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Arizona’s
Natural Regions
All of Arizona lies in the Intermontane Plateaus, which form one of the major
physiographic divisions of the United States. The state can be divided into
three physiographic regions: the Colorado Plateau, the Basin and Range region,
and the Transition Zone, or Central Highlands.
The Colorado Plateau is a flat,
dry, semi-desert region that covers the northern two-fifths of the state. Many
rivers have carved deep canyons into this region, with the most famous example
being the Grand Canyon. The
river in the Grand Canyon is as much as 5000 feet below the level of the
surrounding plateau. Canyon de Chelly and Oak Creek Canyon are also beautiful,
but lesser known.
About 2 billion years ago this area, now mostly 5000 to 8000 ft
above sea level, lay under a vast sea. Through the ages the land emerged and resubmerged repeatedly, and many different rock types, including igneous,
sedimentary, and metamorphic, formed. Rivers cut through layers of soil and rock
to reach the ancient granites, quartzites, and rocks now revealed. Water and
wind have eroded the edges of canyons and the surface of the plateaus, carving
isolated, steep-sided, flat-topped hills called mesas. Underlying rocks such as
sandstones, shales, and limestones have also been exposed by erosion, creating a
kaleidoscope of brilliant colors on canyon and mesa walls.
North of the Grand Canyon is the Kaibab Plateau, which is an area extending from
Utah into Arizona. The Kaibab Plateau resembles a peninsula.
The section of the Colorado Plateau located in the northern and northeastern
part of the state is a maze of valleys and mesas. Carved knobs, rounded domes,
and tall rock spires that pierce broad valleys earned the area its name of
Monument Valley. The Painted Desert, where red, yellow, purple, blue, brown, and
gray rocks alternate in a vivid display of colors, extends south from the Grand
Canyon to the Mogollon Rim. Within the Painted Desert is the Petrified Forest
National Park, an area of giant, ancient fallen trees that slowly petrified over
thousands of years.
South of the Grand Canyon lies the San Francisco Plateau, which is covered by
ancient lava flows and dotted with extinct volcanic cones such as Humphreys
Peak, 12,633 ft tall, the highest point in Arizona. The southeastern
part of the Colorado Plateau in Arizona is part of the Datil section, noted for
its solidified lava flows and other volcanic features. The southern border of
the Colorado Plateau is distinguished by an extensive volcanic escarpment known
as the Mogollon Rim. The Rim, which extends from central Arizona toward the
southeast and terminates in the Mogollon Mountains, was originally created by
tectonic pressure, uplift of the plateau, and, most important, erosion of the
Transition Zone. The steep rock wall reaches about 2000 ft high in
some places. To the south of the Mogollon Rim is a narrow strip of land known as
the Transition Zone.
The Transition Zone is characterized by mountain ranges so close together that
the area appears as a cluster of rugged peaks separated by steep, narrow
valleys. The Mazatzal, Santa Maria, Sierra Ancha, and White mountain ranges are
found in this zone, which occupies part of the area once known as the Central
Highlands. So uninviting was the landscape that prospectors did not explore the
region until the late 19th century. Since then, more than 90 percent of
Arizona’s mining activity has taken place in this area.
The Basin and Range region, known to Arizona residents as the Sonoran Desert,
occupies most of the southern part of Arizona. It is composed of a series of
smooth-floored desert basins separated by mountain ranges that extend from
northwest to southeast. Mountains in this region include the Chiricahua, Gila,
Pinaleno, Huachuca, Hualapai, Santa Catalina, Santa Rita and Superstition
ranges. The portion of the Basin and Range region that lies to the south and
west is a low, dry landscape. Elevations in this area range from as low as 141 feet at Yuma to 10,717 feet
atop Mount Graham. While the land has little rain, along the western border of
Arizona farms irrigated with waters from the Most of the loose material on the
mountains in this region has been carried down by infrequent but violent
cloudbursts to form thick fans of sand and gravel where the steep slopes meet
the basin floor. When irrigated, this area produces excellent crops. Also, the
state’s largest cities are located in this region. Most elevations in the Basin
and Range region are from 500 to 5000 ft, but some mountains
rise to more than 11,000 feet. The region is generally higher in the east than
in the west.
Arizona’s Rivers and Lakes
The most important river in Arizona is the
Colorado, which enters the state in
the north, flows southwestward to the state’s western boundary, and then follows
the boundary south into Mexico. In the Colorado Plateau, its main course is
joined by the Little Colorado, which runs from south to north. Because of the
rain-shadow effect of the Mogollon Rim, the Little
Colorado draws very little
water from a relatively large watershed, usually containing a mere trickle of
water in its riverbed. The River’s principal tributary is the Gila
River, which flows all the way across the southern part of the state from New
Mexico to the California border. From the mountains and plateaus of central
Arizona, the Gila River receives the Salt, Agua Fria, and Hassayampa rivers. The
Salt River is itself fed by the Verde River. The Gila River also is joined by
rivers draining the Mogollon Rim and other mountains in the Central Highlands,
as well as mountain ranges such as the Sierriata and Santa Catalinas in the
southern part of the state. Heavy rainfalls typical in the summer months over
the Mogollon Rim drain into the Black, White, and Verde rivers. In February,
March, and April melting snow from the same regions occasionally creates flood
conditions along the Salt River and around Phoenix. These three rivers are
important tributaries to the Salt River, which is a primary source of water for
the highly-populated Phoenix metropolitan and surrounding agrarian areas.
Eleven dams control run-off from the Mogollon Rim, creating the state’s largest
lakes. Arizona has few natural lakes, and those that do exist are small.
Enormous lakes have formed behind dams built for flood control, irrigation, and
power development on major rivers such as the Verde, Agua Fria, Salt, and Gila.
The largest lake is Lake Mead, with an area of 233 sq mi, formed by
the Hoover Dam. It lies partly in Arizona and partly in
Nevada and backs into the lower portion of the Grand Canyon. Other large lakes
created by dams include Theodore Roosevelt Lake, behind the Roosevelt Dam on
Salt River; San Carlos Lake, behind the Coolidge Dam on the Gila River; Lake Havasu, behind the Parker Dam and Lake Powell, which is
partly in Utah and formed behind the Glen Canyon Dam. |
