Declining snowpack from Utah to Pennsylvania will make it harder to find reliable water in the backcountry, enjoy decent ski runs, and paddle classic rivers.
For hikers, a snow-draped peak is a summit goal, a perfect photograph, or a tantalizing daydream. What we sometimes forget is snow's critical role in sustaining life on earth. Water released from melting ice and snow supports plants, wildlife, and millions of human beings from L.A. to Boston. But as global warming continues to melt glaciers, spawn more winter rain, and dissolve snow earlier in the spring, the runoff that flows over Yosemite Falls–and out of your faucet–may slow to a trickle. Here are the factors threatening snowpack and the water it generates.
 Melting up high
Rising alpine temperatures are causing mountain snow to melt faster, which means less groundwater to supply late-season runoff for many Western drainages. Although some precipitation flows directly into streams and lakes, a significant portion leaches into the ground to recharge subterranean aquifers and rivers. This groundwater can seep through rock layers for years before it returns to the surface, providing a slow-release reservoir that keeps mountain streams flowing in summer and fall. "More than half the water flowing over Yosemite Falls has been underground for more than 10 years," says Mike Dettinger, a USGS hydrologist. "Without it," he adds, "the falls would slow to a trickle soon after the spring melt."
 Freezing ground
Frozen soil can block meltwater from infiltrating the ground, keeping more runoff on the surface and increasing the probability of erosion and floods. During a typical winter, heavy snow insulates the ground and prevents it from freezing. But when drifts are patchy or shallow, or temperatures fluctuate rapidly, a layer of surface frost can develop and block water penetration. Global warming could increase occurrences of surface frost by thinning snowpack at lower elevations and producing spring thaws and rainstorms that wash away the snow before the soil warms up.
 Darkening snow
Fresh snow reflects 90 percent of the solar radiation that hits it. But snow darkened by pollution and dust melts earlier and faster, according to Thomas Painter, a hydrologist with the National Snow and Ice Data Center. His fieldwork in Colorado's San Juan Mountains showed that dust-covered snow melted 25 days earlier than clean snow in 2005, and 30 days earlier in 2006, following a surge in dust storms. Most of the dust blew in from the adjacent Colorado Plateau, but some storms originated in China. Painter is concerned that large dust storms could become more frequent as global warming makes landscapes more arid, creating a positive-feedback loop that darkens and melts Rocky Mountain snow a bit earlier each spring.
 Accelerating runoff
In Western streams, peak runoff–the date when snowmelt is at its highest level–has sprung forward by almost 3 weeks in the last 50 years, according to the USGS. It's happening because average temperatures in winter and spring are rising faster than in other seasons. This warmer, moister air results in more winter rainstorms that turn snow to slush. The Pacific Northwest has seen the biggest shift in peak runoff, but drainages in the Sierra, Rockies, and southeast Alaska are experiencing similar cycles. Across the West, these changes are causing larger spring floods, plus reduced stream flow during the drier summer months.