The landscape of eastern Washington is varied and includes prairies, pine forests, the shrub-steppe, channeled scablands, and vast areas of irrigated and dry land agriculture. Likewise, the hydrology and climate of eastern Washington also vary considerably.

Hydrology in eastern Washington is highly influenced by landscape, topography, and precipitation. Across the region, much of the winter precipitation falls as snow which does not melt until warmer temperatures of spring cause high-runoff to occur from April through June. By July, most of the mountain snow has melted and the streamflow becomes low (USGS, 2013). Seasonal irrigation and dams have altered the natural hydrology of some streams.

The 2004 Ecology Stormwater Management Manual for Eastern Washington (2004 SWMMEW) has classified eastern Washington into four climate regions (Ecology, 2004). Figure 1.1 depicts the four climatic regions which include: East Slopes of Cascade Mountains (Region 1); Central Basin (Region 2); Okanogan, Spokane, Palouse (Region 3); and Northeastern Mountains and Blue Mountains (Region 4). The following text provides discussion of the precipitation patterns in each region.

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Figure 1.1 -  Eastern Washington Climate Regions
Approximate eastern Washington climate regions.
Source: Ecology Stormwater Management Manual for Eastern Washington (2004) and AHBL, Inc.