Editor’s note: An earlier version of this story stated that land above the Oso landslide zone was logged in 2005. The site was logged in 2004 and replanted in 2005.
Seattle just wrapped up its wettest March on record, with 9.4 inches of rain reported at Sea-Tac International Airport.
Geologists say near-record rain in the Cascade foothills was key in triggering the fatal landslide near the town of Oso, Wash., on March 22. But they say clear-cutting nearby could also have worsened the risk of the hillside collapsing.
"We're standing on top of a landslide that's geologically similar to the one that occurred near Oso, Washington," said geomorphologist Paul Kennard, looking out over a crumbling cliff face in Seattle’s Discovery Park. The slumping hillside rises above, and occasionally falls onto, a beach where Seattle meets Puget Sound.
Kennard studies the way water and other forces shape the land, including the deep-seated landslides that often hit the slopes and shorelines of Washington state.
"It's almost universally accepted by scientists that water is a dominant control on these large landslides,” he said. “Generally, the more you lubricate, once the water percolates down to the clay, that's when these things start to move more frequently and more quickly."
Rain is the fuel for landslides in wet western Washington. Darrington, just east of the Oso slide, just had its second-wettest March on record. The town reported 19 inches of rain in March, nearly three times its normal rainfall.
The tall trees of the Evergreen State help hold the ground together, not just with their roots, but also by soaking up rain before it goes deep underground. That's why the state essentially prohibits clearing forests in places where groundwater can pool beneath a landslide zone.
"The clearing just makes it much more vulnerable for when you have a wet winter like now," Kennard said.
Drag the map down to see the pizza-slice-shaped clear-cut at the top of the March 22 Oso landslide. Credit: Produced by Esri with imagery from the Washington State Department of Transportation.
‘Whoa, That Seems Awfully Close’
Kennard works at Mt. Rainier National Park now, but in the 1980s he worked for the Tulalip Tribes north of Seattle. Even back then, tribes and state officials knew the Oso site had a long history of landslides large and small.
In 1988, Kennard helped the tribes stop a clear-cut on the same slope that gave way last week. The tribes saw it as a victory for the salmon they depend on in the Stillaguamish River at the base of the muddy slope.
"We considered it a victory because the Department of Natural Resources, who has say over these things, this was the first time, to our knowledge, that they actually prohibited clear-cutting of trees because there seemed to be a connection with landslides," Kennard said.
On the night of the latest slide, Kennard was surprised when he looked up the area on Google Earth. He saw an aerial photograph with a clear-cut from 2004 right at the edge of the landslide zone. "I thought, whoa, that seems to be awfully close," he said.
Aerial photos taken of the Oso slide last week show a wall of exposed earth hundreds of feet high, almost directly below that 2004 clear-cut.
State officials say the cut they approved was safe: It was just outside the no-logging area intended to keep groundwater from building up. But those aerial photos also show that the 7-acre clear-cut crossed the line.
"It appears, though we continue to investigate, that there's been an intrusion into the no-harvest area," said Washington State Forester Aaron Everett.
That intrusion apparently happened at the hands of the landowner, a small timber company called Grandy Lake Forest Associates.
The company has not responded to interview requests. "We, as everyone else, have not been able to get a hold of the landowner. I don't know their status," said Everett.
While his agency prohibits logging in the groundwater zones of landslide areas, Everett said he doesn't see logging playing a big role in this slide.
"The information we have is that the primary factors in this landslide were the erosion of the hillside below by the river, the nature of the soils themselves, being an area that's had landslides for going on over 10,000 years, and the amount of rain that happened," he said.
The state allowed the 2004 clear-cut right on the boundary of a no-logging zone that didn't reflect the latest scientific information. State-funded research from eight years before the clear-cut showed the groundwater danger zone clearly overlapping with the area the state OK'd for clear cutting. Everett said the DNR is investigating what happened.
"The protections that the department puts in place to ensure that timber harvest do not contribute to landslide hazards are rigorous and are designed to protect public safety and natural resources,” he said. “It's a difficult thing to talk about in the face of a tragedy like this."
Geologists say identifying where groundwater moves beneath a landslide zone is hard to do precisely.
"Perhaps what is sort of the legal definition of the groundwater recharge area is actually a small part of the real groundwater recharge area," said Kennard. "All that area has been cut and it takes decades to recover."
Kennard urges more cautious land use anywhere near one of these deep-seated landslide zones.
But geologists also urge people not to jump to conclusions about the role of one small clear-cut in the approximate 300-acre landslide. They say detailed study of this slide is needed before concluding how much of a role, if any, the recent clear-cutting played.
The groundwater danger zone defined in this 1997 geological study of the Oso, or Hazel, slide was not adopted by the Department of Natural Resources.