322 American Semidesert and Desert Province

Southeastern California, southwestern Arizona, southern Nevada, 87,700 mi2 (227,100 km2)

Land-surface form.--The American Desert includes the Mojave, Colorado, and Sonoran Deserts. Its topography is characterized by extensive plains, most gently undulating, from which isolated low mountains and buttes rise abruptly. Elevations range from 280 ft (85 m) below sea level to 4,000 ft (1,200 m) in valleys and basins, with some mountain ranges reaching as high as 11,000 ft (3,400 m). The mountains are rocky and rise abruptly from their outwash aprons and alluvial faces. There are areas of interior drainage, such as the Salton Trough, but a large part of the province drains to the sea through underground seepage or through washes that are dry most of the year. The Colorado River, which crosses the eastern part of the province, is the only sizable stream.

Deeply eroded ranges alternate with smooth basin floors in the Mojave Desert of southern Nevada. Deeply eroded ranges alternate with smooth basin floors in the Mojave Desert of southern Nevada. Note playa in middleground. (Photo: John S. Shelton.)

Climate.--Summers are long and hot; the highest temperature ever measured in the United States was 134F (57C) in 1913 at Death Valley. The average annual temperature is 60 to 75F (15 to 24C). Though winters are moderate, the entire province is subject to occasional frosts. In winter the rains are widespread and usually gentle, but in summer they are usually thunderstorms. In the Colorado and Mojave Deserts of southeastern California, there are virtually no summer rains. No part of the province has regular rains, and a year or more may pass without measurable rainfall, especially in the region's western part. Average annual precipitation is 2 to 10 in (50 to 250 mm) in the valleys, but may reach 25 in (610 mm) on mountain slopes. The evaporation rate in summer is very high.

Vegetation.--Vegetation is usually very sparse, with bare ground between individual plants. Cacti and thorny shrubs are conspicuous, but many thornless shrubs and herbs are also present. On the Sonoran Desert plains, the most widely distributed plant is the creosote bush, which covers extensive areas in nearly pure stands. On some parts of the plains the arborescent cacti (cholla) are also common. Mesquite is less widespread and grows only along washes and watercourses.

Saguaro cactus and desert shrub Saguaro cactus and desert shrub near the Superstition Mountains of Arizona, American Desert Province (Photo:Soil Conservation Service.)

At the base of the mountains, on the gentle rocky slopes called bajadas, the vegetation is dominated by paloverde, ocotillo, and saguaro, but bitterbrush is also a common shrub. Vegetation below 3,000 ft (900 m) in the Mojave Desert is mostly creosote bush and various Atriplex (saltbush) species. The desert mountains are exceptionally barren, and many are almost devoid of vegetation.

Along the higher northern edge of the province is a belt where the Joshua tree is prominent. At a still higher level is a belt of junipers and pinyons.

Interior basins characterized by ephemeral shallow playa lakes are a conspicuous feature of the Mojave Desert. Soils near these playas contain alkali in quantities varying with distance from the lake, resulting in a zonation of several species of vegetation according to their tolerance for salts.

Soils.--Gravel or bare rock covers the ground near the bases of some mountains, and much bare rock is exposed on the mountains because the heavy, violent desert rainstorms allow little soil to accumulate on the steep slopes. Entisols occur on the older alluvial fans and terraces and in the better-drained basins. Aridisols dominate throughout the rest of the province.

Fauna.--Large ungulates are almost absent from the desert. Desert mule deer and peccary live chiefly in the paloverde-cactus shrub community. The Sonoran pronghorn antelope is classified as an endangered species; few are left in southern Arizona. Carnivores, including the desert kit fox and coyote, are small and usually nocturnal. The western spotted skunk is common. Nocturnal burrowers, particularly kangaroo rats and pocket mice, dominate. Merriam kangaroo rat is closely associated with creosote bush. Other important species are the longtail pocket mouse and antelope ground squirrel.

Many desert birds are very selective in their type of habitat. Greasewood may furnish a permanent residence for the loggerhead shrike. Areas where tall cacti are plentiful furnish homes for many birds, including the Gila woodpecker, elf owl, and purple marten. Gambel's quail, the cactus wren, and the roadrunner are common in the southern part of the region. The masked bobwhite quail is an endangered species that has been reintroduced.

Reptiles include numerous species of snakes and lizards, such as the Gila monster, the only poisonous lizard in the United States. The desert tortoise is becoming increasingly rare and is everywhere protected.

Endemic species, common in the Mojave Desert, include five species of desert pupfish living in highly saline lakes in Death Valley.