In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Groveland Ranger Station in Groveland, Calif.
TUOLUMNE CITY, CALIF. (CBS/AP) -- Crews are finally gaining ground on a massive wildfire burning near Yosemite National Park, with fire officials expressing optimism and containment jumping to 20 percent -- even as the blaze grew larger.
Flames lapped at the edge of the main reservoir that supplies San Francisco, but fears that the Rim Fire could disrupt water or power to the city diminished.
"It looks great out there. No concerns," Glen Stratton, an operations section chief on the blaze, said of the Hetch Hetchy Reservoir.
Nearly 3,700 firefighters battled the roughly 252-square-mile fire, the biggest wildfire on record in California's Sierra Nevada.
The U.S. Forest Service said the fire was threatening about 4,500 structures and had destroyed at least 23, including 4 homes, reports CBS Sacramento, Calif. station KOVR.
Weather conditions forecast for Wednesday may bring challenges in the morning as heavy smoke settles low to the ground, limiting visibility, but higher humidity was expected in the afternoon, which could help dampen the flames, said Matt Mehle, a National Weather Service meteorologist assigned to the fire.
Crews remained confident they could protect hydroelectric transmission lines and other utility facilities at the reservoir, the chief source of San Francisco's famously pure drinking water.
"I don't foresee any problems," Stratton said.
Utility officials monitored the basin's clarity and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the city.
So far, the ash that has been raining onto the reservoir 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 that the city will begin filtering water for customers if problems are detected.
Power generation there was shut down last week so firefighters would not be imperiled by live wires. San Francisco is buying replacement power from other sources to run City Hall and municipal buildings.
It has been at least 17 years since fire ravaged the northernmost stretch of Yosemite that now is under siege.
Park officials cleared brush and set sprinklers on two groves of giant sequoias that were less than 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 with such intense heat.
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 spent Monday bulldozing firebreaks to protect Tuolumne City, several miles from the fire's edge.
Stratton said they would continue working to burn vegetation in the fire's path to slow its march toward town. He said that, while the community remains in harm's way, "I'm pretty optimistic."
Hundreds of locals packed a Sonora, Calif. church with concern etched on their faces, wanting to know where the fire is and where it was going, says KOVR. "Is it gonna come down?" asked Jan Colombani. "I think it's a real possibility and I wanna know the truth."
Meanwhile, biologists with the Forest Service are studying the effect of the 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.
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.