<P> 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 agricultural civilizations--some of the most sophisticated indigenous cultures in North America--which eventually faded due to a combination of severe drought and poor land use practices . Most native peoples that inhabit the basin today are descended from other groups that settled in the region 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 later 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 . </P> <P> After most of the Colorado River basin became part of the U.S. in 1846, the bulk 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 . Lesser numbers settled in the upper basin, which was the scene of major gold strikes in the 1860s and 1870s . </P> <P> 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 . </P> <P> 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 . </P>

Who built the large dams in the colorado river basin
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