What large water reservoir, which serves much of California, is located in Yosemite National Park?

Deep within Yosemite National Park lies the primary water source for 2.7 million people in the San Francisco Bay Area. The O'Shaughnessy Dam, completed in 1923 on the Tuolumne River, flooded the magnificent Hetch Hetchy Valley to create this massive reservoir. The system delivers some of the cleanest municipal water in the United States, flowing over 160 miles to the Bay Area via the Hetch Hetchy Aqueduct.

The creation of the reservoir was one of the most significant and contentious environmental debates in American history. Naturalist John Muir, founder of the Sierra Club, led a passionate national campaign to prevent the damming of what he called a "second Yosemite Valley." He and other preservationists argued that a national park should remain untouched for future generations to enjoy.

Despite their efforts, the devastating 1906 San Francisco earthquake and fire intensified the city's need for a reliable water supply, and Congress ultimately passed the Raker Act in 1913, authorizing the project. The battle over Hetch Hetchy galvanized the American conservation movement, and the debate continues today, with some groups advocating for the dam's removal and the restoration of the valley.