||Mountain snow fields act as natural reservoirs for many western water-supply systems, storing precipitation from the cool season, when most precipitation falls and forms snowpacks, until the warm season when most or all snow-packs melt and release water into rivers. As much as 75 percent of water supplies in the western United States are derived from snowmelt. Thus, water-resource management of western rivers commonly is planned around the knowledge that much of the runoff to reservoirs and lowlands occurs during the early parts of the warm season, when water demands for irrigation and other uses are at their greatest. During the cool season, water demands are low and, in West Coast states, the potential is high for winter storms to cause disastrous floods. Separation in time between the cool-season risks of flooding and the warm-season benefits of snowmelt runoff is a fundamental assumption of water-resource management strategies in the West.