Regional setting: Yakima County occupies approximately 4,300 square miles in south-central Washington State, the second largest land area and seventh largest population area in Washington State. County population is approximately 222,500. SMA streams in the county are all within three of the State’s 62 Water Resource Inventory Areas (WRIAs). These are: WRIA 37 (Lower Yakima Basin); WRIA 38 (Naches Basin); and WRIA 39 (Upper Yakima Basin).
Climate: The headwaters of the Yakima River and its tributaries receive substantial precipitation, mostly as winter snowfall that persists until late spring and early summer. Lower areas of the county are relatively arid. Mean annual precipitation ranges from less than 10 inches in the Yakima River valley to 140 inches along the Cascade crest. Chinook winds and rain-on-snow events occasionally cause rapid melting and a surge in stream flows. Annual spring snowmelt provides substantial stream flow for a limited period.
Topography and Drainage: At higher elevations of the Cascade Mountains there are glaciated peaks and deep valleys. In valleys the Yakima River and tributaries include both narrow canyons and broad meandering stretches with floodplains and terrace features. The county includes approximately 300 miles of SMA streams, almost all of which drain to the Yakima River. The largest tributary, the Naches River, receives flow augmentation during the summer from Rimrock Lake and Bumping Lake. The river flows through a gap in the basalt ridges at Union Gap. The lower Yakima River receives flow from several streams and drains that often are augmented with agricultural return flow.
Land use: Agriculture dominates the Yakima County landscape in the valleys, principally tree fruit, mint, hops and dairy. Forestry dominates the upper watersheds.
Vegetation: Vegetation in the county includes forest, range and cropland. Forests in the subbasin are naturally heterogeneous in species composition, age and size class because of the sharp gradient in precipitation zones. Between the forests and farmed valley are extensive shrub-steppe habitats.
Sources: Haring, D., 2001, Rinella et al. 1992
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