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Geologist: Mudslide still moving, even bigger than first thought

Geologists have determined the Oso mudslide is 1500 feet long, 4,400 feet wide and over 600 feet high, with a debris field 30-40 feet deep. (Ted S. Warren/AP)

Geologists say they still have far more questions than answers about the devastating mudslide that wiped out a small town and killed at least 14 people. And they caution the ground is still moving.

Dave Norman, Washington state geologist with the Department of Natural Resources, says preliminary assessments have determined the slide is 1500 feet long, 4,400 feet wide and over 600 feet high, with a debris field 30-40 feet deep.

"This is one of the biggest landslides I've seen," says Norman. He says at this point, conditions remain too dangerous for crews to operate in the slide zone, and he doesn't know when things will stabilize.

"We're monitoring the movement on the landslide and I think as it settles down if we can see that there's less movement then we would have a better feel for that."

Norman cautions the area that gave way Saturday could slide again, and neighboring mountainsides remain a threat as well.

"Anything that has just had that much movement, there would be additional possibility of more movement off of the face of that cliff," he says.

Norman says the area is made up of glacial sediments and has been the site of numerous landslides through history, most recently in 2006 and 1969. He says heavy rains saturated the sandy soil, acting as both a lubricant and providing pressure that triggered the massive slide.

Geologists are currently mapping the area with laser detection systems and aerial photography to determine where it is safe for crews to operate.

Norman says they've also deployed a number of gauges upstream in the mud blocked Stillaguamish River to determine the potential for flooding. But he says the river has begun cutting a channel through the blockage and is slowly flowing over the top.

"This is a good thing that the pressure behind the impoundment is starting to be relieved by that water flowing through it, and that is something that we are looking closely at at this point," he says.

The National Weather Service has extended a flash flood watch for the Stillaguamish River through Tuesday afternoon. Meteorologists warn water building up behind the blockage could still burst through, but the alert says flooding is not imminent.

Geologists are also keeping a close eye on the weather. Scattered showers are forecast starting Monday night, with heavy rain forecast for the end of the week, raising the possibility for more slides and flooding.

About the Author


Josh Kerns is an award winning reporter on KIRO Radio 97.3 FM. He covers everything from May Day riots in Seattle to the latest Boeing news.

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