Wednesday, August 28, 2013

Yosemite fire threatens San Francisco water supply

Sacramento Metropolitan firefighter Matt Owston works the Rim Fire line near Camp Mather, California.

Sacramento Metropolitan firefighter Matt Owston works the Rim Fire line near Camp Mather, California. Photo: Reuters

San Francisco: The giant wildfire burning in and around Yosemite National Park struck the shores of a reservoir that serves San Francisco, threatening the famously pure mountain water that feeds the taps of 2.6 million Bay Area residents.  

San Francisco water managers, who operate the reservoir, reported that water quality had not been compromised. Fears that falling ash could pollute the supply have been addressed -- at least for now -- by diverting unsullied water from the 12.8 kilometres Hetch Hetchy Reservoir to smaller back-up reservoirs in the Bay Area, should it be needed. 

Moreover, water is drawn from 79 metres below the surface to avoid unwanted debris.  

But wildfire experts say problems for San Francisco's water agency may come later. Hetch Hetchy's pristine waters will be vulnerable to eroding hillsides as the fire leaves behind torched soil that can absorb autumn rains and levelled forests that no longer anchor steep mountain slopes. 


''Landslides are absolutely a concern,'' said Keith Gilless, dean of the College of Natural Resources at UC Berkeley.

''Trees promote stability, and if you lose the trees you may get mass movement of the soil. And if the fire is a really hot one and it scorches the soil surface, the soil becomes less penetrable.'' 

All this adds up to more sediment flowing into the water, Gilless said, which not only degrades quality but can pile up in a reservoir and decrease its capacity. 

The Rim Fire, which began August 17 in a remote section of the Stanislaus National Forest, has since burned into the western edges of Yosemite, where the 442-billion litre Hetch Hetchy reservoir has held drinking water for the Bay Area since the canyon was dammed in 1923.

By late Tuesday, the fire had scorched 180,000 acres -- or about 450 squares kilometres -- making it the seventh largest wildfire in California history. Firefighters reported moderate progress Tuesday, estimating the blaze as 20 percent contained.  

''The weather is just not cooperating with us,'' said Lynn Tolmachoff, a spokeswoman for the California Department of Forestry and Fire Protection, citing gusty winds and warm temperatures. 

At least 31 residences and 80 other structures had burned, many at the city of Berkeley's Tuolumne Camp west of Yosemite. More than 4500 residences remained threatened, and officials said the cost of the firefight had swelled to $US27 million. 

San Francisco Chronicle
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