Outdated map used to allow logging on slope

SEATTLE — State regulators have been using outdated boundaries to restrict logging above the Snohomish County slope that collapsed March 22, failing to incorporate newer research that would have protected a swath of land that wound up being clear-cut, according to a Seattle Times analysis of documents and geographical data.

Because trees intercept and absorb water, removing them can contribute to the risk or size of a landslide by increasing the soil’s saturation, according to geological reports. The impact can linger for years.

In 1997, a report commissioned by the state Department of Ecology used “newly developed computational tools” to map the plateau atop the unstable hill outside Oso. That report was prepared by geologist Daniel Miller and hydrologist Joan Sias; Miller’s portion drew boundaries for where groundwater could feed into the slope and increase the risks of landslide.

When the Department of Natural Resources issued logging restrictions later that same year, the agency cited the Miller-Sias report and treated it as state of the art, saying any future study should emulate its methods. But instead of adopting Miller’s map, the agency used boundaries that had been drawn up in 1988.

“We did the work. It was cited in the prescriptions as what you should do. And it appears from your comparison of the maps that it didn’t get done,” Miller said. “I suspect it just got lost in the shuffle somewhere.”

Had Miller’s map been used for the plateau, called the Whitman Bench, an additional 12 acres to the west would have been placed under protection, according to a Seattle Times analysis.

In 2004, DNR approved the clear-cutting of 7 1⁄ acres

2 on the plateau — about five of which would have been protected under Miller’s boundaries.

Grandy Lake Forest, the owner of that property, finished harvesting the acreage by August 2005. The 71⁄ acres took the shape

2 of a pizza slice — with its tip just touching the part of the slope that fell away this month, releasing millions of cubic metres of sand, silt and clay.

The hill that collapsed had a long history of slides, including ones in 1949, 1951, 1967 and 1988. Geologists studying the slope have cited multiple factors that could have contributed to the slides, from excessive precipitation to erosion of the hill’s base by the Stillaguamish River to logging.