Arizona

Introduction

Arizona is a land of contradictions. Although widely reputed for its hot, low-elevation desert covered with cacti and creosote bushes, more than half of the state lies at an elevation of at least 4,000 feet (1,200 metres) above sea level, and it possesses the largest stand of evergreen ponderosa pine trees in the world. Arizona is well known for its waterless tracts of desert, but, thanks to many large man-made lakes, it has many more miles of shoreline than its reputation might suggest. Such spectacular landforms as the Grand Canyon and the Painted Desert have become international symbols of the region's ruggedness, yet Arizona's environment is so delicate that in many ways it is more threatened by pollution than are New York City and Los Angeles. Its romantic reputation as a wild desert and a place of old-fashioned, close-to-the-earth simplicity is at variance with the fact that after the 1860s the state's economy became industrial and technological long before it was pastoral or agrarian.

Arizona is located in the southwestern quadrant of the coterminous states, bordered by California to the west, Nevada to the northwest, Utah to the north, New Mexico to the east, and the Mexican state of Sonora to the south. The Colorado River forms the boundary with California and Nevada. Phoenix, situated in the south-central part of the state, is the capital and largest city.

Area 113,999 square miles (295,256 square km).

Pop. (2000) 5,130,632; (2007 est.) 6,338,755.

Land

Plate tectonics—the shifting of large, relatively thin segments of the Earth's crust—and stream erosion have done the most to create Arizona's spectacular topography. Specifically, the Pacific Plate and the North American Plate came into contact and created the major tectonic forces that uplifted, wrinkled, and stretched Arizona's geologic crust, forming its mountain ranges, basins, and high plateaus. Over the course of millennia, rivers and their tributaries have carved distinctive landforms on these surfaces.

Relief

To Arizona's two major physiographic divisions, the Colorado Plateau and the Basin and Range Province, geologists add the Transition Zone (or Central Highlands). The northeastern two-fifths of Arizona is part of the scenic Colorado Plateau. Far less rugged than adjacent portions of the plateau in Utah, these tablelands in Arizona consist mainly of plains interrupted by steplike escarpments. Although they are labeled mesas and plateaus, their ruggedness and inaccessibility have been exaggerated. The incomparable Grand Canyon of the Colorado River provides the major exception to what has proved to be an area easily traversed. Forest-clad volcanic mountains atop the plateaus provide the state's highest points: Humphreys Peak, 12,633 feet (3,851 metres), in the San Francisco Mountains, and Baldy Mountain, 11,403 feet (3,476 metres), in the White Mountains.

More than 200 miles (320 km) of the southern border of the Colorado Plateau is marked by a series of giant escarpments known collectively as the Mogollon Rim. West and south of the rim, a number of streams follow narrow canyons or broad valleys south through the Transition Zone and into the Basin and Range Province. The Transition Zone bordering the plateaus comprises separated plateau blocks, rugged peaks, and isolated rolling uplands so forbidding that they remained mostly unexplored until the late 19th century. The zone marks the ecological border between the low deserts and the forested highlands; it combines elements of both with, for example, the Spanish bayonet of the Sonoran Desert growing alongside the juniper characteristic of higher elevations.

The Basin and Range region of the southern and western third of the state contains the bulk of the population but none of the large canyons and mesas for which Arizona is famous. It consists largely of broad, open-ended basins or valleys of gentle slope. Isolated northwest-to-southeast–tending mountain ranges rise like islands in the desert plain.

Contrary to desert stereotypes, sand dunes are nearly nonexistent, and stony desert surfaces are seldom visible except in the far southwestern portion of the state. The younger soils of river floodplains provide the more-desirable soils for agriculture.

 Drainage

Virtually all of Arizona lies within the Colorado River drainage system. The Gila River, with its major feeder streams—the Salt and the Verde—is by far the Colorado's main Arizona tributary.

The Black, White, and Verde rivers are the primary perennial tributaries of the Salt River, which enters the Gila River southwest of Phoenix. Only during the infrequent—and occasionally devastating—flood periods does runoff water advance downstream past the numerous dams built on the Salt's system. The Gila River rises in that part of the Mogollon Rim located in western New Mexico, and it includes another and smaller Mogollon Rim tributary, the San Francisco River. Two intermittent southern Arizona streams, the Santa Cruz and San Pedro rivers, flow northward into the Gila, while two other intermittent streams, the Agua Fria and Hassayampa rivers, drain central Arizona southward into the Gila. Dams and irrigation systems, except on rare occasions, leave the Gila River dry for most of its length.

The Little Colorado River—which drains the Mogollon Rim's lee side and flows from southeast to northwest into the Colorado River between Marble Canyon and the Grand Canyon—draws and transports little water from its large watershed. Because of the rain shadow effect on the Mogollon Rim's lee side, the Little Colorado usually is no more than a trickle and often is dry. Several other small and intermittent streams, such as the Bill Williams River, drain a large but arid part of western Arizona.

Climate

About half of Arizona is semiarid, one-third is arid, and the remainder is humid. The Basin and Range region has the arid and semiarid subtropical climate that attracts most winter visitors and new residents. January days in Phoenix receive more than four-fifths of the possible sunshine and have a mean maximum temperature of 65 °F (18 °C). Occasional light frosts occur at most locations in the Basin and Range region in winter, and some precipitation interrupts the exceedingly dry springs and mildly dry falls. Daily maximum readings average 106 °F (41 °C) in Phoenix in July, and nighttime temperatures drop to an average of 81 °F (27 °C).

Moisture-laden air from the Gulf of California and the eastern Pacific Ocean appears in July, bringing more than two months of irregular but sometimes heavy thundershowers that are locally referred to as the “summer monsoon.” Phoenix and Tucson receive about 1 inch (25 mm) of precipitation in July and about 3 inches (75 mm) total throughout the summer months. Winter rains come from the Pacific.

The Colorado Plateau has cool to cold winters and a semiarid climate. Average mile-high elevations and direct exposure to polar air masses can produce January mean high and low temperatures as divergent as the 46 °F (8 °C) and 19 °F ( 7 °C), respectively, in Winslow. Year-round temperatures in Flagstaff are generally 30 °F (17 °C) cooler than those of Phoenix. Most of the region receives from 10 to 15 inches (250 to 375 mm) of precipitation annually, with the Mogollon Rim and White Mountains receiving the state's largest average, 25 inches (625 mm).

Because of the great diversity of relief within the Transition Zone, climatic conditions there vary widely over small areas. Much of Arizona's humid area lies in this zone and in the adjacent high southern edge of the Colorado Plateau. There, perennial streams flowing through shaded riparian corridors contribute to atmospheric moisture, resulting in temperatures that are several degrees cooler than those of the nearby deserts.

Plant and animal life

Considering the variety in relief and climate, it is not surprising to find similar diversity in the state's vegetation. About one-tenth of Arizona is forested, one-fourth is woodland, one-fourth is grassland, and the rest is desert shrub. Elevations above 6,000 to 7,000 feet (1,800 to 2,100 metres) host forests of ponderosa pine, topped in the highest areas by Douglas and other firs, spruces, and aspen. From 4,500 to 7,500 feet (1,375 to 2,300 metres) in the northern half of the state, piñon pine and juniper predominate, while evergreen oak and chaparral grow between 4,000 and 6,000 feet (1,400 and 1,800 metres) in the central mountains. Plains grasses cover about one-third of the Colorado Plateau, and Sonoran or desert grass carpets the higher elevations of the basins. Mesquite trees have invaded many former grasslands in the south. Cacti grow throughout the state, with the greatest variety below 2,000 feet (600 metres). Foothills in the Tucson-Phoenix area carry giant saguaro cacti of the Sonoran Desert, matched in areas of the northwest Basin and Range by dramatic stands of Joshua trees. Shrubs dominate the lowest portions of all areas: big sagebrush and saltbush in the Colorado Plateau, creosote bush in the Basin and Range.

Animal life is even more varied, with representatives of the Rocky Mountain, Great Plains, and Mexican ecological communities. Important larger mammals are black bears, deer, desert bighorns, antelope, and wapiti (elk). The tropical coatimundi, a raccoonlike mammal, has spread northward into Arizona, while the javelina, or peccary (wild pig), is a favourite game animal in the south. Among the several cats, the bobcat and the mountain lion are most characteristic of Arizona. Coyotes, skunks, and porcupines abound, as do cottontails, jackrabbits, and several varieties of foxes. The state's southern border area lies along a major flyway and is rich in birdlife, which attracts thousands of watchers. Game birds include turkeys and a variety of quails, doves, and waterfowl. Among native fish are the Arizona trout and the Colorado squawfish. Venomous animals include rattlesnakes, scorpions, and Gila monsters.