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. Saturday's deadly slide was the latest in a long string of landslides to hit the area known as the Hazel or Oso slide along the North Fork Stillaguamish River. State and tribal officials have known about and tried to block landslides on that spot for half a century. Despite the known hazards, the slopes above the slide area have been clearcut multiple times. Clearcutting is known to aggravate the risk of deep-seated landslides like the one that destroyed Steelhead Drive neighborhood in Oso, Wash., on Saturday.

A clash over logging next to the Hazel slide in the 1980s even shaped, over the course of a decade, Washington's current statewide restrictions on logging of land prone to deep-seated landslides. Tribes Block Clearcut To Prevent Landslides The Tulalip Tribes were so concerned with landslides hitting the Stillaguamish River and its prime salmon habitat that they blocked a proposed timber sale above the Hazel slide in 1988. "There were some very large clearcuts planned for that area, which made us very concerned," said Kurt Nelson, a hydrologist with the Tulalip Tribes. "That reach of the North Fork has multiple, ancient, deep-seated landslides," said Nelson. "There's a lot of unstable terrain in that area."

Summit Timber of Darrington had cleared part of the slope above the landslide zone. The tribes appealed the company's application to log more of the slope. "This had been known at least since the '50s as one of the more problematic areas on the Stillaguamish for perennial landsliding," geomorphologist Paul Kennard said of the Hazel slide. Kennard worked for the Tulalip tribes in the 1980s and now works for the National Park Service at Mount Rainier. "It was our contention that the harvest in the area just above the slide would increase the amount of groundwater, and essentially the largest factor in causing the slide was an increase in groundwater." Summit Timber argued that the river undercutting its bank at the toe of the slide zone was the main factor behind slides there. The state forced the company to cancel its logging plans. "At that time, they almost never prohibited [timber] harvest for any reason," said Kennard. "It was a big deal."

Water: Landslide Fuel The town of Darrington, just east of the slide, has received 16 inches of rain to date in March — double the normal amount for the entire month of March, according to the National Weather Service. Geologists say without big trees to suck up rain water, more water runs underground, where it can lubricate the already unstable soil in deep-seated landslide zones. "Trees intercept a lot of water," said engineering geologist Dan McShane of Bellingham. "With a mature Douglas fir, 30 percent of the water never makes it to the ground." It's the reason the state has restricted logging above landslide-prone slopes like Hazel since 2000 under the state's forest practices act.

"The rule came out of our Hazel lawsuit," Kennard said. The rule requires a detailed geotechnical analysis for logging on any land that could funnel groundwater deep beneath a landslide zone. Since timber companies rarely do such analysis, Kennard said the rule is tantamount to a prohibition of logging such areas. Landslides, or "failures," have followed logging in the Hazel slide area at least four times. "There was cutting in the 1940s; it failed in the '50s," said Kennard. "There was cutting in 1960, then it failed in the mid '60s. There was cutting in '88; it failed in '91. There was cutting in [2004], and it failed in 2006 and in 2014." The 2004 clearcut took place on 7.5 acres of land owned by Grandy Lake Forest Associates of Mt. Vernon. It was much smaller than earlier cuts. Kennard said there was no doubt the clearcut sat within the slide's so-called groundwater recharge zone but that it would take a geotechnical study to know whether the recent logging affected Saturday's slide.