(Adds details on scope of fire, background on reservoir)
By Laila Kearney
SAN FRANCISCO, Aug 24 (Reuters) - Firefighters on Saturday battled to gain control of a fast-moving wildfire raging on the edge of Yosemite National Park that is threatening power and water supplies to San Francisco about 200 miles (322 km) to the west.
The so-called Rim Fire, which had grown to just over 125,000 acres (50,585 hectares) as of early Saturday, remained largely unchecked with extreme terrain hampering efforts at containment.
"We are making progress but unfortunately the steep terrain definitely has posed a major challenge," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.
"Today we're continuing to see warm weather that could allow this fire to continue to grow very rapidly as it has over the last several days," Berlant said. He described the fire as the 16th largest on record in California.
California Governor Jerry Brown on Friday declared a state of emergency, warning that the fire had damaged the electrical infrastructure serving the city, and forced the San Francisco Public Utilities Commission to shut down power lines.
There have been no reports of blackouts in San Francisco.
Ashley Taylor, a spokeswoman for the U.S. Forest Service, said high humidity and firefighting efforts through the night had helped some.
"Firefighters are still working with the same difficult situation, and they're really taking every opportunity they can to take hold of this fire," Taylor said. "They're working very hard to take this down."
The Hetch Hetchy Reservoir provides water to 2.6 million customers in the San Francisco area, and Brown in his declaration said the city's water supply could be affected if the blaze affects the reservoir.
The Hetch Hetchy watershed provides about 85 percent of San Francisco's total water needs, according to the Western Urban Water Coalition, organization of largest water suppliers in western United States.
The fire on Saturday remained about 4 miles (6.4 km) west of the reservoir and more than 20 miles (32 km) from Yosemite Valley, the park's main tourist center, Yosemite spokesman Scott Gediman said.
The fire was consuming brush, oaks and pine and was about 5 percent contained as of Saturday.
The fire had blackened about 12,000 acres (4,856 hectares) in the northwestern corner of Yosemite as of Friday.
Berlant said about 2,700 firefighters were expected to be on the front lines on Saturday to fight the fire, which started on Aug. 17 in the Stanislaus National Forest.
Yosemite, one of the nation's major tourist destinations, attracted nearly 4 million visitors last year. The park has been posting updates and alerts on its website. (Alerts: http://www.nps.gov/yose/index.htm)
The blaze in the western Sierra Nevada Mountains is now the fastest-moving of 50 large wildfires raging across the drought-parched U.S. West that have strained resources and prompted fire managers to open talks with Pentagon commanders and Canadian officials about possible reinforcements.
Taylor said there had been one reported injury, a heat-related injury to a firefighter, due to the fire. About 4,500 residences, three commercial buildings and 1,000 outbuildings are currently threatened by the fire, she said.
An American Red Cross shelter at the Mother Lode Fairgrounds in Tuolumne County located north and west of the fire had 184 people on Saturday, spokesman Jordan Scott said.
Campers forced from Stanislaus National Forest had taken shelter there earlier in the week, but the fairgrounds shelter now had people forced to leave area homes, he said.
"People coming in are obviously concerned about their home and their situation," but their spirits have been "generally upbeat," Scott said in a telephone interview.
Taylor said there is no plan to close Yosemite National Park due to the fire. However, park officials have closed areas in its northwestern edge throughout the week, including the Hetch Hetchy Reservoir area, Lake Eleanor, Lake Cherry and Tuolumne Grove of Giant Sequoias.
Highway 120, one of four access routes to Yosemite, which is known for its waterfalls, giant sequoia groves and other scenic wonders, was temporarily closed. The highway leads to the west side of the 750,000-acre (300,000-hectare) national park.
The 2013 fire season has already drained U.S. Forest Service fire suppression and emergency funds, causing the agency to redirect $600 million meant for other projects like campground and trail maintenance and thinning of trees to reduce wildfire risks, agency spokesman Mike Ferris has said.
The service has spent some $967 million to protect lives and properties amid a season that has seen fires in Idaho, Utah, Colorado and California threaten homes and communities that border forest and wild lands where fire is more dangerous and costly to fight, Ferris said. (Additional reporting by Noreen O'Donnell in New York; Editing by David Bailey, Vicki Allen and Gunna Dickson)