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Ash from Yosemite wildfire drops into San Francisco reservoir

| Monday, Aug. 26, 2013, 9:57 p.m.

TUOLUMNE CITY, Calif. — A raging wildfire in Yosemite National Park rained ash on the reservoir that is the chief source of San Francisco's pure drinking water, and utility officials on Monday scrambled to send more water toward the metropolitan area before it becomes tainted.

Nearly 3,700 firefighters battled the approximately 230-square-mile Rim fire, the biggest wildfire on record in California's Sierra Nevada. They reported modest progress, saying the fire was 15 percent contained.

“We're not there yet, but we're starting to get a little bit of a handle on this thing,” said Lee Bentley, fire spokesman for the U.S. Forest Service. “It's been a real tiger. He's been going around trying to bite its own tail, and it won't let go, but we'll get there.”

Utility officials monitored the clarity of the Hetch Hetchy Reservoir and used a $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the big city. The Hetch Hetchy supplies water to 2.6 million people in the San Francisco Bay area, 150 miles away.

“We're taking advantage that the water we're receiving is still of good quality,” said Harlan Kelly Jr., general manager of the city's Public Utilities Commission. “We're bringing down as much water as possible and replenishing all of the local reservoirs.”

At the same time, utility officials gave assurances that they have a six-month supply of water in reservoirs near the Bay area.

So far, the ash that has been raining onto the Hetch Hetchy has not sunk as far as the intake valves, which are about halfway down the 300-foot O'Shaughnessy Dam. Utility officials said the ash is nontoxic but the city will begin filtering water for customers if problems are detected.

On Monday, the fire was miles away from the steep granite canyon where the reservoir is nestled, but several spot fires were burning closer, and firefighters were protecting hydroelectric transmission lines and other utility facilities.

“Obviously we're paying close attention to the city's water supply,” said Glen Stratton, an operations chief on the fire suppression team.

Power generation at the reservoir was shut down last week so that firefighters would not be imperiled by live wires. San Francisco is buying replacement power from other sources to run City Hall and other municipal buildings.

It has been at least 17 years since fire ravaged the northernmost stretch of Yosemite that is under siege.

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