Chapter 36
Ecological Subregions of the United States

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Colorado Plateau Semi-Desert

Five Sections have been delineated in this Province:

These Sections are located in the southwestern conterminous States, including parts of Arizona, New Mexico, Utah, and Colorada. The area of these Sections is about 75,300 mi2 (195,000 km2).

Section 313A--Grand Canyon

Geomorphology. This Section is in the Colorado Plateau physiographic province. Grand Canyon lands are in the south-central part of Utah and the northern portion of Arizona. It extends into the southwestern corner of Colorado. This area is eroded by the Colorado River and its tributaries. Deep sheer-walled canyons, lines of cliffs, elevated plains, low plateaus, mesas, buttes, and badlands dominate landscape. Major landforms are the Grand Canyon and Colorado Plateau. Elevation ranges from 4,200 to 7,800 ft (1,300 to 2,400 m).

Lithology and Stratigraphy. There are Cretaceous, Jurassic, and Triassic aged sedimentary rock with some Permian aged sediments in northern Arizona. Eolian deposits occur on the east side of the Section. Zion National Park occurs in this Section. All sedimentary rocks are dominantly shales and sandstone, with some limestone.

Soil Taxa. Soils include Torriorthents, Torrifluvents, Ustochrepts, and Haplustalfs in combination with mesic and frigid soil temperature regimes, and ustic soil moisture regimes. A few Haplargids occur with thermic soil temperature regime and aridic soil moisture regime. Some soils are saline-sodic affected. Areas of very sandy soils exist.

Potential Natural Vegetation. This area consists of pinyon-juniper woodland with a small area of Great Basin sagebrush, and blackbrush vegetation. The area has a cold desert shrub and steppe woodland vegetation, with some paleoendemic blackbrush.

Fauna. Reserved.

Climate. Precipitation ranges from 3 to 18 in (80 to 458 mm) annually, with more than half of the annual precipitation falling during the winter. Summers are dry with low humidity. Temperature averages 47 to 55 oF (8 to 23 oC). The growing season lasts 110 to 180 days.

Surface Water Characteristics. Water is scarce. The area is drained by the Colorado River and its tributaries. Ground water supplies are deep and limited. Summer rain storms cause flash flooding in much of the Section. Few lakes and reservoirs occur, with Lake Powell being the largest.

Disturbance Regimes. Fire is cyclical. Grazing for sheep and cattle is the major land use. Hay and pasture lands also occur to a very limited extent along drainage ways. Climate is very dry and hot in the summer and cold and moist in the winter, indicative of a cold, desertic condition.

Land Use. Reserved. Cultural Ecology. The earliest human occupation of the Grand Canyon Section likely occurred between 11,000 and 13,000 years ago and was characterized by an emphasis on big game hunting, supplemented by the gathering of wild plant foods. Evidence for these activities is sparse and is restricted to the discovery of Paleo-Indian projectile points. Between 8,000 and 10,000 years ago, there appears to have been an increase in summer and annual temperatures, and a shift from the winter dominant to a summer dominant or split precipitation regime. These changes led to alterations in subsistence patterns over the next 6,000 years, evolving to a culture in which highly mobile hunter and gatherer groups exploited a wide range of resources in many environmental zones, likely in a structured seasonal round. The upper elevations seem to have been particularly favored during this period, especially during the hot summer months. The use of horticulture as a primary subsistence strategy began in the area about 2,000 years ago, but was not adopted uniformly across the region until about 1,000 to 1,200 years ago. In general, however, human populations began to slowly grow and become increasingly sedentary. The proximity to arable land appears to have been a primary determinant for habitation site locations during this period, continuing to the end of the prehistoric sequence about 1600 A.D. During most of this period, habitation sites tended to be concentrated on low ridges, terraces, and knolls overlooking adjacent alluvium. Numic-speaking hunter and gatherer groups entered the western portion of the area from the Great Basin by about 1200 A.D. Hunting and gathering Athabaskan speaking peoples arrived in the northeast and eastern portions of the area by about 1500 A.D. Both groups exploited many of the same wild resources as the sedentary horticulturists.

The earliest historical records for the Grand Canyon lands suggest that few Anglos were present in the area before about 1776, although members of Coronado's expedition reportedly viewed the Grand Canyon in the early 1540's. The Spanish presence was transitory and consisted of expeditions searching for routes to and from California. The area was settled by Anglos beginning in the 1850's, through the colonization efforts of the Mormon Church. During the late 19th century, ranching, homesteading, and some lumbering were primary activities throughout the area. By the turn of the century, tourism and recreation had begun to play an important role, particularly near the Grand Canyon and southern Utah. At the present time, the area continues to be characterized by small rural communities, often separated by fairly great distances. Lumbering, ranching, fuel wood gathering, hunting, recreation and related tourism, provide the primary subsistence base. The establishment of the National Park system after 1916 has been a major influence on the area's economy.

Compiled by Southwestern Region.

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Section 313B--Navajo Canyonlands

Geomorphology. This Section is in the Colorado Plateaus physiographic province. Navajo Canyonlands are in the northeast part of Arizona and southeast Utah. Geomorphic processes active in this area are deep canyon formations as the result of plateau disSection. Volcanic mountains exist in this Section, but block-fault structural mountain ranges do not. Major landforms are canyonlands, plateaus, plains, and hills. Major landform features are the Painted Desert, Vermillion and Echo Cliffs, Glen Canyon Recreation Area, and Canyonlands National Park. Elevation ranges from 4,000 to 8,000 ft (1,210 to 2,425 m).

Lithology and Stratigraphy. There are Cretaceous, Jurassic, and Triassic aged sedimentary rocks with some Quaternary and upper Tertiary sedimentary deposits and valley fills.

Soil Taxa. Soils include Haplustalfs, Calciorthids, Haplargids, and Ustochrepts, and a few Haplustolls, Calciustolls, and Argiustolls in combination with mesic soil temperature regimes, and ustic and aridic soil moisture regimes. Potential Natural Vegetation. Vegetation consists of pinyon-juniper woodlands at higher elevations. Grama and galleta grasses are found at lower elevations; greasewood and saltbrush are found on calcareous and salt affected soils.

Fauna. Species include pronghorn antelope, jackrabbits, desert mouse, and rattlesnakes.

Climate. Precipitation ranges from 8 to 18 in (200 to 458 mm) annually, with more than half of the precipitation falling during the winter. Temperature averages 45 to 57 oF (7 to 13 oC) and winters are cold. The growing season lasts 110 to 180 days.

Surface Water Characteristics. Water is scarce. The Little Colorado River drains most of the area, but its flow is intermittent; water is commonly stored in small reservoirs.

Disturbance Regimes. Fires are variable in frequency and intensity. Flash floods and drought are common. Approximately 90 percent of this area is rangeland. It is grazed by both cattle and sheep.

Land Use. Reserved.

Cultural Ecology. Although Paleo-Indian and Archaic hunting and gathering people utilized this Section for thousands of years, it was the Anasazi farmers who left the most striking marks upon the land. Their settlements were located near water in or adjacent to pinyon-juniper woodlands, which offered abundant plant and animal resources used to supplement their crops. Most areas were somewhat marginal for agriculture, and communities continued to utilize nearby mountains and lower elevation areas for hunting and gathering activities. Eventually the Anasazi constructed impressive towns such as those in Chaco Canyon, Mesa Verde, and Tsegi Canyon. Paleontological studies indicate that the short duration of many sites may have been related to the need to re-locate due to depletion of wood resources needed for fuel and building materials. Local population pressure and a long period of drought are considered factors in the abandonment of most of this unit by the early 1300's and the subsequent aggregation of pueblo populations at Hopi, Zuni, and along the Rio Grande.

At the time of Spanish contact, the area was largely uninhabited except for the Hopi villages. Early Navajo and Apache and Ute peoples used the area for hunting, gathering, and some horticulture. Spanish use of the area was limited, and it was not until the mid 1800's that the American government led campaigns against the Navajo people and opened the area for settlement. Eventually, much of this Section was included in the Hopi and Navajo Reservations. Overgrazing by sheep and a natural period of downcutting around the turn of the century contributed to the erosional characteristics visible today. Coal, oil, and gas resources in the northern portions of the Section have been exploited in boom-and-bust cycles. Today the area is largely rural, with a few sizable towns. Grazing, agriculture, mineral development, and tourism contribute to the economy. The Four Corners area offers unsurpassed scenic and heritage values.

Compiled by Southwestern Region.

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Section 313C--Tonto Transition

Geomorphology. The Tonto Transistion Section lies between the Basin and Range and Colorado Plateaus physiographic provinces. The Tonto Transition Section is located in central and northwest central Arizona. Precambrian through Mesozoic volcanic activity and sedimentary deposition were major geomorphic processes. Lava flows, plugs, dikes, and relatively flat sedimentary deposits resulted. Major landforms are mountains, hills, scarps, and some plains. Major landform features include the Mazatzal Mountains, Black Hills, Aquarius Mountains, Bradshaw Mountains, and the Superstition Mountains. Elevation ranges from 3,000 to 7,400 ft (915 to 2,255 m).

Lithology and Stratigraphy. There are Precambrian metavolcanics, and metamorphic and sedimentary rocks with Tertiary and Quartenary volcanic and sedimentary rocks.

Soil Taxa. Soils include Ustochrepts, Ustifluvents, Ustorthents, Haplustalfs, Argiustolls, Argiborolls, and Eutroboralfs, and a few mesic and frigid soil temperature regimes and ustic soil moisture regimes.

Potential Natural Vegetation. Vegetation consists of interior chaparral of Turbinella oak on coarse igneous parent materials, steep slopes, and fire disturbed regimes. There are pinyon-juniper on elevations higher than about 4,200 ft (1,280 m); ponderosa pine occurs in frigid and limited mesic soil temperature regimes at higher elevations.

Fauna. Reserved.

Climate. Precipitation ranges from 10 to 25 in (250 to 635 mm) annually, with more than half of the precipitation falling during the winter. Temperature ranges from 40 to 70 oF (4 to 20 oC); winters are mild below about 6,800 ft (2,075 m) and cold at higher elevations. The growing season lasts 70 to 170 days.

Surface Water Characteristics. Much of the area is drained by the Verde River into the Salt River. This area supplies much of the water to adjoining irrigated areas. Because more than half of the precipitation falls during the winter, it is commonly stored in reservoirs for livestock and domestic use; there usually is a deficiency of water in the summer. Ground water is limited and usually occurs at great depth.

Disturbance Regimes. Fire climaxes occur on steep slopes and many coarse igneous rocks in mesic and thermic soil temperature regime areas in the interior chaparral community. Frequency is variable, but may range from 25 to 100 years. Flash floods and droughts are common. Land use includes cattle grazing, irrigated crop land, recreation, and harvest of small areas of commercial timber.

Land Use. Reserved.

Cultural Ecology. Humans have utilized this area for the last 11,000 years. Prior to the end of the Pleistocene, small groups practiced diversified hunting and gathering following seasonal rounds. About 2,000 years ago, most of these groups began to practice corn agriculture, initiating a long-term trend toward increasing sedentism and population growth. Prior to about 1100 A.D., sedentary homesteads and villages were more or less confined to those river valleys suitable for irrigation or flood plain farming. Afterwards, with the widespread adoption of rainfall harvesting technologies, temporary, seasonal, and permanent settlements spread into all geographic areas. In many places this resulted in extensive landscape modification for agriculture, including the displacement of native plant species and the initiation of local erosion cycles. By 1300 A.D., the prehistoric population in the Section had again become concentrated in the irrigable valleys, and large portions of the uplands were abandoned. By 1400 A.D., the entire area was essentially abandoned following a succession of debilitating droughts and floods.

Several hundred years later, new groups of hunter-gatherers re-occupied it at much lower population levels. The first Europeans in the area entered it only briefly in the 16th century. They were followed by trappers and prospectors in the middle 19th century. Shortly thereafter, the military established a presence and removed much of the native population onto reservations. The army was quickly followed by miners and cattle ranchers. Several consecutive mining booms for gold, silver, and copper were responsible for most of the urbanization seen in the Section as well as extensive landscape modification for exploration and extraction. Further extensive modifications occurred following the turn of the century, when a large portion of the central part of the area was developed for reservoir-based reclamation projects. By that time, the sparsely populated area had developed a mixed economy based on rural agriculture and cattle ranching, mining, and forest products. In recent decades, this economy has been supplemented by recreation, tourism, and retirement developments to support a rapidly growing population. Contemporary attitudes and beliefs are highly varied, ranging from traditional Native American to those held by residents dependent on modern urban development and amenities. The human environment is primarily characterized by a mixture of urban and rural lifestyles. Large expanses of open and unoccupied space are heavily used for outdoor leisure activities, most of which originate from the Phoenix area, located just outside of the Section. The Tonto Transistion Section encompasses portions of the San Carlos Apache, Fort Apache, and Havasupai Reservations. The modern economy is primarily focused on recreation and tourism and mining, with secondary emphases on commerce and light manufacturing, grazing, forest products, and government employment.

Compiled by Southwestern Region.

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Section 313D--Painted Desert

Geomorphology. This Section is in the Colorado Plateaus physiographic province. Geomorphic processes active in this area are Mesozoic sedimentary deposition followed by tilting and erosion into majestic plateaus. Major landforms are plains, hills, canyonlands, and valley plains. Elevation ranges from 4,000 to 7,000 ft (1,210 to 2,134 m).

Lithology and Stratigraphy. There are Jurassic and Triassic sedimentary rocks and a few areas of Paleozoic and Precambrian sedimentary rocks.

Soil Taxa. Soils include Haplustalfs, Ustochrepts, and some Haplustolls, Calciustolls, and Argiustolls with a mesic soil temperature and ustic soil moisture regime. A few soils are in areas with a mesic soil temperature regime and a few Torriorthents and Calciorthids are in aridic soil moisture regime.

Potential Natural Vegetation. Grama and galleta grasses occur at lower elevations and pinyon-juniper woodlands at higher elevations; saltbrush-greasewood type occur in dry, salt affected, and calcareous soils.

Fauna. Reserved.

Climate. Precipitation ranges from 8 to 20 in (200 to 508 mm) annually, with about 45 percent of the precipitation falling during the winter. Temperature averages 45 to 57 oF (7 to 14 oC) and winters are generally cold. The growing season lasts 100 to 170 days.

Surface Water Characteristics. Water is scarce. The Little Colorado River and Zuni River drain most of the area, but their flows are intermittent. Water is commonly stored in reservoirs.

Disturbance Regimes. Fires are variable in frequency and intensity. Flash floods and drought are common. Most of this area is rangeland. It is grazed by both sheep and cattle.

Land Use. Reserved.

Cultural Ecology. The Painted Desert Section has been home to a variety of different human groups over the centuries. Clovis projectile point fragments have been found in the Little Colorado River Valley itself, as well as near the Flagstaff area, on the west end of the Section. Later hunter-gathering people roamed throughout the area, and developed small, scattered pit house settlements during late Archaic and Basketmaker II times along ridges and hilltops. The open grassy plains in the eastern end of the unit were particularly important Archaic centers of occupation. In fact, one localized Archaic tradition has been named for this area - the Concho Complex. In the northern side of the Section, the environment is composed mostly of sand dunes and outcrops of Moenkopi sandstone that are covered by low scrub. Human use was limited to areas where springs or washes could sustain a seasonal water supply. In the more hospitable areas, a substantial presence of Basketmaker III pithouse villages developed, especially along the first terrace of the Little Colorado River. In the eastern end of the Section, these populations showed more similarities with the Mogollon culture. These populations remained fairly intact until about 900 A.D., after which time there were some major population re-adjustments and a dispersion to new areas. A pattern of aggregation is evident by 1250 to 1300, culminating by 1400 with the the concentration of population into approximately 35 large pueblos. Between 1300 to 1500, this unit was one of the major population centers of the Southwest; but it, too, was mostly abandoned by 1500 as people moved to the Hopi Mesa country to the north, and the Zuni country to the east. Historic Hopi and Zuni traditions allude to problems with insects, disease, and internal stress within the population as factors causing these abandonments.

During historic times, the aridity of the Little Colorado River desert and steep canyons of Canyon Diablo posed major obstacles to people wishing to cross the area. Many stories and diaries that recount the difficulties people had when trying to make such a crossing. Nonetheless, in 1863 a party from Santa Fe made camp at what is now the highway stop of Navajo to officially take charge of the Arizona Territory. Mormon colonists from the White Mountains to the south, aided by others from Salt Lake City, began to settle and farm the area, establishing numerous small towns both in the southeastern side of the Section and at scattered places along the Little Colorado River, such as Winslow, Holbrook, Woodruff, and Joseph City. During the historic period, due to climate change and cattle overgrazing from the extensive Hash Knife cattle outfit, the river valley became dry and alkaline. The main life-line of habitation became first the railroad, and later Route 66 and Interstate 40. The Hopi people consider the entire unit to be within their ancestral territory. To a lesser degree, this also holds for the Zuni, although their ties are strongest with the eastern part of the Section. The south edge of the unit was Apache country, while the Navajo made sporadic use of the north and western sides.

Compiled by Southwestern Region.

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Section 313E--Central Rio Grande Intermontane

Geomorphology. This Section, which is in the Basin and Range physiographic province, is located in central New Mexico. Active geomorphic processes in this Section are basins produced by erosional and depositional action of running water. Major landforms are valleys and lowland and outwash plains, and alluvial fans and terraces. The Rio Grande basin is the major landform feature.

Lithology and Stratigraphy. There are Cenozoic, Pleistocene and Miocene sedimentary rocks and alluvial deposits, with a few late Tertiary-Quaternary volcanic rocks.

Soil Taxa. Soils include Hapustalfs, Ustochrepts, Haplustolls, Ustifluvents, Torriorthents, and Fluaquents with mesic soil temperature regime and ustic soil moisture regimes and aquic conditions.

Potential Natural Vegetation. Grama and galleta grasses and four-wing saltbrush occur along with sand sage at lower elevations; pinyon-juniper woodlands are at higher elevations. A few areas have riparian species such as cottonwood and willow.

Fauna. Reserved.

Climate. Precipitation ranges from 8 to 16 in (204 to 400 mm) annually, with less than half of the precipitation falling during the winter. Temperature averages 40 to 57 oF (4 to 14 oC). The growing season lasts 130 to 180 days.

Surface Water Characteristics. Water is scarce throughout this Section.

Disturbance Regimes. Fires are variable in frequency and intensity, depending on fuel and moisture. Most of this Section is grazed by sheep and cattle.

Land Use. Reserved.

Cultural Ecology. For the past 12,000 years, the basin that makes up most of this Section has offered human groups the resources of a flowing river and broad flood plain, plus a milder climate and more protected environment than adjacent plateaus and plains. Traces of Paleo-Indian hunting camps and abundant remains of Archaic campsites are found along the gravel hills and mesas that border the Rio Grande and Rio Puerco. Starting some 1500 years ago, more permanent settlements began to appear as human groups made the transition to greater reliance on agriculture. It was not until around 1300 A.D., however, that large populations began to aggregate along the Rio Grande, a development coinciding with the abandonment of much of the Colorado Plateau. Early Spanish accounts document many multi-storied pueblos along the Rio Grande, stretching from what is now Socorro north to Bernalillo and beyond. Accounts describe many fields along the river with occasional groves of cottonwood. The Rio Grande pueblos were decimated in the 16th and 17th centuries, however, due to disease, Spanish oppression, and the attacks of other newcomers to the area, the Apache. Today only six pueblos survive in this Section.

Although traders on the Camino Real regularly moved goods between Santa Fe and Chihuahua, Spanish settlement before 1800 was concentrated in the northern part of the basin due to the dangers of Apache raids. The Mexican Period and finally the American takeover opened up the region to trade and settlement. This was vastly accelerated by the arrival of the railroads in the late 1800's. Even today, however, population is concentrated in towns along the Rio Grande, which continues to serve as a transportation and communication corridor. The area is characterized by a mix of old Hispanic communities, urban populations, pueblos, and tourists. Commerce, light industry, tourism, and, to a lesser extent, agriculture and ranching, contribute to the economy. Contemporary issues include growth, water, and conflicting cultural values.

Compiled by Southwestern Region.

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