As the Rim Fire blazed out of control in California’s Yosemite National Park last month, the people of the San Francisco Bay Area, 167 miles west, started getting nervous. The fire, with its billowing plumes of ash, was moving close to the Hetch Hetchy reservoir, which provides 85 percent of the drinking water supply for San Francisco and also meets the water needs of 28 communities in three surrounding counties, serving some 2.6 million customers in all.
The Hetch Hetchy system’s unique character and remarkable purity made the fire especially threatening. Its water, which reaches the Bay Area through a complex system of pipes and tunnels, is so clean that it does not require filtration (though it is disinfected with ultraviolet rays before use). Among municipal water systems with that distinction, San Francisco’s is the third largest in the nation. The quality of its drinking water is superb.
But the lack of a filtering system means something more ominous in the face of a fire that covered nearly 300,000 acres. Had ash falling on the reservoir created sufficient “turbidity” — the water engineering term for “cloudiness,” measured in Nephelometric Turbidity Units (NTU) — the city would be hard pressed to make its water fit to drink. San Francisco does have some filtration systems available for water that comes from sources other than Hetch Hetchy, but it doesn’t have the capacity to treat the nearly 32 million cubic feet that flow down from Yosemite’s pristine heights each day.
Hetch Hetchy has always been a special case. The reservoir was created by damming the Tuolomne River and flooding the Hetch Hetchy Valley in Yosemite National Park, an engineering feat sanctioned by an act of Congress in 1913 and passed to give San Francisco a reliable water source after the 1906 earthquake. Naturalist and conservation pioneer John Muir, founder of the Sierra Club, fought the plan for years — and lost.
Today the dammed river provides not only water but also a significant amount of hydroelectric power to the city’s municipal facilities, including the airport, the MUNI transit system and San Francisco General Hospital. A ballot measure requiring the city to study how to do without Hetch Hetchy water and restore the valley to its original natural grandeur was soundly defeated last fall.
As of this writing, it looks like Hetch Hetchy is out of danger, even though the Rim Fire — the fourth largest wildfire in the state’s history — marched right up to the reservoir’s banks. Two of three hydroelectric power stations harnessing the Hetch Hetchy flow are back in operation. The city’s Public Utilities Commission, which runs the water system, says that the measurement of turbidity this week came in at between 0.2 and 0.3 average NTU, “far below the drinking water standard of 5.0 NTU.”
Other problems, however, could come down the road. The fire’s long-term impacts are still unknown, but one major concern is the risk of erosion on the slopes around the reservoir. The entire southern edge of Hetch Hetchy was burned, and landslides on scorched hills during the upcoming rainy season could pose a new threat to its water.
The Rim Fire is only the latest reminder of the Bay Area water supply’s vulnerability. While the cause of the blaze is still unknown, it is unlikely to be the last major fire in the region.
“We need to have alternative supplies and contingency plans for events that might result from climate change,” says Laura Tam, sustainable development policy director for SPUR, a San Francisco non-profit that has conducted a long-term analysis of the region’s water needs. Tam notes that while it’s not possible to attribute any single event to climate change, the state just endured its driest winter on record. The Sierra snowpack, which feeds the Hetch Hetchy supply, logged in at just 52 percent of its historic norm.
Hetch Hetchy water is at risk of earthquakes as well. All four of the system’s pipelines cross the Hayward Fault in the East Bay, where the U.S. Geological Survey says a quake of magnitude 6.8 or greater “is increasingly likely.”
Tam says that San Francisco is working to diversify its water portfolio and bolster conservation and recycling in order to reduce its dependence on Hetch Hetchy. It could, for instance, take groundwater traditionally used for irrigation and replace it with recycled water for the drinking supply.
Such efforts are urgently necessary, Tam says, as the Rim Fire has demonstrated. “Every community’s water portfolio needs to be evaluated for resilience and alternative sources,” she says. “Emergency planning and disaster planning are really critical.”
Watermark is made possible with the support of the Surdna Foundation.
Sarah Goodyear has written about cities for a variety of publications, including CityLab, Grist and Streetsblog. She lives in Brooklyn.