Story highlights The Rim Fire has burned more than 160,000 acres in California

A top priority is stopping the fire from spreading further in Yosemite National Park

Evacuating residents say they're worried about their homes

Official: "We're going to do everything in our power to protect" Yosemite

A massive northern California wildfire that's threatening Yosemite National Park and San Francisco's key water and power sources grew Monday, becoming the 13th largest in state history, state fire authorities said.

The Rim Fire, which has devoured 160,980 acres, has scorched an area about the size of the city of Chicago while more than 3,600 firefighters try to rein it in.

Susan Loesch and Curtis Evans just started settling into their second home in California's Sierra foothills a few months ago. Now, they're worried it could go up in smoke as the massive wildfire spreads.

"This is kind of a little paradise up here for us. ... To think this would all be gone would be devastating," Evans told CNN on Sunday.

Cradling their Chihuahua, Cuervo, they prepared to leave the area Sunday.

Photos: Rim Fire in Yosemite National Park Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A Hotshot crew member rests near a controlled burn at Horseshoe Meadows as crews continue to fight the Rim Fire near Yosemite National Park in California on Wednesday, September 4. The massive wildfire has burned over 280,000 acres and is now 80% contained, according to a state fire spokesman. Hide Caption 1 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – California Highway 120 is cleared of debris as crews continue to fight the Rim Fire near Yosemite on September 4. Hide Caption 2 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A member of the Bureau of Land Management's Silver State Hotshot crew walks through a burn operation on the southern flank of the Rim Fire on Friday, August 30. Hide Caption 3 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – BLM Silver State Hotshot crew members perform burn operations on the southern flank of the fire on August 30. Hide Caption 4 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter takes a break from battling the Rim Fire near Groveland, California, on August 28. Hide Caption 5 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Firefighters from Sacramento Metro burn around an old cabin in Ackerson Meadow near Yosemite National Park on August 28. Hide Caption 6 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Smoke from the Rim Fire drifts past cattle in a field near Yosemite National Park on August 28. Hide Caption 7 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Firefighter Russell Mitchell monitors the situation during the Rim Fire near Yosemite National Park on Tuesday, August 27. Hide Caption 8 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A videographer records the flames burning through trees as the Rim Fire menaces Yosemite National Park on August 27. Hide Caption 9 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Firetrucks drive through heavy smoke generated by the Rim Fire near Yosemite on August 27. Hide Caption 10 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Sacramento Metropolitan firefighter John Graf monitors the Rim Fire line near Camp Mather, California, on Monday, August 26. Hide Caption 11 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter stands on top of a fire truck at a campground destroyed by the Rim Fire near Yosemite National Park on August 26. The massive wildfire grew during the week and became the 5th largest in state history, state fire authorities said. Hide Caption 12 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Flames from the Rim Fire destroy trees on Sunday, August 25, near Groveland, California. The fire had consumed nearly 219,000 acres as of Saturday, August 31. Hide Caption 13 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – U.S. Fish and Wildlife Service firefighter Corey Adams sits on a tree stump as he monitors the Rim Fire near Groveland on August 25. Hide Caption 14 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Fierce flames from the Rim Fire burn near Groveland, not far from Yosemite National Park. Hide Caption 15 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – U.S. Forest Service firefighters take a break from battling the Rim Fire at Camp Mather on August 25. Hide Caption 16 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Trees burned by the Rim Fire stand charred on August 25 in Yosemite National Park. Hide Caption 17 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A burned car sits on the side of the road near Groveland on August 25. Hide Caption 18 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A Cal Fire firefighter monitors the Rim Fire as it burns through a stand of trees near Groveland on August 25. Hide Caption 19 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter douses flames of the Rim Fire in Groveland, near Yosemite National Park, on Saturday, August 24. Hide Caption 20 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – The Rim Fire burns just outside Yosemite on August 24. A top priority is stopping the fire from spreading further into the national park. Hide Caption 21 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Smoke and flames move along a ridge just outside Yosemite National Park on August 24. Hide Caption 22 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – El Dorado Hills firefighters work to douse a hotspot in Yosemite National Park on August 24. Nearly 5,000 firefighters are attempting to bring the fast-moving fire under control. Hide Caption 23 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter douses a spot fire, as he battles the Rim Fire near Yosemite National Park in California. Hide Caption 24 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Firefighters move in to douse a spot fire on August 24. Hide Caption 25 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A cow walks through a section of forest that was scorched by the Rim Fire outside of Camp Mather on August 24, near Groveland, California. Hide Caption 26 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A truck sits scorched outside of Yosemite National Park on August 24. Hide Caption 27 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Some 2,600 of firefighters were battling the out-of-control Rim Fire on August 24. Hide Caption 28 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Smoke and flame blot out the horizon as the Rim Fire approaches Yosemite Lake near Groveland, California on Friday, August 23. Hide Caption 29 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Charred siding is all that remains of a building consumed by the fire. Hide Caption 30 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Flames eat up trees as the fire continues to burn out of control on August 23. Hide Caption 31 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A coyote walks across U.S. Highway 120, shut down due to the fire, near Groveland, California, on August 23. Hide Caption 32 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Smoke from the Rim Fire is seen near Hardin Flat outside Yosemite National Park on August 23. Hide Caption 33 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighting crew puts out a spot fire that jumped a fire line in Yosemite National Park on August 23. Hide Caption 34 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Giant plumes of smoke rise up from the Rim Fire near the border of Yosemite National Park on August 23. Hide Caption 35 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter from Cosumnes Fire Department monitors a back fire while battling the Rim Fire on August 22, in Groveland, California. Hide Caption 36 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – The Rim Fire has burned 105,620 acres, spreading from Stanislaus National Forest to part of Yosemite, Forest Service on Friday, August 23, spokesman Bjorn Fredrickson said. Hide Caption 37 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – An air tanker drops fire retardant on a ridge ahead of the advancing Rim Fire on August 22, in Groveland, California. The Rim Fire continues to burn out of control and has entered Yosemite National Park. Hide Caption 38 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A firefighter from the Colorado-based South Arkansas Fire District carries a hose as he monitors the Rim Fire on August 22, in Groveland, California. Hide Caption 39 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Los Angeles County firefighters hike in on a fire line on the Rim Fire near Groveland, California, on August 22. Hide Caption 40 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A Tuolumne County sheriff's deputy looks at an incident map of the Rim Fire on August 22, in Groveland, California. Hide Caption 41 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – The Rim Fire burns close to Groveland Ranger Station near Yosemite National Park, California, on August 22. Hide Caption 42 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – A Marin County firefighter works to put out a spot fire that jumped a fire line on the Rim Fire near Groveland, California, August 22. Hide Caption 43 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Sunlight filters through smoke in a grove of trees burned by the Rim Fire on August 22, in Groveland, California. Hide Caption 44 of 45 Photos: Rim Fire in Yosemite National Park Rim Fire in Yosemite National Park – Smoke from the Rim Fire is visible near the Hetch Hetchy Reservoir on August 22, in Yosemite National Park, California. The Rim Fire continues to burn out of control and entered Yosemite National Park on Friday. Hide Caption 45 of 45

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"It's scary," Loesch said. "You worry about the firefighters being on the line. ... It's overwhelming."

The wildfire, which was 20% contained Monday night, was spreading primarily to the east and threatened to grow amid extremely dry conditions and hot weather.

Part of the fire continued to spread Monday toward a key part of San Francisco's water supply: the Hetch Hetchy Reservoir, which lies within Yosemite and is just east of the flames.

The fire also could threaten the area's hydroelectric generators, which provide much of San Francisco's electricity. Because of the approaching flames, officials shut down the generators, and the city -- more than 120 miles to the west -- temporarily is getting power from elsewhere.

"There's a lot of concern, and there's a lot of work to be done," U.S. Forest Service spokesman Lee Bentley said.

Northwest of the reservoir, rocky terrain could stop the fire's growth there, Bentley said.

As many as 20 helicopters and air tankers were aiding firefighting efforts.

A top priority is stopping the fire from spreading further in Yosemite National Park.

"The park is a national treasure," said U.S. Forest Service spokeswoman Vickie Wright, "so no matter what it takes, we're going to do everything in our power to protect that park."

While the Rim Fire has consumed at least 12,000 acres in the northwest section of the park, so far it has had little or no direct impact on Yosemite Valley , a popular spot for tourists and home to many of the famous cliffs and waterfalls in the park.

About 4,500 structures, many of them vacation homes, were under threat, according to InciWeb , a federal website that collects information from agencies such as the U.S. Forest Service and Bureau of Land Management. The fire so far has cost more than $20 million, Bentley said.

The inferno threatened the Yosemite gateway communities of Groveland and Pine Mountain Lake just outside the Stanislaus National Forest.

"Business is slow, very slow," said Corinna Loh, owner of the Iron Door Saloon in Groveland.

Her normal season is Memorial Day to Labor Day.

"This is time we manage to save up money to make it through the winter, so it's really scary for all of us," she said, sitting among empty tables.

The Tuolumne County Sheriff's Office, meanwhile, issued evacuation advisories for the town of Tuolumne and nearby Ponderosa Hill, InciWeb said. It was not clear how many residents were covered by the evacuation advisory.

The fire has destroyed 11 homes and 12 outbuildings, as well as a Yosemite campground owned by the city of Berkeley, officials said.

Authorities say the Rim Fire started on August 17. The cause is under investigation.

Fire, what fire?

The park typically has 15,000 visitors on a busy summer weekend. August sees an influx of visitors from other countries, a few of whom have called to check on conditions.

Still, there's little indication so far it's keeping many people away.

Ranger programs went on as normal over the weekend and campgrounds were full.

"If somebody cancels, there are 10 people that will take their spot," said Yosemite spokesman Scott Gediman.

Yosemite, with hundreds of campground sites and lodging units, had nearly 4 million visitors last year, the National Park Service said.