Crews are finally gaining ground on a massive wildfire burning near Yosemite National Park as fire officials expressed optimism even as the blaze grew larger while containment jumped to 20 percent. 
 
 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... (Associated Press) 
 
 Smoke clouds from the Rim Fire in the western Sierra Nevada loom up some 20 miles behind the famed granite monolith known as Half Dome, left center, in Yosemite Valley in Yosemite National Park, Calif.,... (Associated Press) 
 
 A firefighter looks at the Rim Fire incident report map posted at the command post near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Crews working to contain one of California's largest-ever... (Associated Press) 
 
 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... (Associated Press) 
 
 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. Crews... (Associated Press) 
 
 In this Monday, Aug. 26, 2013 satellite photo released by NASA, a smoke plume trails north from the drought-fueled Rim Fire, center, burning near Yosemite National Park in central California. Red outlines... (Associated Press) 
 
 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. Crews working to contain one of California's largest-ever... (Associated Press) 
 
 The Rim Fire burns along Highway 120 near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. With winds gusting and flames jumping from treetop to treetop, hundreds of firefighters have been deployed... (Associated Press) 
 
 Two firefighters watch trees burn while battling 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... (Associated Press) 
 
 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. Crews working to contain one of California's largest-ever... (Associated Press) 
 
 As flames lapped at the edge of the main reservoir that supplies San Francisco, fears that the inferno 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. 
 
 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." 
 
 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. ||||| GROVELAND, Calif. — On Day 10 of the Rim fire — even as the blaze scorched its way deeper into the northern end of Yosemite National Park — firefighters got their first small break Monday against massive forces of nature. 
 
 As the fire reached higher elevations, climbing out of steep ravines, crews could take their stand on more level ground. Huge air tankers could finally drop retardant on accessible ridges. 
 
 The fire has charred a swath of nearly 161,000 acres — including about 21,000 acres inside the park. It has destroyed at least 23 structures, and threatens two groves of giant sequoias and historical structures in the famed park. But about 25 miles away, the tourist magnet of Yosemite Valley remained safe, surrounded by its famed granite walls. 
 
 The fire is burning hotter and faster than any in modern Sierra Nevada history, firefighters say. Officials say it is the California wildfire they have warned about for years, as modern firefighting techniques have snuffed out forest fires, allowing fuel to build up on the mountain floor. 
 
 "This is it. This is the big one," Yosemite Fire Chief Kelly Martin said. 
 
 Yet containment reached 20% — up from 7% Sunday. 
 
 "The fire will burn until the snow flies," said Tom Medema, a Yosemite National Park interpretive ranger. "But today, we finally had a chance to box it in." 
 
 The most hopeful sign was written in the sky. 
 
 For the last week, each afternoon has brought a towering plume of smoke rising over the flames. Inside the plume are lightning and hail, and when the plume collapses, it sends wind pushing the fire in all directions. They call from noon to 5 p.m. the burning hours. 
 
 On Monday, the weather changed slightly — an inversion layer kept the smoke thick and low just a little longer — but it was enough to chop the plume down to half its size. 
 
 "Today we can finally fight the fire on our terms, said Gary Wuchner, Yosemite National Park's fire specialist. 
 
 From the Rim of the World vista point on California 120, he looked out over a wide swath of the fire's footprint. 
 
 It's a spot where tourists often pull over to take photos of the Tuolumne River and mountains in the distance. There was a kiosk giving the history of fires in the area, but it burned to the ground this week. Still standing were two stone monuments to firefighters killed in wildfires in the area — one in 1987, one in 2004. 
 
 "This is hallowed ground to firefighters," Wuchner said. "We have bad history here." 
 
 The needles on four nearby Ponderosa pines were swept straight back, frozen in the direction the fire pushed them. Far off to the northeast, smoke rose above the Hetch Hetchy Reservoir and around Pilot Peak, a fire lookout tower that is staffed each summer. Due north, the edge of Jawbone Ridge looked like it had been outlined with a black marker where the fire had first spilled over. 
 
 Power lines stretched overhead from still standing towers. But the electricity had been cut. On a hill above the river was the only circle of green in sight: Drew Meadow, the base camp for more than 2,000 firefighters. It has now been attacked by the fire from three sides. 
 
 Wuchner's radio crackled constantly. 
 
 "Need retardant drops immediately," a voice said. 
 
 The fight at the park is to save a historic ranger station, stables, lakeside campsites and two groves of giant sequoias. The ancient trees have withstood many fires over the centuries. 
 
 The fear is that this fire, with so much unnatural fuel sending flames higher, could get to the top of the trees and kill them. Wuchner said they had sprinklers dampening the tops of the sequoias and had wrapped all nearby structures in fireproof material. They do not use fire retardant in the national park. When there are lightning fires within the park's boundaries, officials let them burn, unless they are threatening communities. 
 
 "When it comes to fighting the fire within the park," Wuchner said, "what we're banking on is all the fires that came before." 
 
 diana.marcum@latimes.com 
 
 samantha.schaefer@latimes.com 
 
 joseph.serna@latimes.com 
 
 Times staff writer Robert J. Lopez in Los Angeles contributed to this report. Marcum reported from Groveland, Schaefer and Serna from Los Angeles.
– The giant Rim fire near Yosemite National Park has now scorched an area the size of Chicago and is still growing, but firefighters are making steady progress. Officials say the fire is now around 20% contained and the 3,700 firefighters battling it are gaining ground as the blaze climbs out of steep ravines. "The fire will burn until the snow flies," a park ranger tells the Los Angeles Times. "But today, we finally had a chance to box it in." Only around 2% of the park itself has burned and though more of it will burn, authorities have vowed to do everything in their power to protect the iconic Yosemite Valley. The fire has been raining ash on the reservoir that supplies San Francisco with most of its water, but officials say it has not sunk as far as the intake valves and the city's supply should not be affected, the AP reports. As crews continue to bulldoze firebreaks and burn vegetation in the fire's path, biologists are beginning to assess the blaze's effect on wildlife. Bald eagle nests in the area are being monitored, and a group of endangered turtles has been found stranded near where their marshy habitat burned. "We're hoping to deliver some water to those turtles," a Forest Service biologist says. "We might also drag some brush in to give them cover."