Columbia Plateau

The Columbia Plateau ecoregion is a Level III ecoregion designated by the United States Environmental Protection Agency encompassing approximately 32,100 square miles of land within the U.S. states of Washington, Oregon, and Idaho. The ecoregion extends across a wide swath of the Columbia River Basin from The Dalles, Oregon to Lewiston, Idaho to Okanogan, Washington near the Canada–US border. It includes nearly 500 miles of the Columbia River, as well as the lower reaches of major tributaries such as the Snake and Yakima rivers and the associated drainage basins. It is named for the Columbia River Plateau, a flood basalt plateau formed by the Columbia River Basalt Group during the late Miocene and early Pliocene. The arid sagebrush steppe and grasslands of the region are flanked by moister, predominantly forested, mountainous ecoregions on all sides. The underlying basalt is up to 2 miles thick and partially covered by thick loess deposits. Where precipitation amounts are sufficient, the deep loess soils have been extensively cultivated for wheat. Water from the Columbia River is subject to resource allocation debates involving fisheries, navigation, hydropower, recreation, and irrigation, and the Columbia Basin Project has dramatically converted much of the region to agricultural use. ()