Land on the move

YAKIMA, Wash. -- Call it the shifting road. Or call it, as engineers do, a maintenance problem.

Tieton Reservoir Loop Road, tight on the southeast corner of Rimrock Lake, is a road in motion. Slow motion.

"At either end of the road you get one piece of road that stays in one spot and another that moves laterally or vertically," said Yakima County Engineer Gary Ekstedt.

The problem: The road sits on debris from an old landslide.

"The ground under the road is moving, albeit slowly," Ekstedt said.

For road crews, it means working at least once a year to keep the pavement smooth.

For geologists, it's one more sign of an active landscape.

They've pinpointed more than a dozen places in Yakima County, including Tieton Reservoir Loop Road, that are at risk of being hit by a landslide.

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The sites range from the county's forested mountains in the west to the arid ridges in the east. Most are in remote areas, where homes and other development is light to nonexistent.

But that's not always the case, as demonstrated last month by the massive Nile slide that buried a section of State Route 410 and redirected the Naches River.

Geologist Newell Campbell recalls being appalled at what he saw when he moved to Yakima in the early 1950s.

The now-retired Yakima Valley Community College geology professor saw buildings constructed atop and near old landslides on the steep ridges that define central Yakima County.

Over time, Campbell's worst worries abated after he realized how the region's low rainfall and stable geology reduce the chances of catastrophic landslides.

Reduces, but doesn't eliminate.

Seventeen years ago, Campbell was paid by the county to create a map of geologic hazards -- such as areas considered susceptible to landslides, earthquakes, erosion and other geologic threats.

Not surprisingly, the hills north and south of the Nile community were rated at high risk for slides, based on the fact that slides had occurred within the last 10,000 years.

So, too, were some summer cabin sites along the east side of Rimrock Lake, along Tieton Reservoir Loop Road.

The slide risks aren't always readily apparent.

Longtime cabin owner Larry Judd of Naches Heights said he has seen no evidence of slippage or sloughing around the group of cabins near the loop road.

A well drilled a decade ago is solid and remains unaffected, he said.

"I haven't really seen any problems or have concerns," he said.

Sometimes evidence is clearer.

The Nile slide represents a tiny toe of a six-mile slide so large it's best grasped from an airplane. Known as the Sanford Pasture landslide, it is believed to be about 1 million years old.

Another far more visible landslide is on the east end of Toppenish Ridge, east of Toppenish.

Campbell said he reviewed that area during some consulting on behalf of the Yakama Nation. He theorizes the Toppenish slide was the result of an earthquake, as were other slides along Toppenish Ridge that stretch to south of White Swan.

"We had significant earthquake activity 12,000 years ago on top of Toppenish Ridge," he said.

There is a rumor that an Indian encampment was buried beneath the Toppenish Ridge slide, but Campbell said there are no oral or written histories by Indians suggesting that happened.

No injuries were reported in 1976, when a section of Yakima Ridge just east of Yakima gave way and covered a 175-foot section of Rest Haven Road and plugged an irrigation canal.

That slide happened several days after workers had taken rock from the hillside to shore up a dike on the Yakima River.

While an investigation is far from concluded, some suspect an open gravel mine may have triggered the Nile slide.

Not all gravel work increases the chance of a slide, however. A highly visible gravel operation along Interstate 82 just outside Selah has seen enough gravel removed to reduce the threat of a slide there, Campbell said.

On the ridges in eastern Yakima County, the north-facing slopes tend to be steeper than the south slopes and are more prone to landslides, said David Huycke, who teaches geology at Yakima Valley Community College.

Huycke said geologists generally believe the buildup of these east-west ridges began about 10 million years ago and continued until about 4 million years ago. The ridges, Huycke added, may yet be growing at a rate of a few millimeters per year.

"Because of the nature of the compression, the steeper part is on the north side. That is true of Toppenish Ridge, Umtanum Ridge and the north side of Yakima Ridge," he said. "Those tend to be steeper and more easily destabilized."

This is true of the Horse Heaven Hills east of Prosser. In the 1980s, the opening of a new section of Interstate 82 between Prosser and the Tri-Cities was delayed when the soil began moving beneath a part of the road.

Farther west, another problem area is Wenatchee National Forest Service land northwest of Divide Ridge, which is southwest of Rimrock Lake. There, steep slopes and clay deposits from old volcanic activity make the slopes unstable.

The U.S. Forest Service isn't aware of significant slumping issues on Divide Ridge, officials said, although one Forest Service road was closed due to a slide.

And there have been problems north of the Nile slide in the Milk Creek area, northeast of Whistlin' Jack Lodge, with numerous small slides, said Marge Hutchinson, an engineer in charge of all roads in the Cle Elum and Naches ranger districts.

She said the Forest Service may have to create new access points because of the continual slumping.

Other areas to the east along Ahtanum Ridge near White Swan also are considered at high risk of slides.

* David Lester can be reached at 509-577-7674 or dlester@yakimaherald.com.

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