A large landslide occurred in northwest Washington on March 22, 2014, leading to tragic loss of life and destruction of property. Landslide debris covered about 40 homes and other structures as well as nearly a mile of State Route 530. It also caused 43 fatalities in the community of Steelhead Haven near Oso, Washington.

This event is commonly named the “Oso Landslide” in many official reports. It is also referred to as the “SR530 Landslide,” as named by Snohomish County and Washington State.

Scientists with the U.S. Geological Survey (USGS) continue to support all of those involved in responding to the event to assess ongoing hazards and identify possible future impacts.

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What Do Scientists Know Now?

Over the past year, scientists have acquired new insight on the circumstances surrounding the Oso Landslide. USGS research indicates that the landslide’s average speed was about 40 miles per hour, with maximum speeds likely even higher. The area overrun by the landslide was about one half square mile, and the landslide moved about 18 million tons of sand, till, and clay. That amount of material would cover approximately 600 football fields 10 feet deep.

The slide dammed the North Fork Stillaguamish River to a depth of as much as 25 feet, forming a temporary lake 2.5 miles long, which flooded houses and other structures in Steelhead Haven. In the 6-8 weeks following the landslide, the river slowly eroded back to near its pre-landslide elevation, effectively draining the remaining excess water by the middle of May.

The landslide involved a complex sequence of events—including rotation, translation, and flow mechanisms—and can be referred to as a debris-avalanche flow. Studies indicate that slope failure occurred in two stages over the course of about 1 minute. During the second stage of movement, the landslide greatly accelerated, crossed the North Fork Stillaguamish River, and mobilized to form a high-speed debris avalanche. The leading edge of the wet debris avalanche probably acquired additional water as it crossed the North Fork Stillaguamish River, ultimately transforming into a water-saturated debris flow. Debris flows are liquefied slurries of rock, water and mud that can travel great distances at high speeds, entraining nearly all objects in their paths.

Did Scientists Expect this to Happen?

The Oso Landslide occurred in an area of known landslide activity, but this time, the slide was much larger, traveled much further, and had greater destructive force than others previously experienced at the site.

An incident of this magnitude was not expected based on previous studies of past events in the area as well as comparison with landslides worldwide of this height, volume and type. If the landslide had behaved in the expected range, it would have likely blocked the river and possibly destroyed some houses, which may have caused casualties. Instead it led to numerous fatalities and more large-scale destruction.

The landslide’s high mobility was likely due to several contributing factors, including the soil’s initial porosity and water content from rainfall in the months prior. Precipitation in the area during February and March of 2014 was 150 to 200% of the long-term average, and likely contributed to landslide initiation and mobility. Landslide mobility might have been far less if the landslide material had been slightly denser and/or drier.

Coordinated Response

Response to the Oso Landslide has involved many federal, tribal, state, and local agencies, as well as the private sector. These organizations include Snohomish County; the Washington State Emergency Management Division; the Federal Emergency Management Agency; the Washington State Department of Natural Resources; the Washington State Department of Transportation; NOAA’s National Weather Service; the U.S. Army Corps of Engineers; the Stillaguamish Tribe of Indians; and the USGS.

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Immediate USGS Response

Shortly following the landslide, the USGS and partnering agencies provided near-real-time information on the potential for additional landslides and flood threats to search and rescue personnel.

The USGS deployed three “spiders,” portable instrument packages originally developed for monitoring active volcanoes and landslides. Spiders contain high-precision GPS units for detecting landslide movement as well as geophones for detecting small vibrations. The spiders were placed on and near the landslide with helicopters.The USGS was also able to provide immediate data on water levels and river discharge from an existing permanent streamgage located downstream on the North Fork Stillaguamish River at Arlington. Immediately following the event, the USGS installed three rapid-deployment gages and three buoys to measure flow, sediment, and lake levels.

Ongoing Research and Goals

There is still much to learn about the Oso Landslide, as well as how and why landslides happen and behave in general. With the initial disaster response now over, longer-term questions have arisen. Below is a summary of key USGS projects underway. There are many more projects in progress with partners.

One key research goal is to identify the primary reasons for the landslide's high mobility. This past summer, USGS scientists spent approximately three weeks mapping the geology and geomorphology of the landslide deposit. The USGS also worked with partners to obtain and study soil samples from the landslide area to better understand the landslide’s characteristics and behavior.

USGS research on the North Fork Stillaguamish River is aimed at identifying potential impacts to the aquatic ecosystem and changes to flood risk along the river corridor including newly reconstructed stretches of State Route 530 and downstream bridges.

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The USGS presently operates eight streamgages along the Stillaguamish River, with two being permanent and six as rapid-deployment gages. Scientists are collecting continuous data from those gages in order to analyze downstream movement of sediment and identify possible regions of accumulation. USGS scientists are conducting aerial overflights to collect photographs and monitor river channel evolution through the landslide.

Start with Science for Landslide Hazards

Landslides occur in all 50 states and U.S. territories, and cause $1-2 billion in damages and more than 25 fatalities on average each year.

The goal of USGS landslide science is to help answer questions such as where, when, and how often landslides occur and how fast and far they might move. USGS scientists develop tools and produce maps of areas susceptible to landslides and identify what kinds of conditions will most likely lead to such events. For more information, watch a video about USGS landslide science, and visit the USGS Landslide Hazards Program website.

Scientists at the USGS are also asking the public to help by reporting landslide experiences and sightings at the new USGS “Did You See It?” website. Further, the USGS is working with the National Weather Service on a Debris Flow Warning System to help provide forecasts and warnings to inform community and emergency managers about areas at imminent risk. Such a system would enhance the USGS ability to respond to landslide crises such as the Oso Landslide.

Photographs and Graphics

• View photographs related to the landslide and impacts.
• Watch a computer simulation on the landslide.

Publications and Additional Information

• A USGS project to monitor the evolution of the new channel and the volumes of sediment transported by the North Fork Stillaguamish River
• Earth and Planetary Science Letters article, “Landslide Mobility and Hazards: Implications of the 2014 Oso Disaster”
• USGS article, “Preliminary Interpretation of Pre-2014 Landslide Deposits in the Vicinity of Oso, Washington”
• Governor Inslee’s Joint SR 530 Landslide Commission
• Geotechnical Extreme Events Reconnaissance Report