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California has perhaps the most tightly managed water distribution system in the world. The intensive cultivation of the Central Valley (8% of total US agri output) and the major urban centers of California like LA simply could not exist in their current form without this system.
The Central Valley produces 8% of America's agricultural output (including 40% of the fruit and nuts) on 1% of America's farmland. https://ca.water.usgs.gov/projects/central-valley/about-central-valley.html
Careful management of the Los Angeles River was estimated to be able to support 250k people. LA had grown from 5k in 1870 to 100k in 1900. The need to secure new water supplies became apparent.
Thanks to the machinations of civil engineer, William Mulholland, those new water supplies would be secured. He was able to outmaneuver the Bureau of Reclamation and secure the water of the Owens Valley for LA. Owens Lake and its farming community would vanish.
Owens Valley would not suffice for LA's ambitions. An aqueduct from Lake Havasu impounding the Colorado River at Parker Dam would be finished in 1935 to provide LA with more water. Water began to flow in 1939.
The Colorado River Aqueduct runs for 242 miles (389 km). Average annual throughput is 1,200,000 acre⋅ft (1.5 km3)
The Los Angeles Aqueduct runs for 419 mi (674 km) and delivers an average of 400k acre feet (0.49 km3).
The Colorado River also provides water to the Imperial Valley via the All American Canal. The water impounded at Imperial Dam is moved through the canal to the Imperial Valley.
The All-American Canal is 82 miles (132 km) long providing irrigation for 630,000 acres (250,000 ha) of farmland. The Salton Sea was accidentally restored by water diversions in the valley.
In the early 20th century, the inadequacy of San Francisco's water supply was made apparent. The Tuolumne River in the Hetch Hetchy Valley was impounded by O'Shaughnessy Dam to provide water for the Bay Area.
The Hetch Hetchy Aqueduct runs for 167 mi (269 km) providing 31,900,000 cu ft (900,000 m3) of water per day to the Bay Area.
The State Water Project moves 2,400,000 acre feet (3.0×109 m3) of water in a year through 701.5 miles (1,129.0 km) of canals irrigating 750,000 acres (300,000 ha) and providing drinking water to 23 million people.
The principal feature of the SWP is the 444 mi (715 km) California Aqueduct, which can move up to 13,100 cu ft/s (370 m3/s) of water.
Integrated with the SWP is the Bureau Reclamation's Central Valley Project. The CVP moves 7.4 million acre feet (9.1 km3) of water through 643.6 miles (1,035.8 km) of canals annually irrigating 3,000,000 acres (1,200,000 ha) of farmland.
In total, California's interconnected mix of municipal, state, federal and private water management projects provides 40 million acre feet (49 km3) of water per year to more than 30 million people and irrigates 5,680,000 acres (2,300,000 ha) of farmland.
