An Overview of the Sonoran Desert

by William G. McGinnies

An Essay Developed from a Paper given at the opening session of the
Second Annual Conference of the Consortium of Arid Lands Institutions (CALI)
February 4, 1976, in Tucson, Arizona

The Sonoran Desert is a part of the Great American Desert of western North America, extending from the northern part of the United States deep into Mexico, a nearly continuous continental arid region separated from the Pacific Ocean and the Gulf of Mexico except in the states of Sonora and Baja California in northwestern Mexico. Individual deserts recognized in this area are the Great Basin, Mojave, Sonoran, and Chihuahuan. The term "desert" applied to these areas includes a range of climatic conditions from extremely arid, with a rainfall of less than 75 millimeters, to the boundary between arid and semi-arid, in the vicinity of 375 millimeters.

The Great Basin Desert, sometimes referred to as the Northern Desert Shrub Desert, is classed internationally as a cold desert. Big Sagebush (Artemisia tridentata) and salt bushes (Atriplex spp.) are characteristic plants. The elevation is mostly above 1200 meters with a precipitation of 100 to 300 mm, 60 percent of which falls in winter.

The Mojave Desert is a transitional desert between the warm Sonoran Desert and the cool Great Basin Desert, with the Joshua tree (Yucca brevifolia) the characteristic species. It extends from nearly sea level to 1200 m, with a precipitation of 50 to 125 mm occurring only in winter.

The Chihuahuan Desert, a continental desert separated from the Sonoran Desert by the highlands of the continental divide, lies mostly above 1100 m, with a precipitation of 70 to 500 mm, 70 to 80 percent falling in summer. While the vegetation is similar to that of the Sonoran Desert, many species are not common to both, for example tarbush (Flourensia incana) and white thorn acacia (Acacia vernicosa), which are strictly Chihuahuan Desert species.

The Sonoran Desert, surrounding the Gulf of California in the southwestern United States and northwestern Mexico, covers some 320,000 square kilometers. It includes a part of the state of Arizona and a small portion of California in the United States, western Sonora, and the southern two-thirds of the peninsula of Baja California in Mexico.

The northern extension of the Sonoran Desert is largely determined by cold temperatures. The eastern boundary is delimited biologically, physically, and geographically by high mountain ranges on the south. The Sonoran Desert merges into the semiarid Thorn Forest of southwestern Mexico. On the west it is bounded by mountains in the north and the Pacific Ocean in the south.

The physiography of most of the Sonoran Desert places it within the Basin and Range Province, consisting of inclined plains on which are superexposed many hills and low mountains, mostly less than 900 min elevation. The Pinal Mountains in Southern Arizona and the Sierra Babiso in Sonora are its only ranges rising high enough to support a non-desert vegetation.

Figure 1. Extreme arid desert, northwest Baja California (44 KB)

The Gulf of California is the recipient of all drainage of the Sonoran Desert except a small portion of Baja California, which drains into the Pacific Ocean. The largest rivers rise in the mountains outside the desert. The only river with a perennial flow is the Colorado River, although the Gila River, before the construction of several dams, was a nearly constant stream. The eastern edge of the Sonoran Desert is hedged by high mountains, which are the source of streams and flood discharge throughout much of the desert area. The larger rivers including the Rio Magdalena and the Rio Sonora rarely discharge flood waters to the Gulf, but the Rio Yaqui has a history of annual discharges. There are also a number of undrained basins of which the Salton Sea is the largest.

The climate of the Sonoran Desert is relatively uniform with some differences due to latitude and elevation. The difference of 11 degrees of latitude between the northern and southern ends and the range in elevation from the coasts to the mountains brings about temperature gradients, especially for cold weather and frosts.

Parts of the Sonoran Desert share with Death Valley the highest and most sustained high temperatures in North America. Temperatures of 30° C may occur from February to December and periods of three months or more with maximum temperatures of 38° C are not uncommon. The frostless season varies from eight to twelve months in different parts of the desert, but the duration of the growing season is dependent upon the time and extent of the rainy seasons.

The amount and seasonal distribution of precipitation, the most critical physical condition limiting the boundaries of the Sonoran Desert, also influences plant and animal distribution within the desert. An important feature of rainfall is the change in seasonal distribution from west to east. In Yuma the winter rains are dominant whereas at Tucson, winter and summer rains are about equal. A short distance farther east, winter rains make up less than a quarter of the total. Total precipitation in general increases from west to east with a range of SO to 375 mm.

The soils of the Sonoran Desert are typical warm desert soils, showing modification of the parent materials associated with aridity. Distinguishing features are the low humus content and high content of readily soluble salts. The latter may lead to lime accumulations in the subsoil or the development of alkali conditions where drainage is impaired. The characteristic bajada slopes exhibit a mixture of soil materials ranging from rocky near the top to fine materials at their lower extremities, these often giving way to fine-textured alluvial bottoms.

Forrest Shreve (1951) [1] who devoted 15 years to the study of the Sonoran Desert, provided a comprehensive picture of the vegetation and its ecological relationships wherein he recognized seven subdivisions, each of which showed sufficient differences from the others to deserve separate treatment: Lower Colorado Valley, Arizona Upland, Plains of Sonora, Foothills of Sonora, Central Gulf Coast, Vizcaino Region, and Magdalena Region.

[Note: Some of these regions, such as the Foothills of Sonora or the Vizcaíno and Magdalena Region are no longer included in the discussion of the Sonoran Desert by organizations such as the Arizona Sonoran Desert Museum and The Nature Conservancy. ]

1. Lower Colorado Valley

This is the largest subdivision of the Sonoran Desert, occupying the lower drainages of the Colorado and Gila Rivers, the Salton Basin and the eastern coast of Baja California as far south as Bahia Los Angeles, and all of Sonora below 400 min elevation, as far south as the valley of the Rio Magdalena.

Figure 2. Vegetation simplicity in Lower Colorado Valley (40 KB)

About 85 percent of the area outside the delta of the Colorado River is comprised of bajadas or nearly level plains. This subdivision is one of the most arid parts of the Sonoran Desert with a range in precipitation from 75 to 200 mm.

The vegetation of the Lower Colorado Valley, distinguished by its simplicity, is dominated by creosote bush (Larrea tridentata) and white bursage (Ambrosia dumosa), often making up 90 to 95 percent of the vegetation on the upland; and by mesquite (Prosopis juliflora), ironwood (Olneya tesota), blue paloverde (Cercidium floridum), and smoke tree (Dalea spinosa) along drainageways. Big galleta (Hilaria rigida) occurs on sandy soils, which often support a galaxy of annual plants during the winter rainy season.

2. Arizona Upland

This subdivision comprises the northeastern portion of the Sonoran Desert, mainly in Arizona but partly in Sonora. It ranges in elevation from 150 to 900 m, and in rainfall from 75 to 300 m. The Arizona portion drains largely into the Gila River and the southern portion into several drainages discharging into the Gulf of California. The landscape is studded with small mountains and hills, and the subdivision is flanked by a broken series of mountains rising above desert conditions and supporting less arid types of vegetation.

Figure 3. Slope vegetation in Arizona Upland (50 KB)

The vegetation of the Arizona Upland exceeds that of the Lower Colorado subdivision in number, stature, and density of species. The most abundant species at the lower elevations include creosote bush (Larrea tridentata), triangle leaf bursage (Ambrosia deltoidea), foothill paloverde (Cercidium microphyllum), prickly pear and cholla (Opuntia spp.), various shrubs, and mesquite (Prosopis juliflora). On the upper slopes, foothill paloverde (Cercidium microphyllum), ocotillo (Fouguieria splendens), saguaro (Carnegiea gigantea), brittle bush (Encelia farinosa), white thorn (Acacia constricta), barrel cactus (Ferocactus wislizeni), and several additional species of Opuntia are added. Blue paloverde (Cercidium floridum) is a common tree in drainage ways throughout, and mesquite (Prosopis juliflora) forms thickets in the broader floodplains.

3. Plains of Sonora

This subdivision, lying between the coastal strip and the foo hills in central Sonora, has some physical features in common with the Lower Colorado Valley. The surface is relatively smooth, ranging in elevation from 100 to 750 m. Mountains and hills are low and widely scattered. Precipitation varies from 250 to 375 mm. Frost is less frequent than in the areas to the north and summer temperatures are more moderate than those of the Lower Colorado Valley.

The open type forest vegetation is dominated by trees and shrubs including: foothill paloverde (Cercidium microphyllum), ironwood (Olneya tesota), and mesquite (Prosopis juliflora). Plants of local occurrence include elephant tree (Bursera microphylla), organ pipe cactus (Lemaireocereus thurberi), and tree morning glory (lpomoea arborescens).

4. Foothills of Sonora

This easternmost subdivision of the Sonoran Desert extends from the vicinity of Arizpe to the delta of the Rio Yaqui, falling gradually from 1,000 m to sea level. On the west it is bounded by the Plains of Sonora, and on the south it gradually merges into thorn forest. On the east it extends to oak grassland of the lower mountain slopes.

Rainfall is higher than in adjacent subdivisions with a maximum of 500 mm, with summer precipitation greater than winter. The area is subject to lengthy droughts, accentuated by high temperatures.

The vegetation is characterized by abundant small trees, occasional palms and dense shrubbery, with grass becoming common at the higher elevations. Cacti are uncommon. Many of the plant species of the adjacent subdivisions extend into the Foothills of Sonora.

Notable additions include MacDougal ocotillo (Fouguieria macdougali), lignum vitae (Guaiacum coulteri), pochote (Ceiba acuminata), Mexican palm (Erythea roezlis), and many shrubs.

5. Central Gulf Coast

This subdivision includes the Sonoran Coast from the mouth of the Rio Magdalena nearly to the mouth of the Rio Yaqui, and in Baja California from Isla Angel de la Guarda to San Jose del Cabo. Throughout the two parts of this area the vegetation is almost identical in physiognomy and a large number of the component species are the same. In both cases the vegetation is distinct from that of the interiors.

The rainfall of the Central Gulf Coast is low and uncertain, and the temperatures are very high except close to the shore. Precipitation may occur in the latter half of winter or in mid-summer, but may be lacking for several seasons in succession. The hills are often bare of soil and the streamways are bordered by bands of sand and boulders. Vegetation includes trees with trunks of exaggerated diameter including elephant tree (Bursera microphylla) and boojum (ldria columnaris), but these are outnumbered by ironwood (Olneya tesota), blue paloverde (Cercidium floridum), mesquite (Prosopis juliflora), and ocotillo (Fouguieria spp.).

Figure 4. Vegetation of Vizcaino Region (34 KB)

6. Vizcaino Region

'The Vizcaino Region extends from the vicinity of Rosario to Punta Pequena. It has a hilly or rolling surface with a few small mountain areas and many rough fields of volcanic rock. Soils include a very fine red volcanic clay and extensive areas of sand. Winter rainfall is light and extremely uncertain. Summer storms occur along the mountains on the eastern edge, and although humidity is high, the constant strong winds are responsible for the open and stunted vegetation of the coastal strip.

The distinctive feature of the vegetation is the occurrence of several unusual appearing plants, including the boojum tree (ldria columnaris), datililla (Yucca valida), elephant tree or copal (Pachycormus discolor), the cardon (Pachycereus pringlei), and Shaw agave (Agave shawii).

7. Magdalena Region

This area includes the desert of the southern third of Baja California and lies entirely in the Pacific drainage. Rainfall is scanty and irregular, for the most part less than the 200 mm recorded at La Paz. The coast is bordered by lagoons and sand dunes, and the eastern boundary of the plain abuts on several mountain chains. The interior of the plain has a fine or moderately coarse alluvial soil with innumerable playas. There is a marked difference in soil conditions of the malpais mesas in the north and the Magdalena Plain in the south with an accompanying difference in vegetation. Many of the representative plant species of the Vizcaino Region are uncommon or lacking. The two most characteristic species are palo blanco (Lysiloma candida), and pitahaya agria (Machaerocereus gummaous).

The animal life of the Sonoran Desert is surprisingly abundant considering the heat, dryness, and limited vegetation. Larger herbivores include the desert bighorn sheep, javelinas, deer and antelope; larger predators include the coyote, mountain lion, and bobcat.

Smaller mammals include ringtailed cat, skunks, jackrabbits, cottontail rabbits, kangaroo rats, woodrats (packrats), ground squirrels and several species of mice.

Birdlife includes many residents notably the cactus wren, roadrunner, cardinal, phainopepla, pyrruloxia, mockingbird, gambel quail, turkey buzzard, and several species of owls and doves.

Cold-blooded animals include a variety of snakes and lizards, probably the best known being the gila monster and sidewinder rattlesnake.

The wide variety of arthropods includes tarantulas, black widow spiders, scorpions, centipedes, and a wealth of insects such as the praying mantis, many beetles, termites, butterflies, and moths.

In common with plants and animals of all desert regions those of the Sonoran Desert exhibit interesting adaptations for living under dry and hot desert conditions as shown in table 1.

The greatest and most widespread human impacts within the Sonoran Desert have resulted from grazing use of the fragile desert ecosystem (Table 2). Plant cover has been depleted and the flora changed by elimination of palatable plants and the increase of unpalatable plants. Burning has had some impact on desert vegetation although fires appear to be less common in desert areas than the grasslands adjacent to the desert because of the scarcity of fuel.

Modern man has exerted an even greater impact than in the past Armed with modern techniques and equipment, he has made many changes. While these have been largely aimed toward making the desert ecosystem a more habitable place for man, the results of his activities have in many instances resulted in the deterioration of the desert environment.

In addition to the impacts of grazing on non-urban and uncultivated lands, there has been a large and widespread use of the desert for homesites and mobile homes and recreation. Starting with war training maneuvers during World War II, mechanical vehicles of many kinds have scarred the terrain. The impact of this offroad travel is difficult to measure, but the fact that the track marks left by vehicles are visible for many years would indicate lasting changes in the microenvironment.

Irrigation agriculture has developed into a major enterprise in both the United States and Mexico (Dunbier, 1968) [2], with about two-thirds of a million hectares in the United States and a million hectares in Mexico. Most of the water for irrigation is imported, although underground sources, largely nonrenewable or slowly renewable, have been exploited recently. In the United States portion of the Sonoran Desert there has been strong competition between agricultural and urban uses in recent years with widespread conversions to town and city uses. In the Mexican portion of the Sonoran Desert the land use competition is very slight, but with a growing recreational program it may increase.

In summary, the Sonoran Desert Environment is capable of and no doubt will be used to support a growing population. The full impact of the many uses by man on the native vegetation has not been measured in terms of overall effects, but there can be no question that the desert environment has deteriorated, and unless some control is brought about, the desert ecosystem will suffer severe and permanent damage.

1. Shreve, Forrest. 1951. Vegetation of the Sonoran Desert. Carnegie Institution of Washington, Publication 591. 192 p. [return to text]
2. Dunbier, Roger. 1968. The Sonoran Desert, Its Geography, Economy, and People. Tucson, Ariz.: University of Arizona Press. 426 p. [return to text]