Physical Geography

Arizona lies within two principal physiographic regions of the United States: the Colorado Plateau in the north and east and the Basin and Range in the south and west.

The Colorado Plateau is a tableland of soft sedimentary rock, some 5,000 to 9,000 feet (1,520 to 2,740 m) above sea level. Into it, rivers have cut a fantastic array of deep canyons and gorges—the region's most distinctive landforms. By far the largest and deepest is the Grand Canyon of the Colorado River.

Projecting from the plateau's surface, which in many areas appears quite flat, are scattered high mountains and flat-topped mesas and buttes. Highest are the San Francisco Mountains in the north-central part of the state. They rise to 12,633 feet (3,851 m) in the volcanic rocks of Humphreys Peak, Arizona's highest point.

The plateau is also marked by other land-forms, including sharp spires and pinnacles, natural arches, and great escarpments, or cliffs. A series of high rimlike escarpments runs along the southern edge of the plateau, beyond which lies the Basin and Range region.

The Basin and Range region consists chiefly of northwest-southeast mountain ranges, separated by numerous valleys and basins. The ranges are highest in the northern and eastern parts of the region, where some peaks rise 8,000 to 11,000 feet (2,440 to 3,350 m). Elevations decline southwestward through a desert area (sometimes called the Gila Desert, part of the vast Sonoran Desert), reaching 70 feet (21 m) above sea level where the Colorado River enters Mexico—the lowest point in the state.

Almost all of Arizona lies in the drainage basin of the Colorado River. Like most major streams in the state, the Colorado has been dammed for irrigation, hydroelectric power, flood control, and other purposes. In the far north, near the point where the river enters the state, is Glen Canyon Dam, one of the largest dams in the West. Behind it, mainly in Utah, is Lake Powell. After flowing through the Grand Canyon, where it receives the water of the Little Colorado River, the Colorado is further controlled by Hoover Dam and Lake Mead on the Arizona-Nevada line. Farther down-stream are the reservoirs of Davis, Parker, and Imperial dams.

The Colorado's longest tributary in Arizona is the Gila. It crosses the southern part of the state and joins the Colorado at Yuma. The Salt River is the principal tributary of the Gila. Below the junction of the two rivers there is little or no flow because of heavy use of water upstream. Most other rivers carry water only seasonally or after brief rains. Some dry up in basins having no outlet.

The climate is determined mainly by the state's southerly location and its variation in altitude. In general, rainfall is light and humidity low. Temperature variations between day and night are usually wide. Sunshine is one of Arizona's greatest assets—the state is one of the sunniest in the Union.

The mean monthly temperature in Phoenix varies from about 50° F. (10° C.) in January to about 90° F. (32° C.) in July. Throughout the south, highs of more than 110° F. (43° C.) are recorded nearly every summer. Highs seldom exceed 90° F. (32° C.) on the plateau and in the high mountains. Winters in those areas are cold, and there are occasional periods when temperatures fall below 0° F. (-18° C.).

Total annual precipitation varies from 2 to 14 inches (50–360 mm) throughout most of the state. The mountains receive as much as 30 inches (760 mm), including some snow.

On high mountain slopes are forests of Ponderosa pine, fir, and other evergreen trees. These stands, especially those of Ponderosa pine, are the chief source of Arizona's commercial timber. At lower elevations, trees include pi$non, juniper, and oak.

The Colorado Plateau is largely a treeless area covered with low bushes and drought-resistant grasses. Typical plants in the deserts of the south are cacti, creosote bushes, and yuccas. Stands of giant saguaro and organ pipe cacti have been designated national monuments.