Columbia Plateau, geographic region, northwestern United States. It forms part of the intermontane plateaus and is bordered east by the Northern Rocky Mountains and west by the Sierra Nevada–Cascade region. The plateau covers an area of about 100,000 square miles (260,000 square km) in Washington, Oregon, and Idaho and is noted for its diverse landforms (see ). It is uniformly covered with basaltic lava flows, but significant warping and faulting have caused elevations to range from 200 to 5,000 feet (60 to 1,500 meters) above sea level. The climate is semiarid, and vegetation is limited mostly to shrubs and grasses. The Columbia and Snake rivers drain the region.