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 250-square-mile blaze. They reported modest progress, saying the fire was 15 percent contained.

Utility officials monitored the Hetch Hetchy Reservoir for clarity and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to San Francisco. The Hetch Hetchy supplies water to 2.6 million people in the 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 rained 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 the ash is non-toxic but 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.

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.

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.