The freshwater supplies of the American West rely, for the most part, on snow. The Colorado River, the Rio Grande, and other rivers in the intermountain west—bounded by the Sierra Nevada and Cascade mountains to the west and the Rockies to the east—are the main sources of water for one of the driest parts of the continent, and their flows are predominantly fed by the springtime melt of snow accumulated over the winter. With winter mean temperatures rising in some places by as much as 2.5°C in the past 2 decades, some scientists are concerned that the current hydrological regime of the region could be overthrown, with snow giving way to rain as the dominant form of precipitation. Decreasing snow accumulation and earlier snowmelt onset have been observed in Colorado. Whether these trends extend to the larger intermountain west region, however, is unknown.