The Mojave Desert is defined by numerous mountain ranges creating its xeric conditions. These ranges often create valleys, endorheic basins, salt pans, and seasonal saline lakes when precipitation is high enough. These mountain ranges and valleys are part of the Basin and Range Province and the Great Basin, a geologic area of crustal thinning which pulls open valleys over millions of years. Most of the valleys are internally drained (endorheic basins), so all precipitation that falls within the valley does not eventually flow to the ocean. Some of the Mojave (toward the east, in and around the Colorado River/Virgin River Gorge) is within a different geographic domain called the Colorado Plateau. -Wikipedia