Hills along Naches River prone to slides

NACHES, Yakima County — Sunday's landslide that buried Highway 410 and the Naches River wasn't the first to strike the area and won't be the last, geologists say.

Nor is it anywhere near the largest.

About 2 million years ago, a landslide caused a massive amount of dirt and rock — probably several cubic miles of material — to slide down the steep slope to the north, completely covering the valley floor to a depth estimated at 800 feet.

Sunday's landslide occurred at the very west end of what's called the Sanford Pasture slide, which extends several miles to the east along the base of Cleman Mountain.

It was also near a gravel pit, where cracks in the hillside above prompted a request for geotechnical testing several years ago. Geologists agree the slide's origins stem from combination of steep slopes, undercutting by the Naches River, and a surface layer of basalt sliding atop a deeper formation as if it were on ball bearings, as one geologist described it.

Jack Powell, an Ellensburg-based state Department of Natural Resources geologist who has studied the area for years, said a landslide like Sunday's might not be seen again in our lifetime.

But it will happen again.

"This whole thing had slid before and formed a beautiful little landslide area that rehealed over a long period of time," he said. "Now it has failed again. The cliff will fill back up with broken rock, and after thousands of years it will fail again."

Powell said the basalt visible from the road sits atop an interbed of sand from rivers that flowed there millions of years ago and was later covered by a layer of flowing lava.

"The interbed is the key to this. It is about 20 to 30 feet thick ... it is especially loose and pure. It is like having a 30-foot thick layer of ball bearings."

Authorities have not determined what prompted Sunday's slide. Some residents have pointed to an operating quarry at the south end of the slide site.

The Simmons quarry has been operating there since 1984, according to state geologist Dave Norman.

He said any connection between the quarry operations and the slide would be speculation.

But he did say inspectors identified cracks in the hillside above the quarry in 2005, prompting a request for the operator to conduct geotechnical testing of the hillside. The cracks were not sufficient cause to order the quarry closed.

"There was a landslide there before. They were working in an area that had moved before," Norman said. "There was some ground movement we noticed a few years ago. We were working through a process of getting engineering reports and revisions to their plans."

Norman added the quarry is at the south end of the slide. The quarry did lose some equipment in the landslide.

Paul Hammond, a retired Portland State University geologist who has extensively mapped the upper valley area since 1989, said the presence of rivers such as the Naches that cut through the toe of the steep slopes, plays a role in landslides.

Tom Badger, an engineering geologist with the state Department of Transportation agreed with Hammond's assessment of the impact streams can have on steep rock structures that are filled with fractured basalt rock. But he emphasized the agency has not yet determined the cause of Sunday's slide.

"You have this situation where these rocks are now being undercut by the river," he said. "After a few million years of erosion, that structure is exposed, and that is what triggers final failure."

Powell said geologic features like the steep slopes that border the Naches River could be considered unstable if expressed in the context of the expanse of geologic time.

"In our lifetimes we should never see another event like that in this area," he said. "Once every few centuries, you will have an event of this magnitude."