(Corrects orientation of threatened homes to northwestern flank of fire, not northeastern, in 10th paragraph)
By Laila Kearney
SAN FRANCISCO, Aug 27 (Reuters) - One of the largest wildfires in California history roared deeper into Yosemite National Park early on Tuesday while flames on the opposite side of the sprawling blaze crept closer toward thousands of homes outside the park, fire officials said.
The blaze on Monday burned to within a mile of a reservoir that serves as the primary water supply for San Francisco and surrounding communities some 200 miles (320 km) to the west, dropping some ash onto the surface of the artificial lake.
But officials said testing of samples taken from the Hetch Hetchy Reservoir show that water quality remains healthy. 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, they said.
Meanwhile, a firefighting force of some 3,700 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 footprint of the blaze continued to grow.
"We are making progress," Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection, said on Tuesday, adding that fire managers were looking forward to a cooling trend forecast for the end of the week. "That would bring some much-needed relief," he said.
The blaze was among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West. The fires have strained resources and prompted fire managers to open talks with Pentagon commanders and Canadian officials about possible reinforcements.
The so-called Rim Fire has charred nearly 180,000 acres (74,843 hectares) - an area larger than the land mass of Chicago - since it erupted Aug. 17, most of that in the Stanislaus National Forest west of Yosemite, Berlant said.
It ranks as the largest California wildfire since October 2007, when the Witch Fire torched nearly 198,000 acres and more than 1,600 structures in San Diego County, and one of the 10 biggest in state history, according to CalFire records.
Firefighters hacking through dense, dry brush and trees to create clearings in the rugged terrain rushed on Tuesday to shore up 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 already has destroyed dozens of homes and cabins, Berlant said, but no serious injuries have been reported.
As of Sunday night, the opposite side of the blaze had scorched some 22,000 acres of Yosemite, forcing the closure of some campgrounds in the northern part of the park and the main entrance road from the San Francisco Bay area. (www.nps.gov/yose/index.htm)
Two famous stands of giant sequoia trees, the Merced Grove and the Tuolomune Grove, were in the path of advancing flames and have been shut down for several days.
But 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.
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.
But 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; Writing by Steve Gorman; Editing by Cynthia Johnston, Bernard Orr)