By Nan W. Frobish.

Visitors to Yosemite’s iconic Hetch Hetchy reservoir are doing a double-take. Instead of seeing the majestic backdrop of the Sierra Nevada reflected in the pristine mountain water, they are now greeted by millions of black balls that cover the surface.

After four years of record-setting drought and statewide low reservoir levels, concerns developed about evaporation losses and the drought’s effect on water quality for San Francisco’s premier water source. Plans to protect the drinking supply and reduce reservoir evaporation began in 2014, when another year of dry conditions was predicted with no end to the drought in sight.

Inspired by similar measures taken at Ivanhoe Reservoir and the Los Angeles Reservoir in Southern California, 96 million black balls were poured into Hetch Hetchy to limit sunlight penetrating the water surface. Limiting sunlight on the reservoir will reduce both evaporation and the growth of potential contaminants. Given the emergency measures required to mitigate the drought’s effects on municipal water supplies, covering the reservoir was deemed more cost-effective and easier to achieve than constructing additional treatment facilities or implementing additional water conservation actions.

Milly Pore, a spokesperson from the San Francisco PEC, explained, “We are facing long-term concerns about water quality and water supply reliability and estimated that we could save a lot of water and water quality this way.”

“It’s an eye-sore,” said Louie Swan, a recent visitor to the reservoir. “It looks like an oil slick. If this is the best we can come up with to fix water quality, we might as well take the dam down altogether.”

Average 2100 deliveries, scarcity, and scarcity cost associated with O'Shaughnessy Dam removal

Average 2100 deliveries, scarcity, and scarcity cost associated with O’Shaughnessy Dam removal. Null and Lund (2006)

Previous research suggests that dam removal may be a contentious idea whose time has come for the politically charged reservoir. Dr. Sarah Null, a former researcher at the Center for Watershed Sciences, published a study of the effects of dam removal on San Francisco’s water supply. The findings suggest that the dam could be removed with little loss to water supply, but would require additional water treatment costs.

With more and prolonged droughts predicted due to climate change, those water treatment costs are becoming a reality. As water managers and conservationists are becoming aware of the “new normal” for water quality in Hetch Hetchy, dam removal is quietly being revisited.

As awareness grows of the recent management activities, local San Francisco residents are voicing additional concerns.“It’s a sensitive issue,” said one state agency representative with knowledge of the dam removal talks. “Frankly, we never thought the barriers to dam removal would [be eliminated] by natural conditions. But now that they are, it gives dam removal advocates a stronger position.”

“What are the balls made out of, anyway?” asked Matthew McPhee, a long-time Bay Area resident who was attracted to the city because of its environmental consciousness. “Are they BPA-free? Will sunlight degrade their material? We’re just trading one water quality issue for another.”

As well as addressing water quality issues, the ‘shade balls’ are also a pilot project to address broader goals set by Governor Brown to reduce evaporation in all of California’s reservoirs.

“Following last year’s 25% reduction in urban water use, we considered requiring that all reservoirs be covered to further save water,” says Rex Kransrose of the Governor’s office. “This year’s wetter conditions delayed this move, but we are glad to see San Francisco leading the way on this.”

“Water supply losses due to evaporation are a major issue,” says Dr. Mollie Luna, a researcher at the Center for Watershed Sciences. “We have enough storage, it’s preventing loss that’s the issue. We’re not sure how effective these shade balls might be, but the drought has shown us that we need to proactively consider many different approaches to secure our water supply.”

Balancing the ‘shade balls’ with recreational uses is another concern. One option is to phase out municipal supply from reservoirs with the poorest water quality, and gradually transition to solely recreational purposes. The experiment on Hetch Hetchy will provide guidance on whether the ‘shade ball’ approach can be effective for California’s extensive reservoir system.

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Originally posted at California Water Blog.

Nan W. Frobish is an occasional contributor to the California Waterblog and director of life enrichment for the UC Davis Center for Watershed Sciences.