SAN FRANCISCO, Aug 13 (Reuters) - Hundreds of firefighters battled a pair of wildfires that burned out of control in Northern California for a second day on Monday, threatening a clothing-optional resort and forcing the evacuation of some 500 homes.

Meanwhile, authorities said a U.S. Forest Service firefighter was killed in Idaho while battling a blaze there.

In California, the Wye Fire and the Walker Fire had charred a total of 5,000 acres (2,000 hectares) by Monday evening in Lake County, a tourist area some two hours north of San Francisco, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

The flames forced the closure of a portion of Highway 20, a major road that runs around Clear Lake, a popular summer destination.

It also prompted authorities to order the evacuation of 480 homes and at least several ranches in the Spring Valley area, Berlant said, and threatened nearby Wilbur Hot Springs, a resort that offers clothing-optional bathing in mineral water.

The resort closed on Sunday for 12 days of maintenance and it was not clear if anyone had been forced to flee. The resort could not be reached for comment on Monday.

"We haven't had rain in months, so the grass and the brush were incredibly dry. We had triple-digit temperatures, which create the perfect burning conditions for a wildfire," Berlant said.


CAUSE OF FIRES INVESTIGATED

The Wye Fire has burned 3,000 acres (1,200 hectares) and is roughly 25 percent contained, he said. Three structures were reported destroyed. The Walker Fire has swept through 2,000 acres (800 hectares) and is 30 percent contained.

One firefighter was injured by the Walker fire, and one civilian was hurt by the Wye fire.

The Red Cross had set up a shelter for evacuees in a senior center in Clear Lake, and at least 30 people were expected to spend Monday night there.

Investigators are looking into what sparked the blazes. Both started at about 4 p.m. on Sunday within a couple of miles of each other.

"They started in the grass, burned in the brush and right into the timber as it gets higher into the ridge," Berlant said.

The fires raged as western states baked under a heat wave that has pushed temperatures into triple digits Fahrenheit.

In southern Oregon, 201 structures were threatened by a wildfire that has burned 28,791 acres (11,651 hectares) of rugged, forested land and is roughly 25 percent contained, officials said.

Twenty homes are subject to evacuation orders and another 30 have been told to prepare for evacuation.

More than 1,000 firefighters are battling the blaze in temperatures in the 90s F, according Renee Snyder, spokeswoman for the fire management team. The Barry Point fire was sparked last week by lightning. No structures have been destroyed.

"Mother Nature is not lending us a hand on this one yet," Snyder said.

In northern Idaho, a U.S. Forest Service firefighter died on Sunday when she was crushed by a falling tree while working to contain a 50-acre (20-hectare) blaze in steep, forested terrain, Forest Service spokeswoman Elizabeth Slown said on Monday.

An investigation into the death of Anne Veseth, 20, of Moscow, Idaho, who was in her second season as a federal firefighter, was to begin Monday afternoon.

Veseth was the first Forest Service employee to die fighting a wildfire since July 2009, Slown said. (Additional reporting by Teresa Carson in Portland and Laura Zuckerman in Idaho; Editing by Cynthia Johnston, Mary Slosson and Eric Beech)