BELLINGHAM - While Whatcom County has no known landslide threats that compare to the one that devastated the Oso community along the Stillaguamish River's North Fork on March 22, hilly portions of the county are dotted with areas where small-but-dangerous slides have occurred in recent years.
Dan McShane, a geologist and former Whatcom County Council member, said geologists were well aware of the hazards at the Stillaguamish site before the Oso disaster.
As McShane described it, the river in that part of Snohomish County has cut through a deep accumulation of unstable sediments that accumulated at the bottom of a late Ice Age lake created by a glacial dam. The lake drained once the glacier melted, and the river has been cutting through the deep sediment deposit and triggering massive slides ever since, most recently in 2006.
While the Nooksack River poses no comparable dangers near populated areas, there is an unstable stretch of glacial deposits known as the Clay Banks, between Deming and the Mount Baker Highway bridge at Nugents Corner. On Feb. 21, 2014, big blocks of gray clay broke loose from a steep bank along the channel and tumbled into the river, cutting off its flow for a few minutes before the water found a new route through the obstruction.
Nobody was in harm's way when the most recent slide occurred in the Clay Banks area. A single house originally built 300 feet from the bluff had been abandoned years earlier as the river's channel moved too close for comfort. That house had tumbled down the bluff long before the February slide. County officials condemned one other home in the area years ago because of the danger.
Scott Linneman, a landslide geomorphologist at Western Washington University, said the Clay Banks slide zone is less dangerous than the one on the Stillaguamish because the unstable sediments are not as deep and the hill is only about half the 700-foot height of the slope that triggered the slide that devastated Oso.
But the Clay Banks area of the Nooksack still needs close monitoring. There is a risk that a larger slide could divert the river into a new course that would threaten farms and homes, Linneman said.
On a smaller scale, Whatcom County's steep, wooded slopes can turn treacherous whenever heavy rains swell the creeks. Debris can form temporary dams that unleash log-and-rock-laden torrents of mud when they rupture. Such events have damaged homes repeatedly in recent decades, although no lives have been lost.
"We've been very lucky no one's been killed," McShane said.
In January 2009, several homes in the Van Zandt area of the South Fork Valley were damaged by debris that surged down creeks swollen by heavy rain. At about the same time, a flow of mud and debris roared across Mount Baker Highway and damaged homes in the Marshall Hill Road area.
Similar debris flows damaged homes in the south Lake Whatcom area in 1983. McShane said the county began toughening its regulations on development in dangerous areas in response to that episode.
Today, the county has identified hazardous areas where regulations prohibit the platting of new building lots, and buildings proposed on existing lots receive extra scrutiny to minimize the risks for occupants of those buildings.
McShane observed that mountainous areas are inherently unstable. The town of Glacier is built atop the remains of an enormous landslide from Church Mountain that happened more than 1,000 years ago. Is Glacier a safe place to live?
"The likelihood (of a similar epic slide) on a year-to-year basis is really low," McShane said. "You can't completely rule out that it could happen again."
WWU's Linneman said the prehistoric slide in the Glacier area was different from the Stillaguamish disaster: It involved the collapse of a massive amount of bedrock that slid off the mountainside, and traveled about six miles. Similar bedrock collapses have occurred on Slide Mountain off of Mosquito Lake Road, Linneman added.
Roland Middleton, special projects manager in the county's Public Works Department, said the county makes efforts to protect residents of hazardous areas.
A levee on Canyon Creek was relocated last year to better protect the Glacier Springs subdivision, Middleton said. A debris flood wiped out three summer cabins there 24 years ago.
The Public Works Department is also looking at ways to protect Acme from a potential debris flow down Jones Creek, Middleton said.
In recent weeks, county workers have been trying to keep up with the ongoing Swift Creek landslide on the western flank of Sumas Mountain, worsened by recent rains. They removed about 3,000 cubic yards of debris to keep Goodwin Road open, Middleton said.
The state Growth Management Act of 1990 restricts new development along "critical areas," which include steep slopes. The critical areas map on the county website shows the Cascade foothills in the east county are riddled with hazardous slopes.
"The county would require you to hire a geologist to take a look before you build in those areas," Middleton said.
As for the uncounted number of homes already built under or on top of unstable slopes, Middleton advised hiring a geologist to assess the threat. Property owners should also be on the lookout for warning signs.
"If people ... go hiking around their property and take a look, they're going to be the ones to see it first," Middleton said.
Signs of possible danger include trees that start leaning, fissures in the soil, and streams that run dry after heavy rains, a sign that a blockage could be temporarily holding back a torrent.