S.F.’s hydropower supply is under threat

Dry conditions may force us to switch from water sources to fossil fuels

San Francisco’s water and power supply are intimate bedfellows.

Nearly 20 percent of The City’s energy is generated by water stored at Cherry Lake and Hetch Hetchy Reservoir, a glacial valley in the northwestern corner of Yosemite National Park – the same water that spills from our taps.

By putting our water to work through the use of gravity, The City has provided fossil-fuel-free hydropower to residents for over a century. But as the state enters its third year of extreme drought and reservoir levels across the state dwindle, it’s not just The City’s water supply that’s under threat: so too is its energy.

This could spell bad news for residents and municipal services that rely on the San Francisco Public Utilities Commission’s (SFPUC) Hetchy Power Program, including public schools, fire stations, hospitals, Muni and large developments like the Salesforce Transit Center.

“The less it snows, the less water there is behind hydroelectric dams, and the less ‘fuel’ there is for producing hydropower,” said Jordan Kern, a forestry and environmental resources professor at North Carolina State University.

It also means that if reservoir levels get low enough, The City may need to look to alternative – and likely, dirtier – power sources in the short term, costing ratepayers and setting our emissions reduction goals further behind.

“The drought has cut in half, over the last two years, the amount of hydropower we’ve gotten,” said Dr. Peter Gleick, a scientist and president emeritus of the Oakland-based Pacific Institute. “And that’s a very significant impact on the renewable energy we get, the greenhouse gas emissions that we emit, and on California ratepayers’ pocketbooks.”

Touted as a low-cost and reliable energy source, the SFPUC’s Hetchy Power Program consists of nine reservoirs and three power stations along the Tuolumne River watershed that store and process water destined for the Bay Area.

At Hetch Hetchy, The City’s most well-known reservoir, our water’s journey begins at the O’Shaughnessy Dam and is then funneled through miles of tunnels and powerhouses, dropping 1,300 feet through large pipes called penstocks, designed to control flow.

As water rushes downward, this pressure turns the blades of turbines connected to generators and converts energy into electricity. That power is then transported through power lines and transmission towers before reaching The City.

California accounts for 13% of the country’s hydropower capacity, but as the state experiences more intense drought, wildfires, and a shrinking Sierra snowpack, mountain streams like the Tuolumne River are ebbing, reducing the force of water pressure needed to turn the turbines.

“The droughts we’re experiencing now are not normal. They’re abnormal,” said Gleick. “They’re a clear indication that human-caused climate change is influencing California’s water supply for the worse.”

These shutdowns are not theoretical. Last August, California’s second-largest reservoir at Lake Oroville hit a historic low, prompting the Edward Hyatt Power Plant to go offline for the first time since 1967.

But it’s not all bad news. As Northern California entered this water year, a smattering of intense October rainstorms and heavy snowpack has helped restore some of the capacity.

“We are expecting a recovery for hydrogeneration in California this year,” said Lindsay Aramayo, an industry economist with the U.S. Energy Information Administration (EIA). “This year has been much better. The precipitation was higher than average, at least until mid-January – which has added more water supply.”

Still, climate scientists say that the total precipitation hasn’t been enough to make a dent in the overall drought conditions. On Monday, as the state entered the driest three months on record, Governor Gavin Newsom issued an executive order calling on local water authorities to activate stricter conservation measures and proposed a ban on decorative turf irrigation.

While Hetch Hetchy is faring better than other reservoirs in the state, if hydroelectric power capacity continues to decline, San Francisco’s reliance on fossil fuels is poised to increase – along with the cost of energy — at least in the short term.

This is because a shortfall in hydropower, a relatively cheap power source, increases the demand for other more expensive sources of power, like natural gas, said Tyson Brown, an economist at EIA. But how residents might feel this is more complicated since agencies like the SFPUC are in charge of setting rates.

Supplementing hydropower with natural gas could also mean that The City’s emissions reduction goals face short-term setbacks. “California already gets more than half of our electricity from noncarbon energy sources, and we’re moving forward on that. But the drought and cuts in hydropower make it that much harder,” said Gleick.

Still, it’s not time to write off hydropower altogether, said Brown. Unlike wind or solar, which only generate energy when the sun is shining, or the wind is blowing, hydropower can provide critical power during high-demand periods.

“Even in drought years, hydroelectric assets provide a really important service to the grid in California,” said Brown.

And it’s unlikely that San Franciscans will be entirely without hydroelectric power in the future.

“We’ll always get rain and snow. It may become more irregular and unreliable, but we always have some hydropower. But what this does tell us is that we cannot rely on any single source of energy,” said Gleick. “San Francisco should be encouraged to diversify its electricity sources as well.”


A breakdown of The City’s water supply system storage as of March 14, 2022. (SF Public Utilities Commission)

A breakdown of The City’s water supply system storage as of March 14, 2022. (SF Public Utilities Commission)

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