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Celebration and Concern: Hetch Hetchy Reservoir Turns 100, But Climate Change Complicates its Future

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A Black woman speaks at a podium in the middle distance as others listen to her, in a space bounded by a large concrete barrier with sweeping views of mountains rising from a massive lake in the background
San Francisco Mayor London Breed speaks during a centennial celebration of the Hetch Hetchy Reservoir in Yosemite National Park on May 2, 2023. The reservoir – San Francisco's main water source – may not be as reliable a tap of Sierra Nevada snowmelt in the coming century. (Beth LaBerge/KQED)


an Francisco Mayor London Breed and a gaggle of water officials gathered in the northwest corner of Yosemite National Park on Tuesday to celebrate the centennial of the creation of the Hetch Hetchy Reservoir and the O’Shaughnessy Dam.

“Hetch Hetchy is one of the reasons why San Francisco is such a resilient city,” said Mayor London Breed with a sweeping view of mountains soaring above the water behind her, their reflections mirrored on the reservoir’s surface.

“This [dam] was created and is a testament to the creativity, and the innovation of San Franciscans,” she said.

The water system, San Francisco’s main water source, provided a stable supply of pristine Sierra Nevada snowmelt for city residents through most of the 20th century. But as human-caused climate change worsens, some water experts say the stability of San Francisco’s mountain tap is losing its surety for the 21st century and beyond.

“I no longer think it will be a reliable water system,” said Samuel Sandoval Solis, an expert in water management at UC Davis. He said the ping-pong of drought and deluge are challenging the current system and will require alterations to the water system in the future.

“It is a privilege that San Franciscans have this high-quality water,” he added.


Hetch Hetchy was a glacier-carved valley located 15 miles north of Yosemite Valley until the dam project was completed in 1923. Now it’s a massive reservoir that holds 117 billion gallons of water. A true triumph of engineering, the system relies on a 167-mile pipeline and gravity to push water down the height of Sierra Nevada, through the Central Valley, over the golden hills and into the Bay Area.

“We do shoot it with ultraviolet light to kill the bugs,” said Christopher Graham, Hetch Hetchy water operations and maintenance manager with the San Francisco Public Utilities Commission. “We don’t have to filter out the dirt from the water because it’s so clean.”

A white man in a bright yellow jacket descends concrete stairs, surrounded by metal pipes, with more concrete and a few trees visible in the far distance across the unseen water surface
San Francisco Public Utilities Commission Manager Christopher Graham descends into the O’Shaughnessy Dam at the Hetch Hetchy Reservoir in Yosemite National Park on May 2, 2023. (Beth LaBerge/KQED)

This year he said 1.4 million acre-feet of water is likely to melt into into the Hetch Hetchy Reservoir, but only a fraction of that will remain at the end of the dry season.

The water passes over three faultlines and is used for about 85% of the water needs for 2.7 million people in San Francisco and parts of Santa Clara, San Mateo and Alameda counties.

But water experts believe the next 100 years of supplying a finite resource to millions of people will be much more complicated than storing water in a mountain bathtub and piping it to the bay. Water officials will need to conserve more and might need to make engineering upgrades so that the system can store more water.

The Hetch Hetchy system has been able to handle recent severe droughts and this winter’s powerful storms, but Newsha Ajami, the president of the SFPUC, said “part of that has been driven by luck.”

“We have some of the lowest water use in the state,” she said. “San Franciscans use about 40 gallons per person daily, which is very low.”

The SFPUC operates the reservoir and expects the Hetch Hetchy system will be tested by even more extreme drought and deluge.

“This year was one of the wettest years we’ve ever seen. Right before we had the driest three-year sequence we had ever seen,” said Graham. “We are seeing what the climate change models are forecasting, wetter wet periods and drier dry periods, which makes managing all this quite a bit more difficult.”

The water supply may seem stable in a wet year, Graham said, but managing the runoff from a massive Sierra snowpack takes constant attention, especially with no guarantee future years will also be wet. Come August, he said, the reservoir will remain glassy and brimming.

“For now, all I’m trying to do is maximize what we got and the storage capacity we have,” he said. “Climate change is going to make managing this water supply much more difficult.”

A smiling Black woman speaks while gesturing with one hand, wearing a fingerless glove and gray jacket, as the curving concrete arc of a massive dam spreads out at eye level behind her
San Francisco Mayor London Breed looks out from the O’Shaughnessy Dam at the Hetch Hetchy Reservoir during Tuesday’s centennial celebration. (Beth LaBerge/KQED)

While San Franciscans don’t use much water, other parts of the Bay Area overuse the resource. Laura Feinstein with the non-profit public policy group SPUR analyzed water use within the system and found that communities like Hillsborough in San Mateo County that receive water from the Hetch Hetchy system use 190 gallons of water per person per day.

“They use water generously even in the winter when it rains,” she said of residents who continue to water large yards and landscapes regardless of season. “Those types of inefficiencies put a lot of pressure on the system. Making communities like that more water-efficient would mean a lot of savings and make the whole system more climate resilient.”

Hetch Hetchy’s success will depend on rethinking its use

Susan Leal wants to ensure that Hetch Hetchy exists as a thriving water resource in the face of human-caused climate change. She is a former general manager of the SFPUC.

She sees three possibilities for the future of water originating from Hetch Hetchy: it will become more expensive over time, officials begin to recycle it on a large scale or they raise the reservoir level.

Regarding recycling water, Leal said there is “no alternative,” and city officials must give serious thought this year to creating more recycled water plants. Recycled water, she said, costs more but could relieve pressure on the system.

“To get people off bottled water, we kept telling them how good their Hetch Hetchy water is,” she said. “We have to let people understand that recycled water is like distilled water. It’s very pure water.”

the concrete top of a dam stretches away at odd angles into the distance on a cloudy day as dark water can be seen far below, with mountains in the background
The O’Shaughnessy Dam holds back the Tuolumne River, forming Hetch Hetchy Reservoir in Yosemite National Park. (Beth LaBerge/KQED)

Another option for increasing the water supply is to raise the dam to hold more water. In 1938, officials raised it from 227 feet to 312 feet. Leal said O’Shaughnessy Dam, which holds back the Tuolumne River, could be built 55 feet taller than its height today.

“We may need to build the dam higher to impound more water,” she said. “We have to consider this because we never planned for the extreme storms.”

Raising the dam and increasing the holding capacity of the reservoir would likely come with substantial pushback from environmentalists and would likely be tied up with lawsuits.

“I don’t know if it’s being considered, but whether it’s recycling or impounding more water at Hetch Hetchy or other dams, all these things you have to start thinking about yesterday,” she said.

100 years marks the ‘sacrifice of the environment’

The story of Hetch Hetchy isn’t all about free flowing, pure drinking water, and its construction kicked off one of the first major U.S. fights over land use and conservation. When officials constructed Hetch Hetchy, the reservoir cut off waterways to the fish, flora and other fauna that relied on flowing water that is now largely stored behind a cement wall deep in the Sierra Nevada.

UC Davis’ Sandoval Solis said the building of the reservoir, while good for many people in the Bay Area, has devastated riparian freshwater ecosystems.

“The environment has been compromised already for 100 years,” said Sandoval Solis. “The centennial also marks the centennial of the compromise of the environment or the sacrifice of the environment, and some of the native communities displaced for the sake of the well-being of people living in San Francisco.”

Hetch Hetchy is the ancient homeland of as many as a dozen Indigenous peoples; across Yosemite many were forcibly removed or killed in the mid-1800s.

A sweeping panorama photograph shows a massive lake reflecting the shapes of steep rounded granite peaks rising above it, with alpine forest also visible
The Hetch Hetchy Valley has been inundated with water since the construction of the O’Shaughnessy Dam 100 years ago. (Beth LaBerge/KQED)

Damming rivers across the Sierra Nevada, like the Tuolumne River at Hetch Hetchy, have ramifications on the river systems that unite in the San Joaquin River, flow into the San Joaquin-Sacramento Delta and push into the bay.

The large amounts of freshwater that once flowed helped mix natural algae into the salty water. Still, lesser flows into the bay mean the algae sits on top of the water, creating the perfect habitat for toxic algae blooms. Olivia Yip, an associate professor at San Jose State University who studied algae blooms, said the last outbreak that killed thousands of fish was partly due to decreased freshwater flows.

“Imagine a little algae floating around in the bay,” said Yip. “If there’s more mixing, then it’s less likely that it can just sit there on the top and grow like crazy.”

The future of the system must include equitable use of water, say advocates

The Hetch Hetchy pipeline runs through communities that need clean drinking water themselves in the Central Valley and the Bay Area. UC Davis’ Sandoval Solis said access to clean drinking water should “not be a luxury because it is a human right.”

“The pipes are passing through the middle of their communities, but they cannot use it,” he said. “I don’t think there should be a problem with providing clean water to disadvantaged communities.”

Around two-thirds of the pure Hetch Hetchy water in the Bay Area is used outside San Francisco’s bounds.

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The pipeline travels across the Dumbarton Bridge and emerges right next to the community of East Palo Alto, which gets around 80% of its water from the source. The other 20% comes from local sources, and residents question its purity, said Miriam Yupanqui, executive director of the local advocacy group Nuestra Casa de East Palo Alto.

Yupanqui dreams of the day when 100% of the water for this town of more than 90% people of color is pure piped in mountain fresh. She said many of the more than 15,000 residents her agency represents want all of the city’s water sourced from Hetch Hetchy.

“Our community members want quality water,” she said. “They want to feel comfortable drinking the water. Many of our community members feel that our neighbors in Palo Alto and Menlo Park are not receiving second-rate water. So why should they?”

Palo Alto and Menlo Park receive all of their water from the SFPUC.

In the meantime, groups like Climate Resilient Communities in East Palo Alto are working to provide hyper-local water sources, like cisterns connected to gutters to catch rainwater.

“That’s a practice that should be used here with the uncertainties of rainfall patterns and extreme events,” said Violet Wulf-Saena, founder and executive director of the nonprofit. “Every home should have a water tank to capture and hold the water that will reduce not only the flooding but also help conserve and use it when there are droughts.”

But to have any real impact on water conservation, reducing flooding risk and increasing water supply, Wulf-Saena said they “need thousands of rain gardens.”


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