One of the largest California wildfires on record roared east into Yosemite National Park on Tuesday, frightening away many late-summer visitors, while the sprawling blaze also crept closer to thousands of homes west of the park. The so-called Rim Fire, which has burned for 11 days mainly in the Stanislaus National Forest west of Yosemite, nearly doubled its footprint in the park overnight after encroaching on a reservoir that serves as the primary water supply for San Francisco some 320km to the west. Video: Ancient sequoia trees could be affected by California wildfires Officials said some ash from the fire had drifted onto the surface of the Hetch Hetchy Reservoir, but testing of samples taken from the artificial lake showed that water quality remained 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, officials from the city Public Utilities Commission 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 per cent 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. 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 “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 US West. More favourable weather this week in the Pacific Northwest, including lower temperatures, diminished winds and even some rain, has helped firefighters gain an upper hand on a number of stubborn blazes in Oregon and Washington state. Those gains have, in turn, 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 centre for wildland firefighting, in Boise. The Rim Fire has charred nearly about 75,000 hectares - an area larger than the land mass of Chicago - since it erupted Aug. 17, most of that in the Stanislaus Forest, Berlant said. It ranks as the biggest 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 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 on Tuesday 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 morning, the blaze had scorched nearly 42,000 acres of Yosemite - almost double the number from late Monday, 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 . 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,” park spokeswoman Kari Cobb said. “It’s just the northern part of the park is smoky.” 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 fire also ravaged a 91-year-old summer camp operated by the Bay-area city of Berkeley - the Tuolumne Berkeley Family Camp - west of the Yosemite on Sunday, days after a nearby Jewish camp called Camp Tawonga was damaged. The cause of the blaze remained under investigation.