A massive landslide that closed a section of State Route 410, destroyed at least two homes, blocked and changed the flow of the Naches River and prompted the evacuation of dozens of nearby residents could continue to advance for several more days.

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A swath of Highway 410 east of Chinook Pass near Naches will probably remain closed all winter, says a spokeswoman for the Washington Department of Transportation.

A massive landslide buried a quarter-mile section of the roadway Sunday morning, shoving broken chunks of pavement into the Naches River and changing its course.

Engineers and environmental scientists are examining the landslide today, said Meagan McFadden with the DOT. The hillside that slumped is continuing to move, she said.

It’s not clear yet whether it will be possible to remove the mountain of debris, or whether the road will have to be rerouted, McFadden added. “It’s going to take us some time to get in there and do the work.”

Residents are able to leave the area via the Bethel Ridge logging road, McFadden said. It takes about two hours to navigate the rough, one-way road

Electrical power to the area was shut off shortly after the slide as a precautionary measure. As of this morning, the power remained off, but Pacific Power was evaluating whether to turn it back on.

There was some speculation that operations at a gravel mine might have triggered the slide, but that doesn’t appear to be the case, McFadden said. The pit and several pieces of heavy equipment were completely buried by the slide.

What caused the slide isn’t known. There’s been no discernible rainfall in the area.

The flood danger from the Naches River appears to have subsided, McFadden said. When the river was blocked by the debris, it rose 30 feet before finding a new route. The river is now flowing over a part of the Nile River Road, which runs along the river’s southern bank. About 25 homes were damaged by the floodwaters, and five were hit by the landslide.

The slide destroyed at least two homes. About 80 people were evacuated. A total of about 1,500 people live in the affected area.

While geologists assess the hill’s stability, hundreds of people in the Nile area likely will remain without power for a few days. Pacific Power officials said they don’t want to restore power until the ground stops shifting.

Authorities advised evacuation for all homes within a four-mile radius of the Nile –including a boarding school for troubled youth — although a handful of residents chose to stay. It is unknown when evacuated residents will be able to return home.

State Department of Transportation officials began monitoring the area about 2 p.m. Saturday, when early indications of the slide became evident.

Calling it a “rotational landslide,” Washington State Patrol Sgt. Tom Foster said the blockage appears to be a result of earth shifting under the surface of the hillside — and not a classic landslide.

Soil from the slide area continued to slough off the hillside late Sunday afternoon, according to Ty Brown, a lieutenant with the Nile Fire Department and one of the incident commanders at the scene.

“Our main concern is the river is changing its own channel, trying to find its own way around the slide. We are dealing with flooding in that area,” Brown said. “Our next problem is to try to take care of the folks who live up the valley. They aren’t going to have power for some time.”

The slide took down several power poles, cutting power to about 800 customers in the Nile area, said Pacific Power spokesman Art Sasse in Portland.

Cutting the power was a precaution against a major outage that could affect thousands of people just as overnight temperatures dip well below freezing, he said.

Once incident commanders “have a better handle on the extent of damage to the road and the duration of the closure, we will have a better handle on how long the group of customers will be without power,” Sasse said.

Emergency shelters were set up on both sides of the slide — at the Nile Valley Community Church and Naches Valley High School.

Among those evacuated were 12 boys who live at the Flying H Ranch, a Christian residential program for troubled boys, situated south of the Nile Loop Road off State Route 410.

Chris Rodriguez, a counselor at the ranch, described the slide: “It was like a knife had cut through the hill and moved everything to the side.”

Some ranch employees who stayed behind after the evacuation reported some ranch buildings being threatened by water from the Naches River.

As the boys unloaded from a van at Naches Valley High School, a dazed-looking family wandered into the parking lot.

“Our house got hit,” said a woman in the family, before making her way into the school to speak with American Red Cross volunteers. “We knew we were going to get hit — we’re right up against the hill.”

Citing safety concerns, authorities cut all access for the general public to the damaged area from the Yakima side.

Emergency personnel and residents attempting to leave the area are using Bethel Ridge Road, a gravel road that connects the Nile area to U.S. Highway 12 to the south.

Transportation officials closed a 47-mile stretch of State Route 410 — from Mount Rainier National Park’s Lake Tipsoo to its junction with U.S. Highway 12 at the Y west of Naches.

Emergency personnel described the landslide as a “heaving, moving mass” that pushed chunks of earth and pavement into the Naches River, damming its width.

“The slumping of the hillside and the uplifting of the valley floor continues,” DOT regional administrator Don Whitehouse said in a news release after 4 p.m. Sunday. “It will take several weeks before we can have a new roadway constructed and ready for traffic.”

Initial signs that something was happening on the hillside began around 2 p.m. Saturday, when it began moving, said McFadden, the DOT spokeswoman.

A Nile Fire Department firefighter, Assistant Chief Steve Smith, noticed heaving in the driveway that leads to his home near the slide, officials said. Concerned about the movement, Smith left his home Saturday.

The home was engulfed in the slide Sunday morning.

The ground under a second home in the area collapsed, destroying the structure.

Several other homes have been damaged as water from the Naches River found its way around the blockage through private property and back into the river channel, said Robert Cunningham, a Bureau of Reclamation foreman who surveyed the area late Sunday morning.

“The water is not backing up. It has rediverted over the Nile River Road and is taking its own course around the slide,” he said.

Material from the Yakima Herald-Republic was used in this report.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com