The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River. In one of various usages, the term "Columbia Basin" refers to more or less the same area as the Columbia Plateau.
During late Miocene and early Pliocene times, one of the largest flood basalts ever to appear on the earth's surface engulfed about 63,000 square miles (160,000 km2) of the Pacific Northwest, forming a large igneous province. Over a period of perhaps 10 to 15 million years, lava flow after lava flow poured out, ultimately accumulating to a thickness of more than 6,000 feet (1.8 km). As the molten rock came to the surface, the Earth's crust gradually sank into the space left by the rising lava.
The subsidence of the crust produced the large plateau—a large, slightly depressed lava plain sometimes also known as the Columbia Basin. The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. Entities found in these lake beds include fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.
Part of the Columbia Plateau is associated with the Columbia Plateau ecoregion, part of the 'Nearctic temperate and subtropical grasslands, savannas, and shrublands' ecoregion of the Temperate grasslands, savannas, and shrublands Biome.
Washington cities in the Columbia Plateau include:
Oregon cities in the Columbia Plateau include:
See also 
- U.S. Geological Survey Geographic Names Information System: Columbia Plateau
- Floyd, Ben, et al. "Glossary". (1998) Hanford Reach Protection and Management Program Interim Action Plan. Prosser, Washington: Benton County Planning Department.
- Description: Columbia Plateau Columbia River Basalt. United States Geologic Survey, accessed October 9, 2007.
- USGS Page on Columbia Plateau
- Geology of Lake Roosevelt National Recreation Area (source of much of this page)
- Guide to digital documents and photographs about the Columbia River area.
- Columbia River Basin Ethnic History Archive