system also contained relatively high concentrations of mineral salts, including carbonates, sulfates and chlorides. The native fishes evolved in a system that was high in dissolved solids and 8