Nearly 3,700 firefighters are battling the approximately 230-square-mile blaze and reporting modestprogress, with the fire 15 percent contained.
The fire is raining ash on the reservoir that's the chief source of San Francisco's drinking water.
Utility officials are moving water to reservoirs closer to the city.
The fire is still several miles away from the steep granite canyon where the reservoir is nestled, but several spot fires are burning closer. Firefighters are protecting hydroelectric transmission lines and other utility facilities.
Meanwhile, 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.