The Colorado River is one of the principal rivers in the Southwestern United States and in northern Mexico. The 1,450-mile-long (2,330 km) river drains an expansive, arid watershed that encompasses parts of seven U.S. states and two Mexican states. Starting in the central Rocky Mountains of Colorado in Grand Lake, Colorado, the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border. After entering Mexico, the Colorado approaches the mostly dry Colorado River Delta at the tip of the Gulf of California between Baja California and Sonora.
Known for its dramatic canyons, whitewater rapids, and eleven U.S. National Parks, the Colorado River, with its tributaries, is a vital source of water for 40 million people. The river and its tributaries are controlled by an extensive system of dams, reservoirs, and aqueducts, which in most years divert its entire flow for agricultural irrigation and domestic water supply. Additionally, some of its waters are transported via a series of dams, tunnels, and canals into the Rio Grande watershed. Its large flow and steep gradient are used for generating hydroelectric power, and its major dams regulate peaking power demands in much of the Intermountain West. Intensive water consumption has dried up the lower 100 miles (160 km) of the river, which has rarely reached the sea since the 1960s.
Beginning with small bands of nomadic hunter-gatherers, Native Americans have inhabited the Colorado River basin for at least 8,000 years. Between 2,000 and 1,000 years ago, the watershed was home to large agricultural civilizations—considered some of the most sophisticated indigenous North American cultures—which eventually declined due to a combination of severe drought and poor land use practices. Most native peoples that inhabit the region today are descended from other groups that settled there beginning about 1,000 years ago. Europeans first entered the Colorado Basin in the 16th century, when explorers from Spain began mapping and claiming the area, which became part of Mexico upon its independence in 1821. Early contact between Europeans and Native Americans was generally limited to the fur trade in the headwaters and sporadic trade interactions along the lower river. After most of the Colorado River basin became part of the U.S. in 1846, much of the river's course was still the subject of myths and speculation. Several expeditions charted the Colorado in the mid-19th century—one of which, led by John Wesley Powell, was the first to run the rapids of the Grand Canyon. American explorers collected valuable information that was later used to develop the river for navigation and water supply. Large-scale settlement of the lower basin began in the mid- to late-19th century, with steamboats providing transportation from the Gulf of California to landings along the river that linked to wagon roads to the interior. Starting in the 1860s, gold and silver strikes drew prospectors to parts of the upper Colorado River basin.
Large engineering works began around the start of the 20th century, with major guidelines established in a series of international and U.S. interstate treaties known as the "Law of the River". The U.S. federal government was the main driving force behind the construction of dams and aqueducts, although many state and local water agencies were also involved. Most of the major dams were built between 1910 and 1970; the system keystone, Hoover Dam, was completed in 1935. The Colorado is now considered among the most controlled and litigated rivers in the world, with every drop of its water fully allocated. The environmental movement in the American Southwest has opposed the damming and diversion of the Colorado River system because of detrimental effects on the ecology and natural beauty of the river and its tributaries. During the construction of Glen Canyon Dam, environmental organizations vowed to block any further development of the river, and a number of later dam and aqueduct proposals were defeated by citizen opposition. As demands for Colorado River water continue to rise, the level of human development and control of the river continues to generate controversy.
The Colorado River and its tributaries often nourish extensive corridors of riparian growth as they traverse the arid desert regions of the watershed. Although riparian zones represent a relatively small proportion of the basin and have been affected by engineering projects and river diversion in many places, they have the greatest biodiversity of any habitat in the basin. The most prominent riparian zones along the river occur along the lower Colorado below Davis Dam, especially in the Colorado River Delta, where riparian areas support 358 species of birds despite the reduction in freshwater flow and invasive plants such as tamarisk (salt cedar). Reduction of the delta's size has also threatened animals such as jaguars and the vaquita porpoise, which is endemic to the gulf. Human development of the Colorado River has also helped to create new riparian zones by smoothing the river's seasonal flow, notably through the Grand Canyon.
More than 1,600 species of plants grow in the Colorado River watershed, ranging from the creosote bush, saguaro cactus, and Joshua trees of the Sonoran and Mojave Deserts to the forests of the Rocky Mountains and other uplands, composed mainly of ponderosa pine, subalpine fir, Douglas-fir and Engelmann spruce. Before logging in the 19th century, forests were abundant in high elevations as far south as the Mexico–U.S. border, and runoff from these areas nourished abundant grassland communities in river valleys. Some arid regions of the watershed, such as the upper Green River valley in Wyoming, Canyonlands National Park in Utah and the San Pedro River valley in Arizona and Sonora, supported extensive reaches of grassland roamed by large mammals such as buffalo and antelope as late as the 1860s. Near Tucson, Arizona, "where now there is only powder-dry desert, the grass once reached as high as the head of a man on horse back".
Rivers and streams in the Colorado basin were once home to 49 species of native fish, of which 42 were endemic. Engineering projects and river regulation have led to the extinction of four species and severe declines in the populations of 40 species. Bonytail chub, razorback sucker, Colorado pikeminnow, and humpback chub are among those considered the most at risk; all are unique to the Colorado River system and well adapted to the river's natural silty conditions and flow variations. Clear, cold water released by dams has significantly changed characteristics of habitat for these and other Colorado River basin fishes. A further 40 species that occur in the river today, notably the brown trout, were introduced during the 19th and 20th centuries, mainly for sport fishing.