The Colorado River is the principal river of the southwestern United States and northwest Mexico. The 1,450-mile (2,330 km) river drains an expansive, arid watershed that encompasses parts of seven U.S. and two Mexican states. Rising in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau before reaching Lake Mead on the Arizona–Nevada line, where it turns south towards the international border. After entering Mexico, the Colorado forms a large delta, emptying into the Gulf of California between Baja California and Sonora.
Known for its dramatic canyons and whitewater rapids, the Colorado is a vital source of water for agricultural and urban areas in the southwestern desert lands of North America. The river and its tributaries are controlled by an extensive system of dams, reservoirs and aqueducts, which furnish irrigation and municipal water supply for almost 40 million people both inside and outside the watershed. The Colorado's 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. Since the mid-20th century, intensive water consumption has dried the lower 100 miles (160 km) of the river such that it no longer reaches the sea except in years of heavy runoff.
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 river and its tributaries fostered large, sedentary agricultural civilizations, which may have been some of the most sophisticated indigenous cultures in North America. These societies are believed to have collapsed due to a combination of severe drought and poor land use practices. Most native peoples that inhabit the river basin today are descended from other groups that settled in the region beginning about 1,000 years ago. Europeans first entered the Colorado River watershed in the 16th century, when explorers from Spain began mapping and claiming the area, which later became part of Mexico upon its independence from Spain in 1821. Early contact between foreigners and natives was generally limited to the fur trade in the headwaters and sporadic trade interactions along the lower river.
After the Colorado River basin became part of the U.S. in 1846, the river's course was still largely unknown, and the whereabouts of its headwaters and mouth were still the subject of myths and speculation. Several expeditions charted the Colorado in the mid-19th century, of which one was the first to run the rapids of the Grand Canyon, led by John Wesley Powell in 1869. American explorers collected valuable information that would later be used to investigate the feasibility of developing 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 and trade along the Colorado and Gila rivers. Lesser numbers settled in the upper basin, which was also the setting of major gold strikes in the 1860s and 1870s.
Major engineering of the river basin began around the start of the 20th century, with many guidelines for development established in a series of domestic and international treaties known as the "Law of the River". The U.S. federal government was the main driving force behind the construction of hydraulic engineering projects in the river system, although many state and local water agencies were also involved. Most of the major dams in the river basin were built between 1910 and 1970, with the system keystone, Hoover Dam, completed in 1935. Because of these developments, the Colorado River is now considered among the most controlled and litigated in the world, with every drop of its water fully allocated. High rates of water removal for irrigation and industry combined with declines in natural runoff due to climate change could lead to severe shortages by the mid-21st century, endangering power generation and water supply.
The construction of dams and associated water export schemes on the Colorado River system have been a flashpoint for the environmental movement in the American Southwest, due to their impacts 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.