The Southwest is drier than the adjoining Midwest in weather. The population is less dense and, with strong Spanish-American and Native-American components, more ethnically varied than neighboring areas. Outside the cities, the region is a land of open spaces, much of which is desert. The magnificent Grand Canyon is located in this region, as is Monument Valley, the starkly beautiful backdrop for many western movies. Monument Valley is within the Navajo Reservation, home of the most populous American Indian tribe. To the south and east, lie dozens of other Indian reservations, including those of the Hopi, Zuni, and Apache tribes.

In the 1860s, the wasteland extended from the Mississippi Valley almost to the Pacific Coast. But settlers learned that the prairies could grow corn and that the grasslands could feed cattle and sheep or yield wheat.  As they continued to cultivate the desert, its size decreased.   Dams on the Colorado and other rivers and aqueducts have brought water to the once small towns of Las Vegas, Nevada, Phoenix, Arizona, and Albuquerque, New Mexico, allowing them to become metropolises.

Source: http://usa.usembassy.de/travel-regions.htm