Panoramic view of the southwestern United States
The geography of the region is mainly made up by four features: the Mojave, Sonoran, and Chihuahuan Deserts, and the Colorado Plateau; although there are other geographical features as well, such as a portion of the Great Basin Desert. The deserts dominate the southern and western reaches of the area, while the plateau (which is largely made up of high desert) is the main feature north of the Mogollon Rim. 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 east, north to south.
Formed approximately 8000 years ago, the Chihuahuan Desert is a relatively dry desert, although it is slightly wetter than the Sonoran Desert to the west. The Chihuahuan Desert spreads across the southeastern portion of the region, covering from southeastern Arizona, across southern New Mexico, and the portion of Texas included in the Southwest. While it is the second largest desert in the United States, only a third of the desert is within the United States, with the rest in Mexico. El Paso is the major city in this desert, with other smaller cities being Las Cruces and Roswell in New Mexico.
The elevation in the Chihuahuan varies from 1970 to 5500 feet, as there are several smaller mountain ranges contained in the area, namely the San Andres and Doña Anas in New Mexico, and the Franklin Mountains, which span Texas and New Mexico in the Southwest region. The Chihuahuan is a "rain shadow" desert, formed between two mountain ranges (the Sierra Madre Occidental on the west and the Sierra Madre Oriental on the east) which block oceanic precipitation from reaching the area. The Chihuahuan Desert is considered the "most biologically diverse desert in the Western Hemisphere and one of the most diverse in the world", and includes more species of cacti than any other desert in the world. The most prolific plants in this region are agave, yucca and creosote bushes, in addition to the ubiquitous presence of various cacti species.
Saguaro cactus in the Sonoran Desert.
When people think of the desert southwest, the landscape of the Sonoran Desert is what mostly comes to mind. The Sonoran Desert makes up the southwestern portion of the Southwest; most of the desert lies in Mexico, but its United States component lies on the southeastern border of California, and the western 2/3 of southern Arizona. Rainfall averages between 4–12 inches per year, and the desert's most widely known inhabitant is the saguaro cactus, which is unique to the desert. 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. Aside from the trademark saguaro, the desert has the most diverse plant life of any desert in the world, and includes many other species of cacti, including the organ-pipe, senita, prickly pear, barrel, fishhook, hedgehog, cholla, silver dollar, and jojoba. 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, and Scottsdale, all in Arizona. 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, which lies just to its north. Within the region, Las Vegas is the most populous city; other significant areas of human habitation include Laughlin and Pahrump in Nevada, and Lake Havasu City, Kingman, and Bullhead City in Arizona. The Mojave is the smallest, driest and hottest desert within the United States. The Mojave gets less than six inches of rain annually, and its elevation ranges from 3000 to 6000 feet above sea level. 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, both only found in the Mojave, as well as the creosote bush.
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. Within the Southwest U.S. region, the Colorado is bordered to the south by the Mogollon Rim and the Sonoran Desert, to the west by the Mojave Desert, and to the east by the Rocky Mountains and the Llano Estacado. 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, having come into existence at least 500 million years ago. The Plateau can be divided into six sections, three of which fall into the Southwest region. Beginning with the Navajo section forming the northern boundary of the Southwestern United States, which has shallower canyons than those in the Canyonlands section just to its north; the Navajo section is bordered to the south by the Grand Canyon section, which of course is dominated by the Grand Canyon; and the southeastern-most portion of the Plateau is the Datil section, consisting of valleys, mesas, and volcanic formations. Albuquerque is the most populous city within the portion contained in the Southwest region, but Sante Fe, New Mexico and Flagstaff, Arizona, are also significant population centers.
Geographer D. W. Meinig defines the Southwest in a very similar fashion to Reed: the portion of New Mexico west of the Llano Estacado and the portion of Arizona east of the Mojave-Sonoran Desert and south of the "canyon lands" and also including the El Paso district of western Texas and the southernmost part of Colorado. Meinig breaks the Southwest down into four distinct subregions. He calls the first subregion "Northern New Mexico", and describes it as focused on Albuquerque and Santa Fe. It extends from the San Luis Valley of southern Colorado to south of Socorro and including the Manzano Mountains, with an east-west breadth in the north stretching from the upper Canadian River to the upper San Juan River. The area around Albuquerque is sometimes called Central New Mexico.
"Central Arizona" is a vast metropolitan area spread across one contiguous sprawling oasis, essentially equivalent to the Phoenix metropolitan area. The city of Phoenix is the largest urban center, and located in the approximate center of the area that includes Tempe, Mesa, and many others.
Meinig calls the third subregion "El Paso, Tucson, and the Southern Borderlands". While El Paso and Tucson are distinctly different cities, they serve as anchor points to the hinterland between them. Tucson occupies a large oasis at the western end of the El Paso-Tucson corridor. The region between the two cities is a major transportation trunk with settlements servicing both highway and railway needs. There are also large mining operations, ranches, and agricultural oases. Both El Paso and Tucson have large military installations nearby; Fort Bliss and White Sands Missile Range north of El Paso in New Mexico, and, near Tucson, the Davis-Monthan Air Force Base. About 70 miles (110 km) to the southeast are the research facilities at Fort Huachuca. These military installations form a kind of hinterland around the El Paso-Tucson region, and are served by scientific and residential communities such as Sierra Vista, Las Cruces, and Alamogordo. El Paso's influence extends north into the Mesilla Valley, and southeast along the Rio Grande into the Trans-Pecos region of Texas.
The fourth subregion Meinig calls the "Northern Corridor and Navajolands". A major highway and railway trunk which connects Albuquerque and Flagstaff. Just north of the transportation trunk are large blocks of American Indian land.
Phoenix, Tucson, and Las Vegas dominate the westernmost metropolitan areas in the Southwest, while Albuquerque, and El Paso dominate the easternmost metropolitan areas.