The Columbia Plateau ecoregion is a Level III ecoregion designed by the United States Environmental Protection Agency in the U.S. states of Oregon and Washington, with little areas over the Washington state border in Idaho. This ecoregion stretches across a wide swath of the Columbia River Basin from the Dalles, Oregon to Lewiston, Idaho to Okanogan, Washington near the Canadian border. It incorporates nearly 500 miles of the Columbia River, as well as the lower reaches of major tributaries. It’s named for the Columbia River Plateau, a flood basalt plateau created by the Columbia River Basalt Group during the late Miocene and early Pliocene. The arid sagebrush steppe and grasslands of the region are bordered by moister, predominantly forested, mountainous ecoregions on all sides. The underlying basalt is up to two miles thick and partly covered by thick loess deposits. Where the 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, recreation, irrigation and hydropower, and the Columbia Basin Project has considerably transformed much of the area of the region to agricultural use.
This plateau ecoregion has been subdivided into at least fourteen Level IV ecoregions. The Channeled Scablands, Loess Islands, Umatilla Plateau, Okanogan Drift Hills, Pleistocene Lake Basins, Dissected Loess Uplands, Yakima Folds, Palouse Hills, Deep Loess Foothills, Nez Perce Prairie, Deschutes/John Day Canyons, Lower Snake and Clearwater Canyons, Okanogan Valley, and the Umatilla Dissected Uplands.