arid physiographic province occupying much of the western and southwestern part of the United States. The region comprises almost all of Nevada, the western half of Utah, southeastern California, and the southern part of Arizona and extends into northwestern Mexico. The province has a varied and remarkable topography consisting largely of numerous small, roughly parallel mountain ranges (trending north-south) separated by nearly flat desert plains, or basins. The basins are generally 4,000–5,000 feet (1,200–1,500 m) above sea level, and the mountain ranges rise 3,000–5,000 feet above the level of the basins.

The northern half of the province is called the Great Basin (q.v.). The Sonoran Desert (q.v.) section extends into Sonora, Mexico, while the Salton Trough extends to the Gulf of California; the Salton Trough is a concave desert basin that descends to 235 feet (72 m) below sea level at the Salton Sea. East of the Sonoran Desert and extending southward from the Colorado Plateau is the Mexican Highland section, which has many of the characteristics of the Great Basin and which covers parts of New Mexico, Arizona, and northern Mexico. To the east of this, the narrow, varied Sacramento section, in New Mexico and western Texas, is a plateau with blocks of eastward-dipping mountains.

The Basin and Range is a semidesert area with an extremely complex geologic history. Its 100 or so mountain ranges are the remains of crustal rocks that were upraised or uplifted by faulting along north-south lines, after which the resulting blocks were tumbled, eroded, and partly buried by alluvial debris that had accumulated in the desert basins at their feet. Death Valley in California is the lowest of these sediment-filled basins. Most of the basins have no outlets for their drainage, and thus rainwater accumulates in the form of salt lakes (e.g., Walker and Pyramid lakes in Nevada and the Great Salt Lake of Utah) or in playas, which are mud flats occasionally covered by a few inches of rapidly evaporating water. In the southern part of the Basin and Range the intermontane basins are connected by through drainage that contributes to the flow of the lower Colorado River.