The population of the area is around 11 million people, with over half that in Arizona, the most populous cities are Phoenix, El Paso, Las Vegas, Albuquerque, and Tucson. Most of the area was part of the Viceroyalty of New Spain in the Spanish Empire before becoming part of Mexico and it became part of the United States through the Treaty of Guadalupe Hidalgo and the Gadsden Purchase. The deserts dominate the southern and western reaches of the area, the two major rivers of the region are the Colorado River, running in the northern and western areas, and the Rio Grande, running in the south. Formed approximately 8000 years ago, the Chihuahuan Desert is a dry desert. The Chihuahuan Desert spreads across the portion of the region, covering from southeastern Arizona, across southern New Mexico. While it is the second largest desert in the United States, only a third of the desert is within the United States, El Paso is the major city in this desert, with other smaller cities being Las Cruces and Roswell in New Mexico. The Chihuahuan is a rain shadow desert, formed two mountain ranges which block oceanic precipitation from reaching the area. The most prolific plants in this region are agave, yucca and creosote bushes, when people think of the desert southwest, the landscape of the Sonoran Desert is what mostly comes to mind. Rainfall averages between 4–12 inches per year, and the deserts most widely known inhabitant is the saguaro cactus and it is bounded on the northwest by the Mojave Desert, to the north by the Colorado Plateau and to the east by the Arizona Mountains forests and the Chihuahuan Desert. The portion of the Sonora Desert which lies in the Southwestern United States is the most populated area within the region. Six of the top ten major population centers of the region are found within its borders, Phoenix, Tucson, Mesa, Chandler, Glendale, also within its borders are Yuma and Prescott Arizona. The most northwest portion of the American Southwest is covered by the Mojave Desert, bordered on the south by the Sonoran Desert and the east by the Colorado Plateau, its range within the region makes up the southeast tip of Nevada, and the northwestern corner of Arizona. In terms of topography, the Mojave is very similar to the Great Basin Desert, the Mojave is the smallest, driest and hottest desert within the United States. The Mojave gets less than six inches of rain annually, the most prolific vegetation is the tall Joshua tree, which grow as tall as 40 feet, and are thought to live almost 1000 years. Other major vegetation includes the Parry saltbush and the Mojave sage, the Colorado Plateau varies from the large stands of forests in the west, including the largest stand of ponderosa pine trees in the world, to the Mesas to the east. Although not called a desert, the Colorado Plateau is mostly made up of high desert, the Plateau is characterized by a series of plateaus and mesas, interspersed with canyons. The most dramatic example is the Grand Canyon, but that is one of many dramatic vistas included within the Plateau, which includes spectacular lava formations, painted deserts, sand dunes, and badlands. One of the most distinctive features of the Plateau is its longevity, the Plateau can be divided into six sections, three of which fall into the Southwest region
Panoramic view of the southwestern United States.
The Chihuahuan desert terrain mainly consists of basins broken by numerous small mountain ranges.