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Rim Fire near Yosemite: Threat to San Francisco water supply not over

The continuing threat to the Hetch Hetchy Reservoir from the Rim Fire near Yosemite underscores the unusually high stakes surrounding the firefighting effort. Moreover, the danger won't be over even after the fire is out, some experts say.

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The Hetch Hetchy Reservoir is hazy with smoke from the Rim Fire fills the air in Yosemite National Park, Calif., Sunday, Aug. 25, 2013. Crews working to contain one of California's largest-ever wildfires gained some ground Monday against the flames threatening San Francisco's water supply, several towns near Yosemite National Park and historic giant sequoias.

Jae C. Hong/AP

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Flames from northern California's Rim Fire – now the size of Chicago, at 234 square miles – came within one mile of the reservoir that supplies 85 percent of San Francisco's water, and water authorities have been racing to fill local water basins before ash reaches intake pumps far below the surface.

For the time being, the iconic Yosemite Valley and stands of giant sequoia appear safe, with the fire licking the fringes of Yosemite National Park. But the continuing threat to the Hetch Hetchy Reservoir underscores the unusually high stakes surrounding the firefighting effort.

“The challenges that the Rim Fire poses for firefighters, and the citizens of San Francisco, are exponentially greater than [with] a typical wilderness fire,” says firefighting expert Char Miller, a professor of environmental analysis at Pomona College in Claremont, Calif. “There can be no mistakes; there is no room for error. The fire ought to make it crystal clear to San Franciscans how critical the Sierras are to their daily life.”

Containment of the fire, which began Aug. 17, had doubled to 15 percent by 11:30 a.m. Monday Pacific time. But some of Yosemite’s backcountry hiking trails have been closed, 23 structures have been destroyed, and 4,500 more are threatened. Four thousand firefighters are assigned to the blaze, with winds hitting 30 miles per hour in an area of difficult and steep terrain that makes movement on the ground arduous.

San Francisco officials say no contamination has occurred of the water supply from Hetch Hetchy, which is renowned for its purity. Some 260 million gallons of Hetch Hetchy water flow 160 miles west, downhill through pipes leading to San Francisco. If the water becomes too turbid – too polluted by particulates from the fire – San Francisco could draw emergency waters from other reservoirs. The turbidity of Hetch Hetchy water is only 0.2 now, Charles Sheehan, a spokesman for the San Francisco Public Utilities Commission, told USA Today. The emergency threshold is 5.0.

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