Defined by its hydrology, the Great Basin is a large area of the American West within which the rivers and streams drain from an irregular ring of topographic divides toward the interior of the basin, never to reach the sea. The west edge of the Great Basin runs along the crest of the southern Cascades, the Sierra Nevada Range, the Tehachapi Range and the San Bernardino Mountains while the east edge follows the crest of the Wasatch Plateau and the westernmost Uintas. Along its northern boundary, an irregular, low divide separates the watersheds of the basin rivers (dominated by the Bear and Humboldt Rivers) from that of the Snake River. The southern edge of the Great Basin is even more complex, separating basin river watersheds from that of the Colorado River, extending southward into northwest Mexico.
Since the Great Basin is surrounded by topographic divides, winds are downsloping from all directions, warming and drying the air; as a result, the Basin floor, with elevations ranging from 6500 feet to below sea level (in Death Valley), is a mosaic of deserts, covered by sage grasslands, salt flats and, in parts of the Mojave, joshua trees. The largest lakes within the Great Basin are the Great Salt Lake and Utah Lake, both in Utah, Pyramid Lake, in northwest Nevada, and Lake Tahoe and the Salton Sea in California.
Within the Great Basin, Earth's crust is being stretched in an east-west direction, caught between the uplifts of the Sierra Nevada, Rocky Mountains and Colorado Plateau. This has created a maze of linear faults, aligned north to south, along which fault-block mountains have risen to produce waves of ranges, separated by broad, flat valleys; while this aptly named Basin and Range Province includes all of the Great Basin, it also extends southeastward into the Sonoran Desert.
