The process sometimes repeats itself. The river washes away soil, a slide occurs and the process begins anew, with water eating away at the remaining foundation that holds up the hillside, said David Montgomery, a professor of geology at the University of Washington.
“It’s pretty clear to me that one thing’s that made that area so unstable over the years is that the river is taking the toe out of that slope,” he said. “Whether that contributed to this recent reaction of this old slide is an open question.”
At least two smaller slides have occurred in recent decades along the same hill — one in 2006, another in the late 1960s.
“There have been many historical landslides in this valley since glaciers receded,” state geologist David Norman said at a news conference here on Monday.
Last week’s heavy rainfall and saturated soil also have been cited as likely factors.
Still, a precise trigger for the landslide has not been determined, Norman said. Geologists are studying the area with aerial photography and laser technology.
“Very little has been understood at this point,” Norman said. “There are lots of question marks.”
Geologists from the U.S. Geological Survey, the state Department of Transportation, the state Department of Natural Resources and Snohomish County are reviewing the information, he said.
Shifting conditions in the slide zone and the threat of more slides have limited the time anyone is able to spend in the area.
Rain is forecast for every day the rest of the week for the Cascade foothills, which is a concern, Norman said.
A lot is known about the geology of the North Fork Stillaguamish River valley, scientists said.
The top layers of the surrounding hills consist of porous glacial soil that becomes easily saturated with heavy rainfall.
Geologists agree that Saturday’s slide was a deep-seated one, meaning several layers of soil gave way.
A state geologic map shows that in many areas just north of the current river route — including along the base of the hill that collapsed — the soil is made up in part of river silt.
There was no way to predict when the hillside would give way, Norman said.
Saturday’s slide left a headscarp, or top of the slide, 600 feet high and 1,500 feet long, he said.
The slide zone is nearly a mile long and a mile wide, he said — it ran 4,400 feet outward from the hillside and spread about the same distance from side to side.
The debris field is 30 to 40 feet deep, Norman said.
Water that accumulated behind the slide was 20 feet deep as of Monday afternoon but holding steady, he said — not increasing or decreasing in size.
The river carved a channel through the slide zone on Sunday, reducing the risk of a flash flood downstream, officials said. River gauges downstream, which showed a dramatic drop in the water level late Saturday and early Sunday, showed the level had rebounded to normal levels by Monday. The U.S. Geological Survey plans to add gauges downstream, Norman said.
The confluence of the north and south forks of the Stillaguamish River, at Haller Park in Arlington, reflected the difference in water between the two streams on Monday.
Water in the north fork was a flat, brownish gray, while water coming in from the south fork was bluish green.
Bill Sheets: 425-339-3439; firstname.lastname@example.org.
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