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.
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.
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.
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.