Colorado River

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 water for irrigation and municipal supplies of almost 40 million people both inside and outside the watershed. The Colorado's steep drop through its gorges is also utilized for the generation of significant 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 dewatered the lower course of the river such that it no longer reaches the sea except in years of heavy runoff.

Nomadic hunter-gatherer peoples first populated the Colorado River basin at least eight thousand years ago. Between 1,000 and 2,000 years ago, native peoples began to form large, sedentary agricultural-based civilizations, which may have been some of the most sophisticated indigenous cultures in North America. A combination of climate change and poor land use practices led to the collapse of these societies, but other native groups remained, many up to the present day. Europeans first entered the Colorado River watershed in the 1500s, with explorers from Spain mapping and claiming the area, which later became part of Mexico with its independence from Spain in 1821. Early contact between foreigners and natives was largely 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 course was still largely unknown, and the whereabouts of its headwaters and mouth were still the subject of myths and speculation. A number of 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 water supply and as a navigation route. Large scale settlement of the lower basin by whites began in the mid-to-late 1800s, with steamboats providing transportation and trade along the Colorado and Gila Rivers. Lesser amounts settled in the upper basin, which was also the scene 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 treaties both domestic and international 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. However, declines in runoff and heavy water use could lead to severe shortages by the mid-21st century, endangering power generation and water supply.