OVER THE PAST 50–60 YEARS, warming temperatures across the western United States have resulted in greater proportions of precipitation falling as rain rather than snow (Knowles et al. 2006) and in earlier snowmelt and streamflow (Mote et al. 2005; Stewart et al. 2005). The years 2004 and 2007 marked two of the earliest spring melts on record (Pagano et al. 2004), and 2007 was one of the driest years on record in California. Glaciers are disappearing across the West, and Glacier National Park (Montana) may cease to live up to its name as early as 2030 (Myrna et al. 2003). Annual precipitation amounts in the western United States have not changed significantly, and predictions of precipitation are uncertain (Dettinger 2005, 2006). However, even without changes in total precipitation amounts, warming temperatures and corresponding shifts from solid to liquid precipitation have profound implications for park water supplies and park management.

“Glaciers have provided a buffer against low flows in dry, warm summers, and their absence could result in perennial rivers becoming ephemeral streams. Streams that are already ephemeral, such as Yosemite Falls, will likely become drier on average earlier in summer.”

Temperature changes will have the greatest influence in mountain parks with a Mediterranean climate such as Yosemite (California), Sequoia and Kings Canyon (California), Lassen Volcanic (California), Crater Lake (Oregon), Mount Rainier (Washington), Olympic (Washington), and North Cascades (Washington) national parks, where nearly all precipitation falls in winter, and where ecosystems and humans depend on snowmelt for water supplies throughout summer. Earlier snowmelt and a greater proportion of rain result in more water flowing into rivers in winter when it is a hazard, and less into rivers in summer when it is a resource.