I | INTRODUCTION |
Colorado (river, North
America), river in the southwestern United States and northwestern
Mexico. The Colorado River is the major source of water for the surrounding
region, which receives very little rainfall. More than a thousand years ago,
native peoples used the river to irrigate crops. Today, people still depend on
the Colorado for irrigation, but they also use it to generate hydroelectric
power and to supply water to urban areas.
The river is 2,330 km (1,450 mi) long. The
Colorado River system, including the Colorado River and its tributaries, drains
an area of 637,000 sq km (246,000 sq mi). All the lands that these waterways
drain make up the Colorado River basin. The Colorado River basin’s total
runoff is approximately 700 cubic meters (about 24,700 cubic feet) per second.
A 1944 treaty requires the United States to
allow a yearly average of 1.5 million acre-feet (1.85 billion cubic meters) of
water to cross the border into Mexico. This amount represents enough water to
cover 1.5 million acres (an area about the size of the state of Delaware) to a
depth of one foot. Mexico consumes most of the Colorado River water that enters
its boundaries, and only a trickle reaches the Gulf of California.
II | DESCRIPTION |
The source of the Colorado River is Grand
Lake, Colorado, in Rocky Mountain National Park. The river initially flows to
the southwest through the state of Colorado and is joined by the Green River in
central Utah. It runs through Utah’s majestic canyons cut in the colorful
sedimentary rocks of the Colorado Plateau, past Arches and Canyonlands national
parks. In southern Utah, Glen Canyon Dam holds back, or impounds, the water of
the Colorado, forming Lake Powell.
The river then continues westward across
northern Arizona through the Grand Canyon. On the border of Arizona and Nevada,
the Colorado runs through Black Canyon, where Hoover Dam has created a reservoir
known as Lake Mead. From this point the river flows primarily southward, forming
the boundary between Arizona on the east and Nevada and California on the west.
Finally, the Colorado crosses into Mexico, where it becomes the border between
the states of Sonora and Baja California.
The Colorado River's major upper tributaries
rise in the central Rocky Mountains in Colorado and Wyoming. These tributaries
include the Yampa, Green, Gunnison, Dolores, and San Juan rivers. They are fed
by rainfall and melting snow from the high mountains. They then merge into the
Colorado and supply most of its water flow.
The lower tributaries of the Colorado include
the Virgin, Little Colorado, Salt, and Gila rivers. They drain semiarid and arid
portions of both the Colorado Plateau and the Basin and Range (see Basin)
region of the Southwest. These tributaries contribute only a modest amount of
water to the Colorado, but they add large amounts of sediment. This sediment
gives the river its characteristic reddish color.
Along the Colorado’s banks, river water
nourishes a narrow band of natural vegetation that includes willow, cottonwood,
and mesquite trees and shrubs such as seepwillow and arrowweed. Along the lower
portion of the river, however, vegetation has diminished because irrigation as
well as residential, commercial, and industrial water use have significantly
reduced the flow of the river. In addition, some native vegetation has been
displaced by newly introduced species, most notably desert shrubs such as
tamarisk (salt cedar) and the Russian olive (Oleaster) which have spread rapidly
through the Colorado River system.
Over thousands of years the river has built
up a tremendous delta at its mouth. A delta is an accumulation of sediment or
silt that a river deposits where it empties into an ocean. At one time, the
Colorado entered the Gulf of California near the present site of the city of
Yuma, Arizona. Because of the growth of the delta, the mouth of the Colorado is
now located approximately 100 km (60 miles) south of Yuma.
III | ECONOMIC IMPORTANCE |
Many people consider the Colorado River
system to be the most important natural resource of the southwestern United
States. Without its water, the region could support very little agriculture, and
major cities such as Phoenix, Arizona, and Las Vegas, Nevada, could not have
grown to their present size. About two-thirds of the water flowing in the
Colorado and its tributaries is used for irrigation, and the rest supplies urban
areas, evaporates, or nourishes vegetation along the riverbanks.
Nearly 17 million people depend on the
Colorado’s waters. The population of the basin has expanded quickly in recent
years. The most rapid growth is in urban areas, where about 80 percent of the
region's residents live. Phoenix, Tucson, and Las Vegas are the largest cities
in the basin, and they use the Colorado and its tributaries as their primary
source of water. The Colorado River offers the only renewable water supply in
the region.
During the 20th century, numerous dams were
built on the Colorado and its tributaries. These dams generate electricity,
control floods, and provide recreational opportunities. They also store water
which is a highly important function because the river’s flow is not evenly
spread throughout the year, nor is it consistent from year to year. Extra water
that flows in wet periods can be stored until it is needed. Together the dams in
the basin can impound more than 86 billion cubic meters (over 304 billion cubic
feet) of water, which is about four times the river’s average annual runoff. The
largest of these facilities is Hoover Dam, located on the border between Nevada
and Arizona and completed in 1936. The second largest is Glen Canyon Dam, which
is in north central Arizona and began operating in 1964. These two dams account
for about 80 percent of the basin’s water-storage capacity.
The water concentrated at dam sites
throughout the Colorado River system is used to provide hydroelectric power
(see Waterpower) for the region. Hydroelectric generation totals about 12
billion kilowatt-hours per year, which is roughly equivalent to one-sixth of the
electricity consumed in Arizona each year. This power is shared among several
states.
Dams have also controlled flooding and
permitted development of land along formerly flood-prone sections of the lower
river. In addition, some of the reservoirs created by dams have been
incorporated into national recreation areas. The Glen Canyon National Recreation
Area includes Lake Powell, while the Lake Mead National Recreation Area is made
up of Lake Mead, formed by Hoover Dam, and Lake Mohave, formed by Davis Dam.
Water from the Colorado River is also
diverted from the river’s main course and transported great distances. In fact,
many users live outside the Colorado River basin. For example, water that would
normally flow in the Colorado River is diverted eastward across the Rocky
Mountains to Denver and other cities in Colorado. The Colorado River Aqueduct
carries water to metropolitan Los Angeles, and the Central Arizona Project
supplies the Phoenix and Tucson areas. The All-American Canal provides water for
the Imperial Valley of southern California, once a desert but now a productive
agricultural area.
IV | HISTORY |
The Native Americans who originally lived in
the region developed a complex urban culture more than 1,000 years ago. These
people, some of whom may have belonged to a group known to archaeologists as the
Anasazi (see Pueblo (people)), constructed elaborate canal and reservoir
systems to divert water from the Colorado and its tributaries. They used the
water to irrigate fields of corn, beans, and other crops. Little is known about
their civilization or why they abandoned the area sometime between ad 1300 and 1400, but their descendants,
the Hopi and Pueblo people, continue to farm and irrigate using the river's
waters.
The first European to visit the river was
probably Spanish soldier and explorer Francisco de Ulloa, who explored the mouth
of the Colorado in 1539. In 1540 and 1541 another Spaniard, Francisco Vásquez de
Coronado, traveled through much of the region around the Colorado River. An
exploring party from the Coronado expedition, led by Garcia López de Cárdenas,
was probably the first group of Europeans to view the Grand Canyon. In
1540 the viceroy of Mexico also sent Hernando de Alarcón, a navigator, to supply
Coronado by sea. Alarcón failed to locate Coronado’s party but traversed parts
of the Colorado River with his own men, twice making the difficult passage from
the river's mouth to the vicinity of the Gila River, near present-day Yuma,
Arizona.
Native people and explorers knew the river by
many different names. Alarcón may have originally called it Rio de Buena Guia
(River of Good Guidance), but the name Colorado probably came from early Spanish
settlers who had noted its reddish color. The color was caused by the large
amounts of sediment the river carried.
In 1869 American geologist John Wesley Powell
made the first detailed exploration of the Colorado River, the Green River, and
the surrounding territory. During that expedition, Powell and his party
navigated through the Grand Canyon in wooden boats. In 1871 Powell embarked on
another dangerous descent of the Colorado. Powell wrote a report of his journey
for the government in which he suggested that the land could support only a
limited amount of irrigated agriculture.
In 1902, however, the U.S. government began
to construct large-scale dams and irrigation systems after the Reclamation
Service (now the Bureau of Reclamation) was established. The water provided by
these projects was divided up by the seven states of the Colorado River basin.
In 1922 these states agreed in the Colorado River Compact to regulate the use of
the river’s water, and Congress approved this agreement in 1929. The compact
assumed that the annual flow of the river was 15 million acre-feet. The 15
million acre-feet were split equally between the upper basin states (Colorado,
Utah, Wyoming, and New Mexico) and the lower basin states (California, Nevada,
and Arizona). The states in each basin then divided the water among themselves.
However, the compact did not resolve all the disputes surrounding water
allocation, and competition among users continues. See also Water Policy
in the United States
The greatest water quality problem in the
Colorado River today is salinity, the amount of solids, primarily salt, in the
water. The river receives dissolved salts from a variety of sources; most of
them, such as soils and rocks, are natural. When river water is used for
irrigation, some evaporates, and salt becomes concentrated in the remaining
water that returns to the river. The salinity problem is also caused by
evaporation from reservoir surfaces and water use by plants along the river. In
the lower river, the concentration of salt in the water is so high that it
cannot be used for human consumption without treatment. A desalination plant
near the border with Mexico removes salt from the river and enables the United
States to provide Mexico with usable water.
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