(Adds fire reaching reservoir, updates acreage burned, adds official’s comment)
By Laila Kearney
SAN FRANCISCO, Aug 27 (Reuters) - One of the largest California wildfires on record roared deeper into Yosemite National Park on Tuesday, frightening away many late-summer visitors and reaching a water reservoir that serves as the primary water supply for San Francisco.
The so-called Rim Fire, which is burning mainly in the Stanislaus National Forest west of Yosemite, nearly doubled its footprint in the park overnight and the sprawling blaze later crept closer to thousands of homes west of the park.
Officials said ash had drifted onto the surface of the Hetch Hetchy Reservoir, which supplies water to San Francisco some 200 miles (320 km) to the west, but testing of samples showed water quality remained healthy.
After advancing on the reservoir for several days, the flames reached the shores of the artificial lake, officials said. But the San Francisco Public Utilities Commission said in a statement there was “little risk for direct impacts” on the reservoir because of the rocky terrain and lack of brush surrounding it.
If the water should become fouled by too much ash and soot and require filtration, it can be diverted through a treatment plant near San Francisco before being delivered to customers, officials from the Public Utilities Commission said.
Meanwhile, a firefighting force of some 4,100 personnel, backed by teams of bulldozers and water-dropping helicopters, continued to make headway in their drive to encircle and suppress the flames.
By late on Monday, containment lines had been established around 20 percent of the fire’s perimeter, nearly triple Sunday’s figure, though the overall area of the blaze continued to grow as much of the firefighting effort focused on structure protection.
“The lines are being built miles long, and in some areas 12 (bulldozer) blades wide,” said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.
Fire managers were looking forward to a cooling trend forecast for the end of the week, which Berlant said “would bring some much-needed relief.”
The blaze was among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West.
More favorable weather this week in the Pacific Northwest, including lower temperatures, diminished winds and even some rain, helped firefighters gain an upper hand on a number of stubborn blazes in Oregon and Washington state.
Those gains freed up some fire crews and other thinly stretched resources, leading federal fire managers to lower the nation’s wildfire threat index a notch.
“Things are really getting rosier,” said Ken Frederick, a spokesman for the National Interagency Fire Center, a support center for wildland firefighting, in Boise.
The Rim Fire has charred nearly 184,500 acres (about 75,000 hectares) - an area larger than the land mass of Chicago - since it erupted on Aug. 17, most of that in the Stanislaus Forest.
It ranks as the biggest California wildfire since October 2007, when the Witch Fire torched nearly 198,000 acres (80 ha)and more than 1,600 structures in San Diego County, and the sixth-largest in state history, according to the records of Cal Fire, a state government site.
Firefighters hacking through dense, dry brush and trees to create clearings in the rugged terrain rushed to improve buffer zones around some 4,500 homes threatened by the blaze on its northwestern flank, Berlant said.
Most of those dwellings have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment’s notice. The fire has already destroyed dozens of homes and cabins, Berlant said, but no serious injuries have been reported.
As of Tuesday, the blaze had scorched more than 40,000 acres of Yosemite, forcing the closure of some campgrounds in the more remote northern part of the park and the main entrance road from the San Francisco Bay area. (www.nps.gov/yose/index.htm)
Tioga Road, the second of four access routes to Yosemite National Park is scheduled to be closed on Wednesday for fire crews to build containment lines along the road before flames can reach it, said Yosemite spokesman Tom Medema.
The vast majority of the 1,200-square-mile park, including the Yosemite Valley area renowned for its towering rock formations, waterfalls, meadows and pine forests, remained open to the public and free of smoke. But late-summer crowds were notably diminished, park officials said.
“There are still people here, but there’s definitely fewer visiting than there normally would be for this time of year, and that just happened within the last couple of days,” said park spokeswoman Kari Cobb. “It’s just the northern part of the park is smoky.”
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August, she said.
The Rim Fire, named for 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 been drawing on reserve power stored for emergencies and purchasing additional electricity on the open market to make up for the difference.
The cause of the blaze remained under investigation. (Reporting by Laila Kearney; Additional reporting by Laura Zuckerman in Idaho and Alex Dobuzinskis in Los Angeles; Writing by Steve Gorman; Editing by Cynthia Johnston and Christopher Wilson)