Chapter 40
Ecological Subregions of the United States
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American Semi-Desert and Desert

Three Sections have been delineated in this Province:

These Sections are located in California and Arizona. The area of these Sections is about 87,700 mi2 (227,100 km2).

Section 322A--Mohave Desert

Geomorphology. This area comprises widely separated short ranges in desert plains. It contains isolated mountains, plateaus, alluvial fans, playas, basins, and dunes. Elevation ranges from 300 ft below sea level to 11,000 ft above sea level (-91 to 3,344 m). This Section is in the Basin and Range geomorphic province.


Lithology and Stratigraphy. There are Cenozoic nonmarine sedimentary and granitic rocks and alluvial deposits, and Precambrian rocks of all types.

Soil Taxa. Soils include Aridisols and Entisols in combination with thermic and hyperthermic soil temperature regimes and aridic soil moisture regime on foothills and valleys. This Section contains areas with salt-affected soils. Aridisols and Entisols in combination with mesic and frigid soil temperature regimes, and aridic and xeric soil moisture regimes occur on mountains.

Potential Natural Vegetation. K\"uchler mapped vegetation as Mojave creosote bush, juniper-pinyon woodland, desert saltbush, and Joshua tree scrub. Predominant potential natural communities include creosote bush, blackbush, greasewood and saltbush series on basins, plains, and hills; Joshua Tree series on plains and hills; and basin sagebrush, western juniper and pinyon pine series on mountains.

Fauna. Mammals include desert bighorn sheep, desert kit fox, coyote, spotted skunk, spotted bat, black-tailed jackrabbit, ground squirrels, kangaroo rat, and white-footed mouse. Birds includes eagles, hawks, owls, quail, roadrunners, finches, warblers, and orioles. Reptiles include desert tortise, several species of rattlesnakes, and chuckawalla lizard.

Climate. Precipitation ranges from 3 to 10 in (80 to 250 mm). It mostly occurs as scattered high intensity storms of short duration. Temperature ranges from 50 to 75 oF (10 to 24 oC). The growing season lasts 200 to 300 days.

Surface Water Characteristics. There are mostly bedrock controlled channels in mountains that carry seasonal flows to alluvial channels below. Most channels terminate in basins within the Section. Some of the eastern part of this Section drains toward the Colorado River. A few reservoirs occur on the Colorado River.

Disturbance Regimes. Areas with less than about 8 in (200 mm) of rainfall rarely support enough vegetation to carry a fire. Fire occurrence in areas receiving more than about 8 in (200 mm) has been influenced by introduced grasses. Fires are variable in frequency and intensity. Flash floods are commonly associated with the irregular occurrence of precipitation events.

Land Use. Composition and successional sequence of some communities has changed because of plant and animal species introduced between the late 1800's and early 1900's related to mining and grazing. Since the early 1900's, significant effects on some plant and animal species occur at widely scattered locations associated with military testing, recreational activities, and rapidly expanding urbanization.

Cultural Ecology. Humans have been utilizing the area for some 10,000 years; the early Lake Mohave Paleo-Indian hunting assemblage is well documented at sites along the shores of Pleistocene Lake Mojave. After the end of the Pleistocene, prehistoric assemblages reflect extensive practice of seasonal rounds for resource gathering. In the Section's southern portion, agricultural practices from Colorado River culture influences spread into the area during late prehistoric times, after 1000 A.D. Historic mining of hard rock minerals and borax altered much of the landscape. Contemporary attitudes and beliefs are varied; lifestyle is rural. The economy emphasizes government employment, mining, ranching, and recreation.

Compiled by Pacific Southwest Region.

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Section 322B--Sonoran Desert

Geomorphology. This Section is in the Basin and Range physiographic province. There are widely separated short ranges in desert plains. This Section, which is located in southwestern Arizona, has as its major landforms, plains, fans, and terraces. Elevation ranges from 300 to 3,500 ft (91 to 1,064 m).

Lithology and Stratigraphy. There are Cenozoic nonmarine sedimentary and granitic rocks and alluvial deposits, and Precambrian rocks of all types.

Soil Taxa. Soil include Calciorthids, Haplargids, and Torrifluvents in combination with thermic and hyperthermic soil temperature regimes and aridic soil moisture regime.

Potential Natural Vegetation. Potential natural communities include palo verde, creosote bush, saguaro, mesquite series, and bursage.

Fauna. Reserved.

Climate. Precipitation ranges from 3 to 8 in (80 to 200 mm). It is bimodal, occurring as winter rain and high intensity summer thunderstorms, with more than half of the annual precipitation falling during the winter. Temperature ranges from 61 to 75 oF (16 to 24 oC) and winters are mild. The growing season lasts 250 to 350 days.

Surface Water Characteristics. This Section has mostly bedrock controlled channels in mountains that carry seasonal flows to alluvial channels below. Channels terminate in basins or the Colorado River.

Disturbance Regimes. Composition and successional sequence of some communities have changed because of plant and animal species introduced between the late 1800's and early 1900's related to mining and grazing. Since the early 1900's, significant effects on some plant and animal species occur at widely scattered locations associated with military installations, recreational activities, and rapidly expanding urbanization. Flash floods are commonly associated with summer precipitation events.

Land Use. Reserved.

Cultural Ecology. Water is the critical resource of the Sonoran Desert, and its availability defines two Subsections. the Lower Colorado River Valley and the Arizona Uplands. The Lower Colorado River Valley, characterized by creosote bush and white bursage, occupies the western portion of the Sonoran Desert. Over time, it has been inhabited by Clovis, Amargosa, Hohokam, Patayan, Yuman, and Hia-Ced and Tohono O'odham populations. The Valley Subsection is low, hot, and dry (except for the Colorado River) and aboriginal settlement patterns involved widespread foraging or agriculture in the basins during the rainy season. During dry seasons, populations concentrated around water sources. The Arizona Uplands are more mesic and are characterized by paloverde and saguaro vegetation. Prior to Euro-American colonization, it was occupied by Clovis, Cochise culture, Hohokam, and Tohono O'odham populations. Aboriginal populations became more dependent on agriculture, and permanent villages were located along major streams such as the Santa Cruz and Salt Rivers. In the Phoenix basin, major Hohokam communities flourished, based on a complex system of irrigation canals.

Although Coronado may have crossed the western edge of the Section in 1540, it was not until the late 17th century that missions were established by the Spanish; settlements appeared even later. Spanish and Mexican settlement did not spread north of Tucson or away from permanent streams, and was based on grazing, mining, and agriculture. Anglo-American settlement has shared this economic base and also focused on the major drainages. Paleo-environmental reconstructions have been hampered by the lack of good dendro-climatological data, and extreme local variability in responses to regional trends. Historic change does appear less extensive than in grassland Sections. Arroyo cutting has occurred, and mesquite bosques have replaced cienegas along streams. Overgrazing, fire suppression, and climatic variation are debated as the cause. Today this Section is characterized by rapid population growth in urban areas where economies are dominated by commerce, tourism, light manufacturing, and the construction industry. Contemporary cultures include Anglo-American, Mexican-American, the Tohono O'odham (Pima and Papago), and the San Carlos Apache.

Compiled by Southwestern Region.

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Section 322C--Colorado Desert

Geomorphology. There are alluvial slopes, basin, dunes, and delta plain (Gulf of California). Elevation ranges from 230 feet below sea level to 1,000 ft (-70 to 304 m). This Section is in the Basin and Range geomorphic province.

Lithology and Stratigraphy. Cenozoic alluvial deposits occur.

Soil Taxa. Soils include Aridisols and Entisols with hyperthermic soil temperature regime and aridic soil moisture regime.

Potential Natural Vegetation. K\"uchler mapped vegetation as Sonoran creosote bush, Salton Sea saltbush, and oasis scrub-woodland. Predominant potential natural communities include creosote bush and mesquite series.

Fauna. Mammals include desert bighorn sheep, desert kit fox, coyote, spotted skunk, spotted bat, black-tailed jackrabbit, ground squirrel, kangaroo rat, and white-footed mouse. Birds includes eagles, hawks, owls, quail, white-winged dove, roadrunner, finches, warblers, and orioles. The Salton Sea provides habitat for a wide variety of waterfowl and shore birds. Reptiles include several species of rattlesnakes.

Climate. Precipitation averages 2 to 6 in (50 to 150 mm). Temperature averages 65 to 75 oF (18 to 24 oC). The growing season lasts 250 to 350 days.

Surface Water Characteristics. There are mostly bedrock controlled channels in mountains that carry seasonal flows through alluvial channels below to the Salton Sea. A few rivers that evolved from irrigation drainage water flow to the Salton Sea from Imperial and Coachella Valleys. The largest surface water feature is the Salton Sea (about 40 by 16 mi (64 by 26 km).

Disturbance Regimes. Strong winds and drifting sand are common in parts of the area. Flash floods are commonly associated with the irregular occurrence of precipitation events. Precipitation does not occur every year.

Land Use. Composition and successional sequence of some communities have changed because of plant and animal species introduced between the early 1800's and early 1900's. These introductions related to grazing, agriculture, and urbanization. Most of the Section is converted to irrigated agriculture. Increased salinity of soil and water occurs in some areas as a result of modified drainage. Rapidly expanding urbanized areas are scattered throughout the Section.

Cultural Ecology. Humans have been utilizing the area for some 10,000 years. After the end of the Pleistocene hunting traditions, Archaic assemblages reflect extensive practice of seasonal rounds for diversified hunting and gathering. Agricultural practices of Colorado River cultures spread throughout the area during late prehistoric times, after 1000 A.D. In recent historic times, large areas have been converted to irrigated agriculture. The recently formed Salton Sea has become a mecca for retirement, recreation, and development. Contemporary attitudes and beliefs are varied; lifestyle is rural. The international border and large Hispanic populations contribute to cultural diversity; Hispanic populations comprise much of the agricultural, often migrant, work force. The economy emphasizes agriculture, government employment, and recreation.

Compiled by Pacific Southwest Region.

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