SILVERTON — The ground under a section of the Mountain Loop Highway washed out after weekend flooding and could cost Snohomish County a bundle to fix — if the whole road doesn’t give way first.

For now, the road is down to one lane about a mile west of Silverton. About 45 people h

ave houses in the former mining community in the Mount Baker-Snoqualmie National Forest and a half dozen of those homes are occupied all year. The road also goes to the Big Four Ice Caves, which draws 50,000 people a year.

The slide affects a steep slope below the shoulder, county engineer Owen Carter estimated. During the weekend flooding, the roadway above collapsed. Repairs could cost $1 million to $1.5 million and take five to eight weeks.

“If the whole road goes, Silverton is pretty much landlocked,” Carter said. “The only other way out is going through Barlow Pass, which gets a lot of snow in the wintertime.”

That means a detour of about 50 miles, traveling the long part of the Mountain Loop to Darrington. That also means that county crews would have to plow snow from the route.

The slide area has been under watch by Snohomish County and U.S. Forest Service officials for about 10 years, Carter said.

It’s enough of an issue that it has its own name — “the Waldheim Slide.”

In 2006, a logjam formed, directing the South Fork Stillaguamish River toward the road. That made the erosion worse.

A landslide in December blocked part of the road. Then, heavy rains this past weekend swelled the river, causing the water to eat into the slope below the road.

“It’s a pretty active landslide right now,” Carter said.

Another spot on the other side of Silverton is a problem as well, he said.

Long-term repairs will require putting rocks at the bottom of the slope.

The county and the U.S. Forest Service maintain the road under a joint agreement from 1958.

“It has the potential to be a huge project,” Darrington District Ranger Peter Forbes said. “What we’re trying to coordinate right now with the county is an emergency fix that would stabilize that slope on the roadway.”

The Forest Service is looking for federal money to pay for the work, Forbes said.

In addition to Silverton and the Big Four Ice Caves, the Deer Creek snow play area is beyond the slide area.

Duncan West, a health-information technology consultant, is one of Silverton’s full-time residents. He frequently commutes to work in Mount Vernon.

“It adds an hour for me to get to I-5 if I have to go around” through Barlow Pass, he said.

Forbes said he was lucky that he works to the north. Things could be even more difficult for his neighbors who work in Verlot, in Everett or points south.

The Mountain Loop is a 50-mile scenic byway between Granite Falls and Darrington. It’s paved on either end, with a 14-mile gravel section in the middle.

The repair work near Silverton is an example of why Lynnwood-based civil engineer Bill Lider has been urging the county to exit its maintenance agreement and leave the Mountain Loop roadwork to the U.S. Forest Service. Lider, who’s also a member of the Pilchuck Audubon Society, spoke to the County Council about the issue at public meetings last year.

Noah Haglund: 425-339-3465, nhaglund@heraldnet.com.