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Unstoppable Slopes, Landslides, Debris Flows, and Rockfalls

Geological Activity Common in Oregon

Landslide on U.S. 101

A landslide is a location where the ground is constantly on the move, part of the natural geology of Oregon, especially along the coast and throughout the Coast Mountain Range. Landslide events are when a new or existing landslide experiences a large pulse of movement. We are continually assessing and taking inventory of unstable slopes.  We currently have 4,143 identified unstable slopes along Oregon highways. These landslides affect about 7% of our entire highway system or 495 miles of road.

Our mission is to keep our roads safe and open for travelers. We also recognize the need to make our system more resistant to earthquakes, which can trigger landslides. The effects of climate change make the system even more vulnerable to slope movement.
  • ODOT is continually taking inventory of unstable slopes. We currently have 4,143 identified unstable slopes along Oregon highways.
    • 64 debris flows
    • 2,237 landslides
    • 1,842 rockfalls
  • The estimated cost to stabilize damage to infrastructure of all inventoried unstable slopes is between $5.3 and $6.3 billion.
  • We typically spend around $6.3 million a year on unstable slope mitigation projects and $11.2 million on maintenance repair.
  • At the current funding rate, it would take 841 years to fix all the unstable slopes. If we spent $177 million a year, we could do it in 30 years. 
There are three slide areas that have the most impact on Oregon highways.

Below you'll find more information including definitions, what we are doing and what you can do.


​A landslide is defined as the movement of a mass of rock, debris, or earth down a slope.  “Landslide” is a broad term used to describe any down-slope movement of soil and rock under the direct influence of gravity. "Landslides" encompass five modes of slope movement: falls, topples, slide​s, spreads, and flows. These are further subdivided by the type of geologic material (bedrock, debris, or earth). Debris flows (commonly referred to as mudflows) and rockfalls are examples of common landslide types.

Common Landslide Modes

  • Slides- downslope movement of earth on a surface of rupture (failure plane or shear-zone). Commonly occurs along an existing plane of weakness or between upper, relatively weak, and lower, stronger soil and/or rock. The main modes of slides are translational and rotational.
  • Flows- mixtures of water, soil, rock, and/or debris that have become a slurry and commonly move rapidly downslope. The main modes of flows are unchannelized or “avalanches” and channelized. Avalanches can consist of soil, rock, and debris as well as snow and ice.
  • Spreads – lateral extension of materials overlying liquefied layers.
  • Topples/falls - rapid, nearly vertical, movements of masses of materials such as soil, rocks, or boulders. Toppling failures are distinguished by forward rotation about some pivotal point below or low in the mass.

Common Rapidly Moving Landslide Types

  • Debris fFlows – fast-moving landslides that are particularly dangerous to life and property because they move quickly, destroy objects in their paths, and often strike without warning.
  • Rockfalls – where a source of rocks exists above a slope steep enough to allow rapid downslope movement of dislodged rocks by falling, rolling​, bouncing, or sliding.
  • Translational and rotational landslides can also have short bursts of movement but are generally less dangerous than flows and falls.
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We continue to work with agency partners at the Oregon Department of Land Conservation and Development, the Oregon Department of Geology and Mineral Industries, Oregon Parks and Recreation Department, Oregon Department of State Lands and other organizations at the federal, state, county, and city levels.
  • The Unstable Slopes Program​ analyzes slopes adjacent to state highways for potential impacts that a failure could cause.
  • We also partner with OSU, PSU, and the U of O, to research new methods for landslide and rockfall detection, monitoring, analysis, and mitigation.
  • Our Maintenance teams evaluate these areas as budget constraints allow. If necessary, they take these extra steps:
    • They make the warning signs clearer.
    • ​They mark off the areas that are affected by erosion so people know to be careful in that area.
    • They patch pavement in sunken areas when weather allows.
    • They increase the frequency of inspections during heavy rainfall. 

If you are traveling in an area prone to landslides, check the route before you go using Tripcheck.com. Make sure you have an emergency ​kit in your vehicle that includes water and snacks in case you get stuck in closure.

If a slide is affecting a route, please use the recommended detour. Think twice before using a gravel road or forest service road. Be patient with crews who are working to clear the slide.

Landslides and rockfalls are the most active during periods of heavy rainfall.  Rockfall is also common in freezing temperatures. Use extra caution while driving in these conditions. Use your headlights to make sure you can see the road in front of you at night or when visibility is low.
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Landslides are large masses of unstable rock, soil and debris that move down sloped areas.
  • “Active” refers to landslides are currently moving. These include active rockfall sites, slow-moving landslides that may only move a few feet over several years, large landslides that also move a few feet per year but are prone to episodes of rapid movement, or stream channels that produce debris flows in certain weather conditions.
  • “Inactive” landslides are those that aren’t currently moving but have moved in the past. Some of these slides are known from historic records while others can only be detected using advanced technology. Inactive slides can become active if disturbed by natural phenomena or human activity.

  • ​Intense rainfall is the primary trigger for landslides but waves and stream erosion can also trigger them. Rain saturates the soil and makes it less stable. 
  • Slow, natural erosion can make the soil less stable.
  • Waves erode sediment from cliffs and shorelines.
  • Rivers and streams also erode their banks, especially during flood events.
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  • ​Landslides can happen on any slope but are primarily found in areas with steep slopes, heavy rainfall, or weak soil and rock.
  • Landslides are most common during winter when heavy rain saturates soil on unstable slopes.
  • Areas prone to landslides: U.S. 101, U.S. 97 near Klamath Falls, roads in the Columbia River Gorge and the Cascade and Coastal Mountain passes. 
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Contact

For more information, contact one of our regional public information officers.

Additional Information