Columbia Plateau

Columbia Plateau, physiographic region of North America, c.100,000 sq mi (259,000 sq km), NW United States, between the Rocky Mts. and the Cascade Range in Washington, Oregon, and Idaho. Most of the plateau is underlaid by deposits, more than 10,000 ft (3,048 m) thick in places, of lava (mainly basalt) interbedded with sedimentary rock; older rocks outcrop in the Blue and Wallowa mts. Young lavas, scattered cinder cones, volcanic ash, and barren landscapes (including Craters of the Moon National Monument) are features of the Snake River plain in the south. Older, decayed lavas, much modified by accumulations of loess, occur in the north in the Columbia basin section; coulees (dry river canyons) and scablands (extensively eroded basalt surfaces), both carved by glacial meltwaters, are features of the region. The Columbia Plateau is an important agricultural and grazing area and is a major source of hydroelectric power.

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Columbia Plateau 

a lava plateau in the North American Cordillera. Located in the northwestern USA, between the Cascades to the west and the Rockies to the east. Area, approximately 500,000 sq km; elevation, 700–1,000 m.

The foundation is composed of Paleozoic and Mesozoic crystalline and sedimentary rock, covered on the surface with Miocene and Pliocene basalt; the plateau’s surface is flat or undulating and covered with loess; in the north it is morainic and dissected by the canyons of the Columbia, Snake, and other rivers with depths to 900 m (on the Columbia River) and 1,500 m (on the Snake River). There are also dry canyons, formed in Anthropogenic times (the largest is the Grand Coulee). The rivers have many waterfalls and rapids, and hydroelectric power plants have been constructed. Most of the plateau is covered with turf and grass steppes, now tilled. Wheat is the main crop.