What is it?
A landslide is a mass of rock, earth, and/or debris that moves down a slope. This can occur quickly or slowly depending on the circumstances. Evidence of the largest landslide to occur in Teton County's recorded history is seen in the picture above and can still be seen today:
Clickable Map of Gros Ventre Slide
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During the 1920's, several persistent small earthquakes shook areas of Teton County, especially the eastern side. These earthquakes increased in number during the spring of 1925, which coincidentally was an unusually wet year. Several Jackson Hole residents in the Gros Ventres reported seeing new freshwater springs pop up in hillsides where they never existed before.
On June 22, 1925 the valley experienced, what is estimated through first-hand accounts to be, a magnitude 4 earthquake. Around 4pm on June 23, 1925, local farmer Guil Huff was on horseback in the Gros Ventre canyon looking for loose cattle. All of a sudden, he saw a 30-to-40 foot high riverbank on the south side of the Gros Ventre river collapse into the water. After that the entire side of the mountain came rushing down, carrying with it boulders, trees, and earth. Guil estimated that the entire event happened in about a minute and a half, and if it weren't for the horse he was riding he would have been buried instantaneously.
The Gros Ventre Slide brought down 50 million cubic yards of debris from the south side of the Gros Ventre canyon. The slide moved 2100 vertical feet while traveling 1.5 miles. The entire debris field was 2,000 feet wide, 225 feet deep at the river, and climbed 350 feet up the opposite north side of the canyon. This huge amount of earth formed a natural dam in the Gros Ventre river that formed Lower Slide Lake.
No one was injured or killed by the Gros Ventre Slide itself, but two years later 6 people in the town of Kelly were killed by the ensuing flash flood when the natural dam formed by the slide broke (Windows Into the Earth; Smith and Siegel, 2000).
Today we still have the risk of landslides in many parts of Teton County. You can check the landslide hazard map to see if there is a risk to your home or routes of travel. As you'll see, areas at risk of landslides cover a large portion of Teton County.
The state’s record snowpack and above-average precipitation in the spring of 2011 caused a massive landslide in Snake River Canyon on May 14, 2011. The slowly moving mass of mud, rock and trees crossed the highway 26/89, blocking about 300 feet of the road, covering the pavement to depths of up to 50 feet and eventually flowing into the Snake River. The length of slide was about 2,000 feet and contractors hauled away an estimated 200,000 cubic yards of debris. The commuter corridor for residents of Alpine and Star Valley remained closed for almost 2 weeks, forcing the average of 5,740 vehicles that commute to and from Jackson every day, to take a 74-mile long detour through Idaho and Teton Pass. Read more about the Snake River Canyon landslide here.
Watch a Time-lapse video of the Snake River Canyon Landslide ("Double-Draw Landslide").
Photo of the Double-Draw Landslide, courtesy of Civil Air Patrol
What are the risk factors?
Landslides can occur of their own accord, but more frequently there are underlying events that trigger them:
Unusually high amounts of precipitation.
High amounts of rainfall are thought to have been one of the causes of the Gros Ventre slide. The water can act as a lubricant, causing already unstable layers of earth to succumb to the forces of gravity and begin moving downhill.
This is the second factor that is thought to have brought about the Gros Ventre slide. Earthquakes can shake up and disturb layers of soil, causing them to become unstable and move downhill. If you add high amounts of precipitation to the mix, you can get an effect known as liquefaction. Click the play button to see liquefaction in action:
Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to ground shaking. If this occurs beneath buildings or structures, it can cause the ground to lose its firmness and the buildings sink. In the case of a landslide, it can cause upper layers of earth and debris to become unstable and through the forces of gravity move downhill.
Recent wildfires on steep slopes.
When trees and other plant life are destroyed following a wildfire, this can destabilize the soil on steep slopes. No longer having protection from rainfall or the root systems for stability, these bare hillsides can more readily form landslides.
Repeated freezing and thawing.
Repeated freezing and thawing of the ground can act like a pry-bar between layers of soil. During periods of thaw, water percolates down into the ground, then when it freezes again, that water forms ice. When water freezes, it expands and can force layers of soil apart causing landslides.
Volcanic eruptions or activity.
Volcanic eruptions, either through the shaking of the ground or lava flows, can cause large amounts of debris to move downhill. Clicking on the map to the left, you will see the outline of the Yellowstone Caldera, which is one of the largest and most active calderas in the world (USGS). A caldera is essentially a collapsed volcano, or a volcanic basin.
Human modification to slopes.
This boils down to people building where they shouldn't be building. Increases in erosion due to development on steep hillsides or building in the path of susceptible areas significantly increase the risk of landslides.
What should I do?
Once a landslide begins, there isn't much anyone can do except get out of the path. The key is to prepare ahead of time and know the signs of an impending landslide:
As for any disaster, have a plan and preparedness kit ready for your family.
If you don't have a family emergency plan and a 72 hour kit, click here to get started.
Do not build near steep slopes, close to mountain edges, near drainages, or natural erosion valleys.
This is common sense, but goes against where most people want to live. If you live in one of these areas, be sure to get a detailed ground assessment of your property. Contact the Town of Jackson or Teton County Department of Planning and Development for more information.
Discuss with your insurance agent coverages for landslides if you live in a susceptible area.
Some policies will cover damages due to landslides, others will not. Some National Flood Insurance Program (NFIP) flood insurance policies also cover debris flows (landslides, mudslides, etc.) Even if your policy states that landslides are covered, if that landslide is originally caused by an earthquake or a volcano, you may need a special rider on your policy to cover that. For all of these issues, contact your insurance agent for more information.
Minimize the hazards at your property.
Have flexible fittings installed on your utilities to avoid breakage in the event of a landslide (or earthquake, for that matter). If your property is on a hillside, plant ground cover to help prevent erosion and build retaining walls to hold back debris.
Recognize the warning signs of a potential landslide.
If you see these signs and suspect a landslide is occurring or beginning to occur, call the Teton County Sheriff's Office at (307) 733-2331. Remember, a landslide can be a very slow process but that doesn't make it any less dangerous. Recognizing these signs and notifying emergency services can save lives through advance evacuations. If it is an immediate emergency, as always, dial 911:
Changes in landscape such as different patterns of storm water drainage on slopes, land movement, small slides, or progressively leaning trees.
Doors or windows stick or jam for the first time.
New cracks appear in plaster, tile, brick, or foundations.
Outside walls, walks, or stairs begin pulling away from the building.
Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
Underground utility lines break.
Bulging ground appears at the base of a slope.
Water breaks through the ground surface in new locations.
Fences, retaining walls, utility poles, or trees tilt or move.
A faint rumbling sound that increases in volume as the landslide nears.
The ground slopes in one direction and may begin shifting in that direction under your feet.
Unusual sounds such as trees cracking or boulders knocking together.
Collapsed pavement, mud, fallen rocks, along road embankments, which are particularly susceptible to landslides.
If in the vicinity of a landslide, immediately evacuate.
Keeping your personal safety in mind, notify your neighbors and immediately evacuate the path of the landslide. Even a slow moving landslide has an incredible amount of force behind it, moving boulders, trees, and buildings. If you are caught in a landslide and escape is not possible, curl into a tight ball and protect your head with your hands.
Stay tuned to EAS broadcasts for instruction before, during, and following the disaster.
Listen to your NOAA All-Hazards Weather Radio or another Emergency Alert System (EAS) broadcaster for instructions from emergency services before, during, and following a disaster.
After a landslide, do not return home until authorities advise it is safe.
One landslide can cause other landslides, flash flooding due to the loss of vegetation on slopes, damage to gas and power lines, plus disrupt other utilities such as sewage making areas uninhabitable. Wait until authorities check the area and say it is safe before returning. Listen to NOAA All-Hazards Weather Radio or your local EAS station for instructions following a landslide.
What are the impacts?
Depending on the magnitude of the slide, the impacts can vary:
Loss of life and damage to personal property
Many landslides, like flash floods, occur at night catching people unaware. If advance warning is given by authorities, having a NOAA All-Hazards Weather Radio with an alert function could wake you up so that you can react accordingly.
Foundations, chimneys, and other rigid structures can be damaged by landslides through only slight shifting. These can be costly to repair, which is another reason to ensure that your insurance covers these events ahead of time.
Damage to routes of transportation.
Roads through canyons, such as the Snake River Canyon and the Hoback Canyon, are especially prone to landslides. These slides can be small causing minor interruptions, or very large taking days to remove. Since these are major arteries into and out of Teton County, they can affect businesses, travelers, and commuters directly.
Increased risk of flooding
Unless the area is able to be rehabilitated, the loss of vegetation can increase the risk of flooding in the area for the foreseeable future.
Damaged underground utilities.
Underground utilities may take a long time to repair depending on the extent of the damage. This can delay your return home following a landslide.