DESCRIPTION:
Washington State Hydrology

Washington has a distinctly varied climate resulting from two features: (1) the Cascade Mountain Range and (2) the prevailing westerly winds from the Pacific Ocean. The north-south trending Cascades divide the State into the western part, which has a marine climate characterized by cool, wet winters and warm, relatively dry summers, and into the eastern part, which has a more continental climate of cold winters and hot, dry summers.

Although the average annual precipitation in Washington is about 40 inches, the western part receives about 70 inches and the eastern part about 20 inches. Locally, average annual precipitation ranges from only 7 inches in the driest part of eastern Washington to about 150 inches in the Olympic Mountains in western Washington. About two-thirds of the precipitation in Washington occurs in the fall and winter (October through March), either as rain at the lower elevations or as snow at the higher elevations. Heavy snowpack and glaciers in the Olympic Mountains and Cascade Range are major sources of water for many rivers in Washington.

The runoff in Washington is greatly influenced by the varied topography and rainfall and falls into two general categories: (1) the snowmelt-runoff regime of the eastern part of the State; and (2) the rainfall-runoff regime that predominates in western Washington. The Naselle River is typical of the coastal streams whose major runoff occurs during the 4 months November through February; only about 10 percent runs off during the 5 months May through September. The runoff is closely related to precipitation and because of the relatively low elevations, the heavy winter precipitation falls as rain and runs off quickly. Thunder Creek (Skagit River basin) is a glacier-fed stream in western Washington draining a high elevation where almost all winter precipitation falls as snow. The distribution of runoff for Thunder Creek is opposite to that of the Naselle River, with the maximum runoff occurring in July and the high-runoff period extending from May through September. The period of low flow lasts all winter. The Colville River exhibits a typical runoff pattern for a stream in eastern Washington. Much of the winter precipitation falls as snow which does not melt until warmer temperatures of spring cause the high-runoff period to occur from April through June. By July the snow has melted and the streamflow becomes low.

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