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River, the East River, and the Animas River. The second phase of the project then evaluated how these hydrologic changes might affect water supply, salinity, and hydroelectricity production throughout the entire Colorado River Basin using the Colorado River Simulation System (CRSS), a reservoir-simulation model developed and operated by the U.S. Bureau of Reclamation. Two types of climate scenarios were used for these sensitivity studies: hypothetical temperature and precipitation scenarios, and scenarios generated by general circulation models (GCMs) of the climate. The hypothetical scenarios included increases in average temperatures of 20 to 40C and increases and decreases in precipitation of 10 and 20 percent. The regional changes in temperature and precipitation from three GCMs were also evaluated. The scenarios chosen reflected both the best understanding and the uncertainty about the expected magnitude of regional climatic changes when the study began. Our results suggest that certain aspects of the hydrology and water.supply system of the Colorado River Basin are extremely sensitive to climatic changes that could occur over the next several decades. Not only are significant changes in runoff possible, but the ability of the existing water supply system to mitigate the worst effects is limited. For example, the major reservoirs of the Colorado Basin lessen the impacts of reduced flows, but only for a short period of time. Under conditions of long-term flow reductions and current operating rules, these reservoirs are drawn almost completely dry, hydroelectricity production drops dramatically, and salinity in the Colorado River increases to the point where It fails to meet legal standards almost all of the time. The results strongly suggest that the current approaches to water management In the basin will have to be modified to balance the many competing demands and priorities under conditions of altered climate, and that current water allocations may well be threatened. Changes In Colorado River Basin Hydrology The principal Impacts of changes In temperature and precipitation on runoff In the Colorado Basin are summarized below. · Increases In temperature of 20C alone, with no change In precipitation, cause mean annual runoff In the Colorado River Basin to decline by ~ to 12 percent. .' A temperature Increase of 40C causes mean annual runoff to decrease by 9 to 21 percent. · Increases or decreases In annual precipitation of 10 to 20 percent result In corresponding changes In mean annual runoff of approximately 10 to 20 percent. · A temperature Increase of 40C would require an Increase In precipitation of 15 to 20 percent merely to maintain annual runoff at historical levels. · Temperature Increases shift the seasonality of runoff In the Colorado Basin, causing a distinct Increase In winter runoff and a decrease In spring runoff. This Is the result of a decrease In winter snowfall and snowpaCk, an Increase In winter rain, and a faster and earlier spring snowmelt. These temperature-drlven changes could Increase the potential for winter and spring flooding In some regions. · GCM temperature and precipitation scenarios modeled as part of this study suggest that Ix