Clean Energy SF: San Francisco, fog then sun, est. 2026

Vol. 1 - No. 14 - Thursday, September 24, 2026 - Free to readers

Hydro ยท Analysis

The city's mountain hydropower, a century on

San Francisco runs a carbon-free hydropower system fed by Sierra snow. Here is how it works, who it powers, and why dry years force a hard choice between water and electricity.

Filed October 6, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. From snowmelt to your streetlight

If you rode a Muni streetcar down Market Street this morning, or walked under a lit intersection in the Mission after dark, the electricity behind that trip very likely started as snow on the Sierra Nevada, roughly 160 miles to the east. San Francisco owns and runs a mountain hydropower system that most residents never think about. The Hetch Hetchy system, operated by the San Francisco Public Utilities Commission (the SFPUC, the city agency in charge of water, sewer, and power), holds Tuolumne River water behind O'Shaughnessy Dam in a glacier-carved valley inside Yosemite National Park, then sends it downhill through a series of powerhouses on its long trip to the Bay.

That water does two jobs at once. It is the drinking supply for roughly 2.7 million Bay Area people at last public report, and on the way down it spins turbines that make carbon-free electricity for city operations. Start at the top: water released from the dam runs through powerhouses the SFPUC has operated for decades, including the Kirkwood, Moccasin, and Holm plants strung along the system, which together generate on the order of a few hundred megawatts of capacity at last public report, enough to cover a large share of the city government's own electricity needs in a normal year.

II. Who the power actually serves

Here is where many San Franciscans get confused, because it is not your apartment. Hetch Hetchy Power serves municipal load: Muni, San Francisco International Airport, public schools, streetlights, libraries, and city buildings. Homes and most businesses are served either by PG&E or by CleanPowerSF, the city's community choice program (a setup where the city buys the electricity supply while PG&E still runs the wires and sends the bill).

The two systems are related but separate, and mixing them up is easy. When you read that San Francisco runs on its own clean hydropower, that is true of the city as an institution, not necessarily of your meter. If you want to know what powers your home, the answer is on your bill and on the CleanPowerSF program page, not in the story of the dam.

III. A century since O'Shaughnessy

Congress opened the door with the Raker Act in 1913, which granted San Francisco the right to build inside a national park, a decision the naturalist John Muir fought bitterly and lost. O'Shaughnessy Dam was finished in the early 1920s, and the first Sierra water reached the Bay Area later that decade, after one of the largest public works efforts the West had seen.

More than a century on, the system is still the backbone of the city's own carbon-free power. The SFPUC describes the electricity it supplies to municipal customers as greenhouse-gas-free, which is a real achievement for a government that size. It is also aging infrastructure in earthquake country, and keeping dams, tunnels, and powerhouses in repair is a permanent, expensive job rather than a finished one.

IV. Water versus power in a dry year

In California, water comes first, and that rule shapes everything about Hetch Hetchy in a drought. When the Sierra snowpack runs thin, the SFPUC manages the reservoir to protect drinking supply and required river releases, which can leave less water to send through the turbines. Generation falls, and the city has to buy replacement power on the open market to keep Muni and the airport lit, often at higher cost.

Climate change makes that math harder to predict. Warmer winters can mean more rain and less snow, earlier melt, and a snowpack that no longer acts like the slow-release battery the system was designed around. The honest answer is that nobody can tell you exactly how much power Hetch Hetchy will make in a given year, because it depends on a snowpack that is itself getting harder to forecast.

V. Worth watching this month

1. The SFPUC posts its commission meeting agendas at sfpuc.gov, where power rates and Hetch Hetchy operations come up regularly, most of it routine but worth a scan.

2. California's Department of Water Resources runs snow surveys through the winter, and the reading around April 1 is the one that best signals how much water, and power, the coming year holds.

3. CleanPowerSF sends rate and enrollment notices to its customers, so if you are served by it, the mail and the program website are where any change will show up first.

4. The California ISO, which runs the state grid, issues grid alerts during heat waves and cold snaps, and those are the days replacement power gets expensive for everyone, the city included.

Frequently asked questions

Does Hetch Hetchy power my home?

Probably not directly. Hetch Hetchy Power serves city operations like Muni, the airport, schools, and streetlights. Most homes are served by PG&E or by CleanPowerSF, the city's community choice program.

Is the power really carbon-free?

The hydro generation itself produces no combustion emissions, and the SFPUC describes its municipal supply as greenhouse-gas-free. In dry years, though, the city may buy market power to cover shortfalls, which is not always as clean.

Why is there a dam inside a national park?

The Raker Act of 1913 gave San Francisco that right, over strong objections from John Muir and others. Proposals to study restoring the valley have surfaced from time to time, but the dam remains central to the city's water and power.

What happens to the power in a drought?

Water deliveries get priority, so generation usually drops when the snowpack is thin. The SFPUC then buys replacement electricity on the market, often at higher cost.

References