Water is a valuable commodity in the arid West, where mountain snowpack provides much of the storage of water for summer use. Recent work by several research groups has pointed out that in much of the West, spring snowpack amounts have declined, peak snowmelt-driven streamflow has drifted earlier in the spring, winter flows have increased and summer flows have decreased. All of these changes are consistent with positive trends in winter and spring temperature. Observations augmented by a hydrological model integrate these hydrologic responses to climate and can distinguish the separate influences of temperature and precipitation in driving trends over different time periods. Regions that are particularly sensitive to further warming can be identified, and implications for future water resources management will be discussed.