August 26, 2013: This photo released by NASA astronaut Karen Nyberg shows a view of the California wildfires from the International Space Station. (AP Photo/NASA, Karen Nyberg)
In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Yosemite National Park, Calif. (AP Photo)
Aug. 25, 2013: Inmate firefighters walk along Highway 120 as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif. Fire crews are clearing brush and setting sprinklers to protect two groves of giant sequoias as a massive week-old wildfire rages along the remote northwest edge of Yosemite National Park.AP
Fire officials said Tuesday that crews are gaining ground on a raging wildfire in Yosemite National Park that rained ash on the reservoir that is the main source for San Francisco's famously pure drinking water.
Glen Stratton, an operations chief on the fire suppression team, said that while the blaze continues to grow, containment numbers are up, as is optimism that crews are making progress.
Stratton says while flames reached the edge of the Hetch Hetchy Reservoir, crews are confident they will be able to protect the infrastructure and expect no water or power disruptions from raining ash.
Nearly 3,700 firefighters were battling the wildfire, which is the biggest on record in California's Sierra Nevada. As of Monday evening, the blaze, which was first reported on Aug. 17, was 20 percent contained, the California Department of Forestry and Fire Protection said.
"We're not there yet, but we're starting to get a little bit of a handle on this thing," said Lee Bentley, fire spokesman for the U.S. Forest Service. "It's been a real tiger. He's been going around trying to bite its own tail, and it won't let go but we'll get there."
Utility officials were monitoring clarity of the Hetch Hetchy Reservoir and using a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the city. The Hetch Hetchy supplies water to 2.6 million people in the San Francisco Bay area, 150 miles away.
"We're taking advantage that the water we're receiving is still of good quality," said Harlan Kelly Jr., general manager of the city's Public Utilities Commission. "We're bringing down as much water as possible and replenishing all of the local reservoirs."
At the same time, utility officials gave assurances that they have a six-month supply of water in reservoirs near the Bay area.
So far the ash that has been raining onto the Hetch Hetchy has not sunk as far as the intake valves, which are about halfway down the 300-foot O'Shaughnessy Dam. Utility officials said that the ash is non-toxic but that the city will begin filtering water for customers if problems are detected. That could cost more.
On Monday the fire was still several miles away from the steep granite canyon where the reservoir is nestled, but several spot fires were burning closer, and firefighters were protecting hydroelectric transmission lines and other utility facilities.
San Francisco is buying replacement power from other sources to run City Hall and other municipal buildings.
It has been at least 17 years since fire ravaged the northernmost stretch of Yosemite that is under siege.
Park officials cleared brush and set sprinklers on two groves of giant sequoias that were seven to 10 miles away from the fire's front lines, said park spokesman Scott Gediman. While sequoias have a chemical in their bark to help them resist fire, they can be damaged when flames move through slowly.
The fire has swept through steep Sierra Nevada river canyons and stands of thick oak and pine, closing in on Tuolumne City and other mountain communities. It has confounded ground crews with its 300-foot walls of flame and the way it has jumped from treetop to treetop.
Crews bulldozed two huge firebreaks to try to protect Tuolumne City, five miles from the fire's edge.
"We've got hundreds of firefighters staged in town to do structure protection," Stratton said. "If the fire does come to town, we're ready."
Meanwhile, biologists with the Forest Service are studying the effect on wildlife. Much of the area that has burned is part of the state's winter-range deer habitat. Biologist Crispin Holland said most of the large deer herds would still be well above the fire danger.
The Associated Press contributed to this report.