East of the Cascades the major runoff occurs during the snowmelt period from May through July. Streamflows gradually rise over a period of a month or more and usually reach peak discharge during June. Fluctuations in streamflows are mostly caused by variations in solar radiation and air temperatures. At times, rainfall adds significantly to the runoff. On streams west of the Cascades, precipitation from predominant winter storms usually falls as rain.
Tributary streams carry the storm rains and water rapidly runs off.
Most of the runoff from these areas comes in winter, but moderate streamflows continue through spring and early summer, fed by late snowmelt from high areas and ground water outflows. Dominating the interior region of the Pacific Northwest, the Columbia River runs through central Washington, then forms a border between Washington and Oregon. The river has one-third of the potential hydroelectric power in the United States. Grand Coulee Dam, constructed and operated by the Bureau of Reclamation, is the largest dam on the Columbia. Additionally the Corps built and operates four dams on the Snake River, the Columbia's largest tributary.