Regional Impacts

While rivers west of the Cascades are primarily fed by rainfall, melting mountain snow sustains rivers east of the mountains during the dry season, particularly the mighty Columbia Sixty percent of water flowing through Washington state began as melting snow.

Global warming threatens to eliminate half the Northwest snowpack resource. This “is likely to be the most important of the consequences of global warming to the Northwest,” University of Washington atmospheric scientist Robert Fleagle says.

Warmer temperatures promise to elevate freezing levels. A Pacific Northwest National Laboratory (PNNL) model shows average Cascades snowline rising from its current 3,000 feet to 4,100 feet by 2050-80. The PNNL model shows the volume of water stored in Northwest snowpack shrinking 50 percent by 2050-80. In the scenario some areas near snowline see snowpack drop by up to 90 percent. Many Northwest mountain areas in the 3,000-6,000-foot range become snow-free.

Only 40-60 percent of today’s average March snowpack is projected to remain in most of the Cascades and interior eastside mountains of Oregon and Washington. The westside Oregon Cascades take an even harder hit — Most slopes retain 20 percent or less of current snowpack. The somewhat higher Idaho and Montana Rockies lose 30 percent of snowpack overall.

With less snowpack, and warmer, rainier spring months, mountains are expected to lose their snow cover earlier in the year, making for earlier runoff.

“Streamflow is reduced at the time you need it most, in July, and August,” PNNL climate modeler L. Ruby Leung notes “The earlier melt effectively lengthens the period between the end of snowmelt and the onset of fall rains,” says Alan Hamlet, a University of Washington streamflow expert. “In hydrologic terms this is like making summer several months longer than it is now.”