Arizona holds parts of three of North America’s hot deserts and a cold high desert besides. The Sonoran Desert covers the southwest and south, including Phoenix, Tucson and Yuma; a slice of the Mojave Desert reaches into Mohave County in the northwest; the Chihuahuan Desert edges into Cochise County in the southeast; and the colder, higher Colorado Plateau in the north contains the Painted Desert, whose sagebrush flats some ecologists group with the Great Basin Desert. Smaller named deserts, such as the Yuma, Lechuguilla and Tonopah, sit inside the Sonoran. Across a state of 295,253 km² (113,998 sq mi), the land climbs from about 21 m (70 ft) on the Colorado River at the Mexican border to 3,852 m (12,637 ft) on Humphreys Peak, and that climb, more than latitude, decides which desert you are standing in.
Arizona divides into the Basin and Range Province of the south and west, the Colorado Plateau of the north, and a mountainous Transition Zone between them. The deserts follow those provinces, so this guide takes them one region at a time, from the lowest corner.
The Lower Colorado Valley: Yuma, Lechuguilla and Cabeza Prieta
In his 1957 study Vegetation of the Sonoran Desert, the botanist Forrest Shreve divided the Sonoran into seven subregions by their plants. The lowest and driest, the Lower Colorado Valley, fills Arizona’s southwestern corner, where it is usually called the Yuma Desert. It stretches from the Colorado River east across sandy plains and dune fields, takes in the lower Gila River, and continues south into Sonora, where it merges with the Gran Desierto de Altar and the Pinacate volcanic field. Average rainfall is under 200 mm (8 in) a year.
The city of Yuma is the measuring stick. It averages only 85 mm (3.36 in) of rain, and among populated places in the contiguous United States it is the driest, the sunniest and the least humid. On average 117 days a year reach 38 °C (100 °F) or more, and snow has been recorded settling on the city only once, on 12 December 1932.
East of Yuma the Basin and Range pattern takes over: bare, jagged ranges separated by wide gravel plains. The Lechuguilla Desert lies north to south between the Gila Mountains and the Cabeza Prieta Mountains, almost entirely inside the Barry M. Goldwater Air Force Range. Its name is a misnomer, since the lechuguilla agave it is named after grows only in the Chihuahuan Desert hundreds of kilometres to the east. Beyond it, Cabeza Prieta National Wildlife Refuge, set up in 1939 to protect desert bighorn sheep, covers 3,480 km² along 90 km (56 mi) of the Mexican border, an area larger than Rhode Island.
Next door is Organ Pipe Cactus National Monument, proclaimed in 1937 and 1,340 km² (517 sq mi) in size. It is one of the few places in the United States where the organ pipe and senita cacti grow wild, and UNESCO named it a biosphere reserve in 1976.
The Arizona Upland: Saguaro Country Around Phoenix and Tucson
Moving north and east, the ground rises and the rain improves. Shreve called this belt the Arizona Upland, and it is the Sonoran Desert most people picture: slopes packed with saguaro, palo verde, cholla and prickly pear. What sets the Sonoran apart from its neighbours is two rainy seasons, Pacific storms in winter and the North American monsoon in summer, which lets it support trees and giant cacti. Across Arizona, California and three Mexican states, the Sonoran covers about 260,000 km² (100,000 sq mi).
The saguaro is its signature. It grows wild only in the Sonoran Desert, mostly below about 1,400 m (4,500 ft), because long frosts kill it; its northern limit is the Hualapai Mountains near Kingman. Saguaros commonly live 150 to 200 years, may not grow a first arm until 75 to 100 years old, and a fully hydrated plant can weigh 1,500 to 2,200 kg. The largest known living one, listed on the National Register of Champion Trees, stands 13.8 m tall in Maricopa County, while an armless giant near Cave Creek reached 23.8 m (78 ft) before a 1986 windstorm felled it. Saguaro National Park, begun as a national monument in the Rincon Mountains in 1933 and made a park in 1994, protects about 37,000 hectares in two districts either side of Tucson.
Tucson, on the upland’s eastern edge, averages 269 mm (10.6 in) of rain a year. Phoenix, lower and farther west in the Salt River Valley, gets 183 mm (7.2 in). West of Phoenix, where the Hassayampa River meets the Gila, the small Tonopah Desert runs for about 48 km (30 mi) north of Interstate 10, framed by the Belmont and Big Horn mountains. The Gila itself, 1,044 km (649 mi) long, runs only intermittently past Phoenix because its water is diverted for irrigation, and the Central Arizona Project canal now brings Colorado River water 541 km (336 mi) inland from Lake Havasu to both cities.
The Mojave Fringe of Mohave County
Follow the Colorado River north past Lake Havasu and the desert changes character. Northwestern Arizona holds a strip of the Mojave Desert, which is centred on southeastern California and southwestern Nevada and covers about 140,000 km² (54,000 sq mi) in all. It is the smallest and driest of the four North American deserts, higher than the Sonoran, with hot, dry summers and cold winters instead of a reliable monsoon. Most of it lies between 610 and 1,220 m (2,000 and 4,000 ft), which is why it is often called the “high desert”.
The tell-tale plant is the Joshua tree, a tree-like yucca that grows mostly within the Mojave at 400 to 1,800 m. In Arizona, large stands grow northeast of Kingman and along U.S. Route 93 south of Meadview, a stretch designated the Joshua Tree Parkway of Arizona. The reservoirs of the lower Colorado, Lake Mead, Lake Mohave and Lake Havasu, all sit within or at the edge of the Mojave, giving this dry corner of the state its surprising expanses of open water.
Cochise County and the Chihuahuan Desert’s Western Tip
At the opposite corner, the southeast, Arizona touches the Chihuahuan Desert. With about 501,896 km² (193,783 sq mi), mostly in Mexico and west Texas, it is the largest hot desert in North America, yet in its current form it is young, only about 8,000 years old. In Arizona it arrives as high grassland and shrub desert in the valleys of Cochise County, where winters are colder and most rain falls in summer. Towns such as Willcox, Benson and Safford lie in an ecotone where Chihuahuan and Sonoran plants mix.
The valley’s landmark is the Willcox Playa, a dry lake bed about 13 by 16 km (8 by 10 mi) in the northern Sulphur Springs Valley. It is the remnant of Lake Cochise, a Pleistocene lake, and it drains water from the Dos Cabezas and Chiricahua mountains to the east and the Dragoon Mountains to the southwest. The playa was named a National Natural Landmark in 1966 for its buried fossil pollen, the thousands of sandhill cranes that roost there in winter, and the greatest diversity of tiger beetles in the United States. Some classifications put the playa itself in the Sonoran ecoregion, so blurred is the boundary here.
West of the playa, the San Pedro River flows north out of Sonora for 230 km (140 mi) through Arizona to join the Gila at Winkelman. It is described as the last major undammed desert river in the American Southwest, and its cottonwood corridor supports about 100 species of breeding birds and close to 300 migrating species.
The Painted Desert on the Colorado Plateau
North of the Mogollon Rim, Arizona is plateau country between 1,524 and 2,438 m (5,000 and 8,000 ft), and the desert here is cold in winter. The northeastern plateau sits in a rain shadow and receives only about 114 to 254 mm (4.5 to 10 in) a year; Winslow, on Interstate 40, averages 178 mm of precipitation a year, including about 21 cm of snow.
The best-known piece of this high desert is the Painted Desert, a belt of badlands running from near the eastern end of Grand Canyon National Park southeast into Petrified Forest National Park. Members of Francisco Vázquez de Coronado’s expedition gave it its name, El Desierto Pintado, in 1540. The colours come from the siltstones, mudstones and shales of the Triassic Chinle Formation, whose iron and manganese compounds weather into reds, greys and lavenders; soft layers erode quickly into rounded, gullied hills, while thin limestone and lava beds cap the mesas.
Petrified Forest National Park, a national monument from 1906 and a national park from 1962, covers about 900 km² at an average elevation near 1,600 m (5,400 ft). Its fallen logs are trees from the Late Triassic, about 225 million years ago, turned to quartz by silica from volcanic ash in the Chinle. Farther north, on Navajo Nation land, the same plateau desert carries the sandstone buttes of Monument Valley, the largest rising about 300 m (1,000 ft) above the valley floor.
Mogollon Rim and the Sky Islands: Where Arizona’s Deserts Stop
The Mogollon Rim, an escarpment of Kaibab Limestone and Coconino Sandstone cliffs about 320 km (200 mi) long, marks the southern edge of the Colorado Plateau. It is named after Juan Ignacio Flores Mogollón, Spanish governor of New Mexico from 1712 to 1715. Its southern slopes catch 380 to 510 mm (15 to 20 in) of rain a year, and ponderosa pine replaces desert shrub. Above it, the San Francisco Peaks, the remnant of a single large stratovolcano just north of Flagstaff, rise to Humphreys Peak, 17.7 km from the city; Arizona’s highest elevations receive up to 760 mm a year.
In the south the deserts are broken up instead by sky islands: isolated ranges such as the Rincon, Chiricahua and Santa Catalina mountains that climb out of the desert floor high enough to carry oak and pine forest. The record temperatures show the range the state spans: 53 °C (128 °F) at Lake Havasu City on the Mojave–Sonoran edge on 29 June 1994 and again on 5 July 2007, and −40 °C at Hawley Lake in the White Mountains on 7 January 1971.
Read together, the four desert regions explain most of Arizona’s map: saguaro country in the low south and west, Joshua trees in the northwest, grassland playas in the southeast, and painted badlands on the northern plateau. To compare the two hot deserts that meet around Lake Havasu, see our guide to the Mojave Desert and the geography of the Sonoran Desert; the Chihuahuan Desert guide follows the southeast’s grasslands into Mexico, and the canyons, peaks and plateaus between all of them are covered in landforms in Arizona.