By Laila Kearney
Aug 27 (Reuters) - A wildfire raging in the northwest part of Yosemite National Park was expected to advance farther into the park on Tuesday and continue to threaten a reservoir that supplies most of San Francisco's water.
The so-called Rim Fire has charred more than 160,000 acres (64,749 hectares), which is larger than Chicago, most of that in the Stanislaus National Forest west of Yosemite.
But the blaze was expected to move east overnight and push deeper into Yosemite, as well as in areas to the north, said U.S. Forest Service spokesman Trevor Augustino.
On Monday the blaze "made a good run to the park," Augustino said. But firefighters had the blaze 20 percent contained toward the end of the day, up from 15 percent earlier Monday.
The weather on Tuesday was expected to remain hot with temperatures in the high 80s Fahrenheit and low 90s Fahrenheit (low 30s Celsius), with winds of 10 miles per hour to 15 miles per hour (16-24 km per hour) from the southwest, Augustino said.
The eastern flank of the fire on Monday burned to within a half mile (0.8 km) of Yosemite's Hetch Hetchy reservoir on the Tuolumne River. It supplies 85 percent of the water consumed by 2.6 million people in San Francisco and several communities in three adjacent counties about 200 miles (320 km) to the west.
With the flames so close, ash fell on the surface of the reservoir on Monday. But water samples from the supply were testing clean by the late afternoon, San Francisco Public Utilities Commission spokeswoman Suzanne Gautier said.
"There continues to be no change or impact to water quality or delivery from the Hetch Hetchy Reservoir," the utilities commission said in a statement.
If fallout from the blaze were to foul the Hetch Hetchy, its water could be diverted to a smaller Bay Area reservoir for filtration at a local treatment plant before delivery to customers, Gautier said.
The Rim Fire, named for a Stanislaus National Forest lookout point called Rim of the World, has already damaged two of the three hydropower generating stations linked to the Hetch Hetchy reservoir that supply electricity for all of San Francisco's public facilities, such as hospitals and firehouses.
The city has drawn on reserve power stored for emergencies and purchasing additional electricity on the open market to make up for the difference.
The blaze has destroyed about a dozen homes, and some 4,500 dwellings remained threatened.
Most of the 1,200-square-mile Yosemite National Park has remained open to the public.
The cause of the wildfire remained under investigation. More than 3,700 firefighters have been assigned to the blaze. (Writing by Alex Dobuzinskis; Editing by Xavier Briand)