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)

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  • Firefighters enter a grove of giant sequoias in Yosemite National Park in this photo made on Sunday, Aug. 25, 2013, and released by the National Park Service on Tuesday. (AP Photo/National Park Service)

  • AP10ThingsToSee - Inmate firefighters walk along state Highway 120 as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong, File)

  • The Rim Fire burns through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • Russell Mitchell

    Firefighter Russell Mitchell, left, monitors a back burn during the Rim Fire near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • A videographer records the Rim Fire burning through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. (AP Photo/Jae C. Hong)

  • A smoke plume from the Rim Fire rises nera Yosemite National Park, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • A helicopter reloads fire retardant as firefighters continue to battle the Rim Fire in Tuolumne City, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • Aerial Water Drop on Rim Fire

    In this Thursday, Aug. 22, 2013 photo provided by the California Army National Guard on Monday, Aug. 26, 2013, Sgt. Chris Boni, crew chief from the 1-140th Aviation Battalion (Air Assault) from Los Alamitos Joint Training Base (JFTB), releases water to help control the Rim fire below near Yosemite National Park, Calif. The The UH-60 Black Hawk Crews are battling the Rim wildfire, one of the largest in California history, in support of the U.S. Forest Service and CAL FIRE. (AP Photo/U.S. California Air National Guard, Master Sgt. Julie Avey)

  • In this Thursday, Aug. 22, 2013 photo provided by the California Army National Guard on Monday, Aug. 26, 2013, Sgt. Chris Boni, crew chief from the 1-140th Aviation Battalion (Air Assault) from Los Alamitos Joint Training Base (JFTB), releases water to help control the Rim fire below near Yosemite National Park, Calif. The The UH-60 Black Hawk Crews are battling the Rim wildfire, one of the largest in California history, in support of the U.S. Forest Service and CAL FIRE. (AP Photo/U.S. California Air National Guard, Master Sgt. Julie Avey)

  • 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 in Yosemite Valley in Yosemite National Park, Calif., Saturday, Aug. 24, 2013. (AP Photo/Tami Abdollah)

  • 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. (AP Photo/Jae C. Hong)

  • A burned SUV sits in front of a campground destroyed by the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. (AP Photo/Jae C. Hong)

  • A helicopter drops water over a hot spot while battling the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A.J. Tevis

    Firefighter A.J. Tevis watches the flames of the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns along Highway 120 near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The Rim Fire burns along Highway 120 near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A smoke rises from smoldering trees as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • Firefighter Charlie Ferris, of Los Angeles, watches a smoke plume from the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • A firefighter watches for spot fires during a burnout operation while battling the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • Inmate firefighters walk along Highway 120 after a burnout operation as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. (AP Photo/Jae C. Hong)

  • The smoke generated by the Rim Fire rises under a star-filled sky near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. Fire crews are clearing brush and setting sprinklers to protect two groves of giant sequoias as a massive week-old wildfire rages along the remote northwest edge of Yosemite National Park. (AP Photo/Jae C. Hong)

  • GROVELAND, CA - AUGUST 28: Cows graze in a field as the Rim Fire burns in nearby hills on August 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite National Park. The fire is currently 23 percent contained. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 28: A CalTrans worker takes a break during a tree cutting operation in an area that was burned by the Rim Fire during a media tour of the fire area on August 28, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: A burned car sits on the side of the road after being consumed by the Rim Fire on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 25: Trees burned by the Rim Fire stand on August 25, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: Flames from the Rim Fire consume trees on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: U.S. Fish and Wildlife Service firefighter Corey Adams sits on a tree stump as he monitors the Rim Fire on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 24: West Stanislaus County Fire firefighters monitor the Rim Fire along highway 120 on August 24, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 24: A firefighter uses a hose to douse the flames of the Rim Fire on August 24, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: Smoke from the Rim Fire lingers over the O'Shaughnessy Dam at Hetch Hetchy Reservoir on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: U.S. Forest Service firefighter Chris Brossard monitors a spot fire while battling the Rim Fire on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 25: Flames from the Rim Fire consume trees on August 25, 2013 near Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 24: A firefighter uses a hose to douse a spot fire while battling the Rim Fire on August 24, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Yosemite National Park, Calif. (AP Photo/U.S. Forest Service)

  • GROVELAND, CA - AUGUST 22: A firefighter from Central Calaveras Fire monitors the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • YOSEMITE NATIONAL PARK, CA - AUGUST 22: Smoke from the Rim Fire is visible near the Hetch Hetchy reservoir on August 22, 2013 in Yosemite National Park, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A firefighting helicopter flies in front of a plume of smoke from the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Groveland Ranger Station in Groveland, Calif. (AP Photo/U.S. Forest Service)

  • GROVELAND, CA - AUGUST 22: Firefighters from the Colorado based South Arkansas Fire District monitor the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: Firefighters monitor a plume of smoke from the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A U.S. Forest Service firefighter monitors a radio while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: A firefighter from the Kern County Rio Bravo Hotshots looks for flying embers while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 22: Firefighters douse hot spots while battling the Rim Fire on August 22, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District uses a drip torch to light a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District monitors a back fire as he battles the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: Firefighters from Ebbetts Pass Fire District monitor a back fire as they battle the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • GROVELAND, CA - AUGUST 21: A firefighter from Ebbetts Pass Fire District uses a drip torch to light a back fire while battling the Rim Fire on August 21, 2013 in Groveland, California. (Photo by Justin Sullivan/Getty Images)

  • BUCK MEADOWS, CA - AUGUST 21: Fire consumes trees along US highway 120 as the Rim Fire burns out of control on August 21, 2013 in Buck Meadows, California. (Photo by Justin Sullivan/Getty Images)