TUOLUMNE CITY, Calif. — 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.

Related on HuffPost:

Loading Slideshow...
  • Firefighters enter a grove of giant sequoias in Yosemite National Park in this photo made on Sunday, Aug. 25, 2013, and released by the National Park Service on Tuesday. (AP Photo/National Park Service)

  • AP10ThingsToSee - Inmate firefighters walk along state Highway 120 as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong, File)

  • The Rim Fire burns through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • Russell Mitchell

    Firefighter Russell Mitchell, left, monitors a back burn during the Rim Fire near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • A videographer records the Rim Fire burning through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • A smoke plume from the Rim Fire rises nera Yosemite National Park, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • A helicopter reloads fire retardant as firefighters continue to battle the Rim Fire in Tuolumne City, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • Aerial Water Drop on Rim Fire

    In this Thursday, Aug. 22, 2013 photo provided by the California Army National Guard on Monday, Aug. 26, 2013, Sgt. Chris Boni, crew chief from the 1-140th Aviation Battalion (Air Assault) from Los Alamitos Joint Training Base (JFTB), releases water to help control the Rim fire below near Yosemite National Park, Calif. The The UH-60 Black Hawk Crews are battling the Rim wildfire, one of the largest in California history, in support of the U.S. Forest Service and CAL FIRE. (AP Photo/U.S. California Air National Guard, Master Sgt. Julie Avey)

  • In this Thursday, Aug. 22, 2013 photo provided by the California Army National Guard on Monday, Aug. 26, 2013, Sgt. Chris Boni, crew chief from the 1-140th Aviation Battalion (Air Assault) from Los Alamitos Joint Training Base (JFTB), releases water to help control the Rim fire below near Yosemite National Park, Calif. The The UH-60 Black Hawk Crews are battling the Rim wildfire, one of the largest in California history, in support of the U.S. Forest Service and CAL FIRE. (AP Photo/U.S. California Air National Guard, Master Sgt. Julie Avey)

  • Smoke clouds and bands of haze from the Rim Fire in the western Sierra Nevada loom up some 20 miles behind the famed granite monolith known as Half Dome, right center, seen from 11,500-foot Mt. Clark in Yosemite Valley in Yosemite National Park, Calif., Saturday, Aug. 24, 2013. (AP Photo/Tami Abdollah)

  • 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. (AP Photo/Jae C. Hong)

  • A burned SUV sits in front of a campground destroyed by the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • A helicopter drops water over a hot spot while battling the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A.J. Tevis

    Firefighter A.J. Tevis watches the flames of the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns along Highway 120 near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns along Highway 120 near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A smoke rises from smoldering trees as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • Firefighter Charlie Ferris, of Los Angeles, watches a smoke plume from the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A firefighter watches for spot fires during a burnout operation while battling the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • Inmate firefighters walk along Highway 120 after a burnout operation as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The smoke generated by the Rim Fire rises under a star-filled sky near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. 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 Photo/Jae C. Hong)

  • GROVELAND, CA - AUGUST 28: Cows graze in a field as the Rim Fire burns in nearby hills on August 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite National Park. The fire is currently 23 percent contained. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 28: A CalTrans worker takes a break during a tree cutting operation in an area that was burned by the Rim Fire during a media tour of the fire area on August 28, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: A burned car sits on the side of the road after being consumed by the Rim Fire on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 25: Trees burned by the Rim Fire stand on August 25, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: Flames from the Rim Fire consume trees on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: U.S. Fish and Wildlife Service firefighter Corey Adams sits on a tree stump as he monitors the Rim Fire on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 24: West Stanislaus County Fire firefighters monitor the Rim Fire along highway 120 on August 24, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 24: A firefighter uses a hose to douse the flames of the Rim Fire on August 24, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: Smoke from the Rim Fire lingers over the O'Shaughnessy Dam at Hetch Hetchy Reservoir on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: U.S. Forest Service firefighter Chris Brossard monitors a spot fire while battling the Rim Fire on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: Flames from the Rim Fire consume trees on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: A firefighter uses a hose to douse a spot fire while battling the Rim Fire on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Yosemite National Park, Calif. (AP Photo/U.S. Forest Service)

  • GROVELAND, CA - AUGUST 22: A firefighter from Central Calaveras Fire monitors the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 22: Smoke from the Rim Fire is visible near the Hetch Hetchy reservoir on August 22, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A firefighting helicopter flies in front of a plume of smoke from the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Groveland Ranger Station in Groveland, Calif. (AP Photo/U.S. Forest Service)

  • GROVELAND, CA - AUGUST 22: Firefighters from the Colorado based South Arkansas Fire District monitor the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: Firefighters monitor a plume of smoke from the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A U.S. Forest Service firefighter monitors a radio while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A firefighter from the Kern County Rio Bravo Hotshots looks for flying embers while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: Firefighters douse hot spots while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District uses a drip torch to light a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District monitors a back fire as he battles the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District uses a drip torch to light a back fire while battling the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • BUCK MEADOWS, CA - AUGUST 21: Fire consumes trees along US highway 120 as the Rim Fire burns out of control on August 21, 2013 in Buck Meadows, California. (Photo by Justin Sullivan/Getty Images)