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Boulder Canyon Project

A large dam on the Colorado River had been envisioned since the 1920s. In 1928, Congress authorized the Reclamation Service (today's U.S. Bureau of Reclamation, or USBR) to build the Boulder Canyon Project, whose key feature would be a dam on the Colorado in Black Canyon 30 miles (48 km) southeast of Las Vegas, Nevada. On September 30, 1935, Hoover Dam was completed, forming Lake Mead, capable of holding more than two years of the Colorado's flow. Lake Mead was, and still is, the largest artificial lake in the U.S. by storage capacity. The construction of Hoover Dam stabilized the lower channel of the Colorado River, stored water for irrigation in times of drought, captured sediment and controlled floods. Hoover was the tallest dam in the world at the time of construction and also had the world's largest hydroelectric power plant.

The Boulder Canyon Project Act also authorized the All-American Canal, which was built as a permanent replacement for the Alamo Canal and follows a route entirely within the U.S. on its way to the Imperial Valley. The canal's intake is located at Imperial Dam, 20 miles (32 km) above Yuma, Arizona, which diverts the majority of the Colorado's flow with only a small portion continuing to Mexico. With a capacity of over 26,000 cubic feet per second (740 m /s), the All-American Canal is the largest irrigation canal in the world. Because the hot, sunny climate lends to a year-round growing season, the Imperial Valley has become one of the most productive farming regions in North America, providing much of the winter produce supply in the U.S. The Imperial Irrigation District supplies water to 520,000 acres (210,000 ha) south of the Salton Sea. The Coachella Canal, which branches northward from the All-American Canal, irrigates another 78,000 acres (32,000 ha) in the Coachella Valley.

Parker Dam was initially built as the diversion point for the Colorado River Aqueduct, planned by the Metropolitan Water District of Southern California to supply water to Los Angeles. The construction of the dam was opposed by Arizona, which feared that California would take too much water from the Colorado; at one point, Arizona sent members of its National Guard to stop work on the dam. Ultimately, a compromise was reached, with Arizona dropping its objections in exchange for the USBR constructing the Gila Project, which irrigates 110,000 acres (450 km) on the Arizona side of the river. By 1941, the 241-mile (388 km) long Colorado River Aqueduct was completed, delivering 1.2 million acre-feet (1.5 km) of water west to Southern California. The aqueduct enabled the continued growth of Los Angeles and its suburbs, and provides water to about 10 million people today. The San Diego Aqueduct, which branches off from the Colorado River Aqueduct in Riverside County, California, opened in stages between 1954–1971 and provides water to another 3 million people in the San Diego metro area.

The Las Vegas Valley of Nevada experienced rapid growth after Hoover Dam, and by 1937 Las Vegas had tapped a pipeline into Lake Mead. Nevada officials, believing that groundwater resources in the southern part of the state were sufficient for future growth, were more concerned with securing a large amount of the dam's power supply than water from the Colorado; thus they settled for the smallest water allocation of all the states in the Colorado River Compact. In 2018, due to declining water levels in Lake Mead, a second pipeline was completed with a lower intake elevation.