A firefighter stands on top of a fire truck at a campground destroyed by the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Crews working to contain one of California's largest-ever wildfires gained some ground Monday against the flames threatening San Francisco's water supply, several towns near Yosemite National Park and historic giant sequoias. In this Sunday, Aug. 25, 2013, photo, the Hetch Hetchy Reservoir is hazy with smoke from the Rim Fire fills the air in Yosemite National Park, Calif. A fire truck drives past burning trees as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Two firefighters watch trees burn while battling the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Operations section chief Glen Stratton points to the Hetch Hetchy Reservoir on the map while briefing the media at the command post near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. (AP) - A raging wildfire in Yosemite National Park rained ash on the reservoir that is the chief source of San Francisco's famously pure drinking water, and utility officials Monday scrambled to send more water toward the metropolitan area before it becomes tainted. Nearly 3,700 firefighters battled the approximately 230-square-mile blaze, the biggest wildfire on record in California's Sierra Nevada. They reported modest progress, saying the fire was 15 percent contained. "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 monitored the clarity of the Hetch Hetchy Reservoir and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the big 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. "Obviously we're paying close attention to the city's water supply," said Glen Stratton, an operations chief on the fire suppression team. Power generation at the reservoir was shut down last week so that firefighters would not be imperiled by live wires. 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. Biologists discovered stranded Western pond turtles on national forest land near the edge of Yosemite. Their marshy meadow had burned, and the surviving creatures were huddled in the middle of the expanse in what little water remained. "We're hoping to deliver some water to those turtles," Holland said. We might also drag some brush in to give them cover. Wildlife officials were also trying to monitor at least four bald eagle nests in the fire-stricken area. While it has put a stop to some backcountry hiking, the fire has not threatened the Yosemite Valley, where such sights as the Half Dome and El Capitan rock formations and Yosemite Falls draw throngs of tourists. Most of the park remained open to visitors. The U.S. Forest Service said the fire was threatening about 4,500 structures and destroyed at least 23. Rugged terrain, strong winds and bone-dry conditions have hampered firefighters' efforts to contain the blaze, which began Aug. 17. The cause has not been determined.