Photo originally published by the Seattle Times. Original article can be seen here: https://www.seattletimes.com/seattle-news/on-4-year-anniversary-of-oso-landslide-family-members-of-victims-announce-memorial/

Glacial Mud and Deadly Landslides: A Legacy of Glaciation in Western Washington

This Naturalist Note was written by Graduate Student Gina Roberti. Gina is a member of the North Cascades Institute M.Ed Residency Program's 17th Corhort, Class of 2019. Last fall, Gina attended a Geology Society of America field trip as a representative of North Cascades Institute. Enjoy this piece about her experience!

It was a rainy morning on a cold, October day when I developed a new lens for reading the landscape of western Washington. I embarked from the Environmental Learning Center, at dark, with my North Cascades Institute nametag and field journal at the ready. The next morning, I found myself amidst a large crowd of unfamiliar faces and introductory chatter. This field trip was a celebratory event, to kick off the Annual Geologic Society of America Meeting in Seattle. Over fifty geologists of varying ages surrounded me, armed with rain gear and umbrellas. All had come eager to discuss the history of landslides in western Washington.

We met along the North Fork of the Stillaguamish River just outside of Darrington, WA. I had driven along this road many times, questioning the large gouge in the hillside, perhaps a gravel pit or mining operation? Hiking up a Forest Service road on the facing hillside, we cleared the valley floor and ascended to where we could see this large scour from across the river. From this vantage, the extent of the destruction was clear.

Our field trip focused on deep-seated landslides in Western Washington. The seemingly benign hills of the Stillaguamish River valley was the first crime scene; the site, we learned, of many past and recent landslides.  Our purpose was to discuss the most recent Oso landslide, and to tease apart the story, and like forensic scientists working backwards at the scene of a crime. It was our task to determine if this destruction was likely to happen again.

Animation combining aerial photography capturing the site of the Oso landslide before and after the event. Note how the course of the river was altered. Source: New York Times, “Search List Expands in Washington State Mudslide” https://www.nytimes.com/2014/03/25/us/search-continues-after-washington-state-landslide.html.

The Oso landslide is the deadliest landslide to date in American history. Forty three people were buried alive. The slide occurred on a sunny Saturday morning, when children were home from school and parents were home from work. Forty three evergreen trees now stand planted along Washington Highway 530 which runs directly through the valley, the highway was also buried by 12 feet of debris during the slide. The fence in front is now decorated with wreaths and teddy bears.

Debris from the Oso slide backed up the Stillaguamish River for multiple days, flooding upstream. The location of Steelhead Haven housing development is circled in red.
An image of the destruction caused by the Oso landslide, which buried 43 people. The final settlement for an estimated $120 million in damages was for $60 million for families of residents. The State of Washington paid $10 million, insurance companies paid $40 million and Grandy Lake Timber associates, a timber company accused of logging that increased the instability zone, paid $10 million.
A slide in 1967 had run into Steelhead Haven buildings as they were being constructed, but building continued after the slide material was removed. Another landslide occurred along the same slope in 2006. This photograph was taken soon after the 2006 slide. For more information on the politics surrounding the construction of Steelhead Haven, see this article in the Seattle Times: http://old.seattletimes.com/html/localnews/2023794966_landslidehistoryxml.html

In the wake of the slide, many fingers were pointed at the logging industry for decreasing the stability of slopes through clearcutting practices. Darrington hosts one of the last major commercial logging mills in an area where logging served as the primary economy for many decades. In the final legal settlement, Grandy Lake Timber Associates was accused of increasing runoff and making the property more vulnerable to landslides from a recent clear cut. Once a tree is cut, it takes about eight to ten years for the root mass to rot and the soil to destabilize. The forested land on the Whitman Bench above the site of the landslide was cut approximately ten years before the Oso slide. Grandy Lake Timber Associates contributed $10 million to the settlement, of approximately $120 million in property damage. Yet from a geologists’ perspective, the picture becomes wider when comparing the site of the Oso landslide to other places in the valley.

A key resource for teaching on this field trip was a set of high resolution maps created with a relatively new technology called “LIDAR.” LIDAR images are created by shooting millions of laser beams to the Earth’s surface, and like bats using echolocation, using differences in the return time of the beams to reconstruct topography. This resolution of this technology is so high that it negates the signal from vegetation, producing a geologist’s true dream: an image of the Earth’s surface without the interference of plants. The following LIDAR image illustrates how common landslides are in the Stillaguamish valley. The most recent Oso landslide is highlighted by a yellow dotted line.

The following image was produced using high resolution laser mapping technique called LIDAR which generates maps of surface topography minus vegetation. The location and extent of the Oso landslide and other landslides in the valley can be clearly seen with this new imaging technology. The Oso landslide headscarp is carved into the Whitman Bench, an upland low-relief surface 650 feet above the valley floor.

In fact, a closer look at the geologic record indicates multiple layers of landslides that have occurred in the Stillaguamish River valley throughout the last 5,000 years.

The geologic record provides evidence for many slides within the bench of glacial deposits lining the Stillaguamish River valley in the last 5,000 years. Figure from A. Duvall, University of Washington, 2017, GSA Field Trip Report.

The fact that landslides are common in this valley hearkens back to its glacial history. Twelve thousand years ago, the Stillaguamish River valley was blocked by glacial ice, from a continental ice sheet over a mile thick that buried the Puget Sound. Channels of the Stillaguamish River have since cut down into geologic materials deposited by glaciers, including sediments of glacial streams or in glacially- dammed lakes.

A schematic representation of how the Stilliguamish valley might have appeared when blocked by ice from the continental ice sheet that covered Puget Sound, 12,000 years ago.

As graduate students in the North Cascades Institute M.Ed. program, we had the opportunity to see some of these glacial sediments up close and personal.

Graduate student Brendan McGarry examines muds and plants preserved in ice-age lakes that left thick layers of sediments filling many of the valleys in Western Washington on a field trip led by North Cascades Park Geologist John Reidal. Classmate Amy Sanchez smiles in the background.

The Skagit Valley, which runs parallel to the Stillaguamish, home to Concrete and Sedro Wolley is also filled with glacial sediments. These layers of mud, sand and gravel sit atop harder bedrock. It is this material that fills the valley floors, but also is steadily sliding away.

Members of the Graduate Cohort 17 stand in front of an exposed section of glacial sediments outside of Concrete, Washington, in the Skagit Valley. These muds, sands and gravels were deposited when the Puget Sound was blocked by ice during the last continental glaciation, 12,000 years ago.

The question after the Oso landslide for geologists was; what made this slide so deadly? The housing development which was buried, Steelhead Haven, sat almost a mile from the valley walls and was not included in landslide hazard zone. Yet any geologic examination of the valley would have revealed that these walls were prone to collapse; a landslide occurred in the exact same hillside in 2006, just seven years earlier. Why did debris from the Oso slide travel so much further than the slide in 2006, far enough to bury the homes in Steelhead Haven?

My field trip to the Oso landslide site with the Geologic Society of America was partially led by David Montgomery, of the University of Washington. He was one of several folks hired to do geologic mapping of the Oso slide directly after the event and determine what happened. The post-slide evaluation produced by his group suggested that the Oso landslide had remobilized sediment that was already unstable from the 2006 slide.

The takeaway: landslides in this area are frequent. According to many geologists on this trip, “The glacial sediments [coating the valley walls] will continue to erode away until it is stripped down to more resistant bedrock. The next slide is only a matter of time.”

Thank you to the North Cascades Institute for providing funding assistance for me to attend this conference!


In October 2017, thousands of geologists from around the US (and the world) descended upon Seattle for the Geologic Society of America Annual Meeting. It is an opportunity to share research, network, and, most importantly, host field trips to sites of geologic interest in the area. As a graduate student at the North Cascades Institute, I was excited to participate in the conference and share my work as an educator, geologist, and now local resident of the Pacific Northwest.

A link to a press release about the conference and research on the Oso slide can be found here:

http://www.geosociety.org/GSA/News/Releases/GSA/News/pr/2017/17-55.aspx

For more information on how scientists are studying geology and natural disasters in Washington State, check out this cool, new resource from the Washington Department of Natural Resources State Geologic survey:

https://wadnr.maps.arcgis.com/apps/Cascade/index.html?appid=36b4887370d141fcbb35392f996c82d9

 

Comments

  1. Emily Ford

    Thanks for this article Gina! I appreciate how your discussion of glacial history adds complexity to an often over-simplified event. Also awesome to hear that you got to go to this conference through NCI! 🙂

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