The Sonoran Desert lies in the arid region west of the Rocky Mountains. It covers about 120,000 square miles of land in southern California, Arizona, and Mexico. The Mojave, Yuma, Pima, Chemehuevi, and Papago Indian reservations are located there. Tucson and Phoenix are two of the desert's largest cities. Much of the area's rain doesn't come until the middle of summer, when it falls in sudden downpours.