San Francisco had secured congressional approval only months earlier when work on what would become one of California's largest water projects began in earnest in 1914. The project centered on Hetch Hetchy Valley, which is a glacier-carved valley inside Yosemite National Park that the city planned to turn into a reservoir. The decision had already triggered a national fight over whether a city should be allowed to build a dam inside a national park. Congress settled that question in December 1913, when President Woodrow Wilson signed the Raker Act, which granted San Francisco access to develop the Hetch Hetchy Valley for water and power. The San Francisco Public Utilities Commission's 2021 Urban Water Management Plan says construction of the broader Hetch Hetchy Project began in earnest in 1914.
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The National Park Service timeline records that construction of the O'Shaughnessy Dam began after the Raker Act, with the first phase completed in May 1923. Another National Park Service account notes that construction activity at the dam site began in 1919 and included a camp that could house up to 500 workers, while a railroad was built to move workers and materials into the remote valley. The first phase of the dam was completed in 1923, but the structure was later expanded. A second construction phase raised the dam to its present height of 430 feet, with the final work completed in 1938.
A fight over a valley inside Yosemite
San Francisco had been looking for a reliable water source beyond its immediate surroundings, particularly as the city grew. The 1906 earthquake and fire increased pressure to find an adequate supply, according to the National Park Service. City officials looked east to the Tuolumne River watershed, while preservationists argued that flooding a valley within Yosemite National Park would set a dangerous precedent.
John Muir became one of the best-known opponents of the project. He and other preservationists fought the proposal for years, while San Francisco officials argued that the reservoir was needed to supply the growing city. The dispute reached Congress and became a national debate over the competing uses of protected land. The National Park Service says the Raker Act passed Congress in 1913 after years of hearings and political battles, giving San Francisco the legal authority to proceed.
The fight did not stop the project, but it changed the way Hetch Hetchy was remembered. The National Park Service describes the controversy as a major early conflict between preservation and development. It also came shortly before the creation of the National Park Service in 1916, when Congress established a federal agency specifically responsible for managing the country's national parks.
The valley became a reservoir
Once the first phase of O'Shaughnessy Dam was finished in 1923, the structure began transforming the valley into Hetch Hetchy Reservoir. But San Francisco still did not immediately receive its new water supply. The wider project required an extensive system to move water from the Sierra Nevada to the Bay Area. The National Park Service says water from the Hetch Hetchy Reservoir first reached San Francisco in 1934, and by then, the project involved far more than a dam. Water had to travel through tunnels, pipelines, reservoirs and aqueduct infrastructure across the state. The system also incorporated hydroelectric facilities, with power generated downstream from the reservoir.
Hetch Hetchy Reservoir currently stretches about eight miles and is the largest single body of water inside Yosemite National Park. The reservoir holds about 117 billion gallons of water, according to the National Park Service . Swimming and boating are prohibited there because it is used as a protected drinking-water source.
Water still travels from Yosemite to the Bay Area
The infrastructure remains central to the Bay Area's water system more than a century after the Hetch Hetchy Project began. San Francisco's 2021 Urban Water Management Plan states that more than 2.7 million people rely on water supplied through the SFPUC's regional system. About 85% of that supply comes from the Tuolumne River watershed, with water collected at Hetch Hetchy Reservoir before entering the larger aqueduct system.
That water then travels roughly 167 miles by gravity toward Bay Area reservoirs and customers, and the remaining supply comes from local watersheds in the Alameda and Peninsula areas. The system built around Hetch Hetchy therefore remains only one part of San Francisco's regional water network, but it is its largest source. The dam completed in stages between the 1920s and 1938 also continues to support hydroelectric generation. Two plants downstream from the reservoir produce power as water moves through the system before continuing toward the Bay Area.
A project whose argument has never fully disappeared
The reservoir continues to provide drinking water and power, while the flooding of a valley inside a national park remains one of the most debated development decisions in the history of Yosemite. It had started as a water project in the years after the Raker Act and is now a system serving more than 2.7 million people across the San Francisco Bay Area. The first phase of O'Shaughnessy Dam was completed in 1923, the structure reached its final form in 1938, and Tuolumne River water has been reaching San Francisco since 1934. Water from Yosemite remains a major part of the region's daily supply even more than 110 years after the Hetch Hetchy Project began in earnest.
