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POSTED: Sunday, Sep. 13, 2009

WWU geologists delve into massive slide at Racehorse Creek

DEMING - Barry Hutchinson was home on the rainy afternoon of Jan. 7 when he heard a roar that grew louder and louder, until he thought Mount Baker was blowing its top.

"It sounded like a 747 tethered to the bridge," he said, referring to the span over Racehorse Creek that is about a half-mile from his home near North Fork Road.

The roar, which started about 3:15 p.m. and lasted for as long as 20 minutes, was neither volcanic activity nor airplane noise. Hutchinson heard what may have been the largest landslide in Washington state unleashed during rainstorms in early January that also caused widespread flooding.

The massive slide on Department of Natural Resources land east of Deming was one of 1,500 recorded in the state from the storms, with 30 in Whatcom County, through aerial and field surveys conducted by the Washington Division of Geology and Earth Resources.

What is known as the Racehorse Creek Landslide has caught the attention of Western Washington University geologists, who have been studying the event. They will present their findings, including why they believe it happened, during the Geological Society of America's annual meeting in Portland, Ore., Oct. 18-21.

The goal is to help anticipate when other landslides like this might happen, said Juliet Crider, associate professor in the WWU Geology Department.

Unlike other slides that came down during the January storms, Racehorse Creek didn't threaten homes or businesses. No one was injured.

By the time it was done, as much as 500,000 cubic meters of debris had rolled off Slide Mountain - with much of it landing in Racehorse Creek, Crider said.

That volume is akin to more than 20,000 dump-truck loads of rock and timber, she explained.

"It must have made a hell of a noise," said Dave Tucker, a researcher in the WWU Geology Department.

A number of factors contributed to the landslide, according to WWU and DNR geologists.

One was the "pineapple express" weather system that blew in from Hawaii on Jan. 7.

It came on the heels of frigid temperatures in December and January that had dumped snow across Washington state. Snow was still on the ground in some of the lowlands when the weather front brought heavy rainfall, from 3 to more than 20 inches over two days, and warm temperatures.

Widespread flooding occurred in Whatcom County and Washington state.

"The rainfall saturated slopes, many already wet from melting snow, triggering debris flows and debris avalanches throughout most of Western Washington," according to a DNR report.

"Areas sensitive to high-intensity storms, such as Glenoma, Concrete and Van Zandt, were the site of numerous large debris flows, blocking roads, limiting emergency response, and destroying homes," the report stated.

The landslide occurred in Chuckanut Formation, which is comprised of layers of sandstone, coal, shale and mudstone that are about 50 million years old. It's the same rock that makes up the Chuckanut Mountains and is found beneath WWU.

It also can be unstable.

"There have been slides in the area in the past, historically," said John Coyle, the northwest region geologist for DNR.

A master's thesis by WWU graduate Courtney Gardner noted that 144 landslides had been mapped within the Racehorse Creek watershed between 1940 and 1995.

The area also bears evidence of prehistoric landslides, according to Crider and Tucker.

DNR geologists also have been looking at whether previous logging in the area may have contributed to the slide, but Coyle said that determination has not been made.

Isabelle Sarikhan, hazards geologist for DNR, noted other possible contributing factors.

"The landslide was also on a steep gorge and by typical geologic processes, you can expect the creek to slowly erode the valley sides, sometimes creating ... greater instability," she said in an e-mail.

She also said a small earthquake recorded on Jan. 6 in the Sumas area could have contributed to the slide.

But nearby residents fear that DNR logging policies could be a cause.

Alexis Zahars, of the Downslope Neighbors Association, said the Racehorse landslide was discussed during a recent community meeting of neighbors in the South Fork Valley.

"We are very concerned that the DNR is still not responding to the growing evidence that the forest practice regulations are leading to more frequent and dangerous landsliding in our county," Zahars said in a statement.

"This is resulting in huge costs to private landowners and the county taxpayers in cleaning up the damages," he said, "and we are especially concerned that the delay of the DNR to respond to public concerns is putting families at risk."

On that January afternoon, the heavy rainfall lubricated the Chuckanut Formation, until a weak coal seam in the bedrock slipped and took sandstone with it. The earth started moving at 2,200 feet above sea level and slid 800 feet down into Racehorse Creek, a stream that supports steelhead, coho, spring-run Chinook, pink and chum salmon as well as cutthroat trout.

It carried vast amounts of rock, mud and timber down the slope of Slide Mountain, forming a massive log jam in Racehorse Creek.

Sarikhan said the slide was "about 100 times greater in size than the average landslide during the January 2009 storm."

It gouged out a chunk of bedrock, leaving about a 75-foot vertical wall in Chuckanut Formation, Tucker said.

"We think it's significant because of the depth of bedrock that was involved," he added.

The slide left behind a scar that can be seen from Mount Baker Highway near Kendall - and a lesson.

"It's a reminder that the mountains around us are not permanent," Crider said. "They're constantly changing."