Faulted Diabase

The Hetch Hetchy reservoir is formed by the O’Shaughnessy Dam. The city of San Francisco set its sights on the waters of the Hetch Hetchy Valley in the 1880s, and after the 1906 earthquake devastated the city, moved to take control of the Tuolumne River here and dam it. John Muir and other environmentalists fought for years to block the dam, but it was authorized by the also controversial Raker Act and completed in 1923. (Their fight and the controversy over the Raker Act helped spur the creation of the National Park Service in 1916.) Water from Hetch Hetchy still provides around a sixth of San Francisco’s water, and the city pays rent of approximately $30,000 a year (plus $8 million or so in “protection and security costs”) for its access. “Hetch Hetchy’s water could simply be stored in Cherry or Don Pedro reservoirs downstream,” Gregory says.

Due to its glacier carved U-shape, Hetch Hetchy was one of the more hospitable locations in California’s rugged Sierra Nevada. The valley was home to many Indigenous peoples both seasonally and year-round.