California blaze deposits layers of ash on surface of reservoir supplying city but water samples test clean
A wildfire raging in the north-western part of Yosemite national park was expected to advance further into the park on Tuesday and continue to threaten a reservoir that supplies most of San Francisco's water.
The so-called Rim fire has charred more than 65,000 hectares (160,000 acres) – an area larger than Chicago – mostly in the Stanislaus national forest west of Yosemite.
But the blaze was expected to move east overnight and push deeper into Yosemite, as well as in areas to the north, said a spokesman for the US forest service, Trevor Augustino.
On Monday, the blaze "made a good run to the park", Augustino said, but by the end of the day, firefighters had managed to contain 20% of the flames – up from 15% earlier in the day.
The weather on Tuesday was expected to remain hot with temperatures of 25-30C (80-90F) and winds of 10-15mph (16-24km/h) from the south-west, Augustino said.
The eastern flank of the fire on Monday burned to within half a mile (0.8km) of Yosemite's Hetch Hetchy reservoir on the Tuolumne river. It supplies 85% of the water consumed by 2.6 million people in San Francisco and several communities in three adjacent counties about 200 miles (320km) to the west.
With the flames edging ever closer, layers of ash fell on the surface of the reservoir on Monday. Water samples from the supply were testing clean by the late afternoon, the San Francisco Public Utilities Commission spokeswoman Suzanne Gautier said.
"There continues to be no change or impact to water quality or delivery from the Hetch Hetchy reservoir," the commission said in a statement.
If fallout from the blaze were to foul the Hetch Hetchy, its water could be diverted to a smaller Bay Area reservoir for filtration at a local treatment plant before delivery to customers, Gautier said.
The Rim fire, named after a Stanislaus national forest lookout point called Rim of the World, has already damaged two of the three hydropower generating stations linked to the Hetch Hetchy reservoir that supply electricity for all of San Francisco's public facilities, such as hospitals and firehouses.
The city has drawn on reserve power stored for emergencies and purchased additional electricity on the open market to make up the difference.
The blaze has destroyed about a dozen homes, and some 4,500 dwellings remained threatened.
Most of the 1,200-square-mile Yosemite national park has remained open to the public.
While the cause of the wildfire remains under investigation, a dry winter and spring have created tinderbox conditions that have fuelled one of the biggest fires in California's history. Flames that leap across treetops creating a "crown fire" are complicating efforts to contain it.
It started on17 August in a remote canyon of the Stanislaus national forest and spread rapidly, torching timber and brush with such power it created its own weather pattern. A total of 8,300 firefighters are battling nearly 400 square miles of fires across California.
