1. What is a Tree Well and Snow Immersion Suffocation (SIS) Accident?

A tree well/ snow immersion suffocation accident can happen when a skier or snowboarder falls – usually headfirst – into a tree well or deep loose snow and becomes immobilized and trapped under the snow and suffocates.

In an inverted position you can become trapped under the snow. Breathing becomes difficult as the loose snow packs in around you. Without immediate help from your partner, you may suffocate.

Prevention of falling into a tree well  or areas of deep snow is all-important because the odds of surviving deep snow immersion are low.

90% of people involved in Tree Well/ SIS hazard research experiments could NOT rescue themselves. If a partner is not there for immediate rescue, the skier or rider may die very quickly from suffocation - in many cases, he or she can die as quickly as someone can drown in water.


2. What is a Tree Well?



A tree well is a void or depression that forms around the base of a tree can and contain a mix of low hanging branches, loose snow and air.  Evergreen trees in particular (fir, hemlock, etc) can have large, deep tree wells that form when low hanging branches block snow from filling in and consolidating around the base of the tree.  These voids can be hidden from view by the tree’s low hanging branches. 

There is no easy way to identify if a particular tree has a dangerous tree well by sight therefore, treat all tree wells as dangerous. 

In simple terms, a tree well is a hole or void in the deep snow, which is clearly marked by a tree.  You can easily identify and avoid these areas. 

The picture from the site of a fatal Tree Well/ SIS accident demonstrates that even very small trees with just their tops visible can be lethal.


Mike standing in ungroomd treed area. (in black jacket & black helmet)
Mike (6'1") Stepping into the tree well (in black jacket & black helmet)


Mike standing in ungroomd treed area. (in brightly colored orange jacket)
Mike (6'1") Stepping into the tree well (in brightly colored orange jacket)

3. Where do Accidents Happen?

Most all Tree Well/ SIS accidents happen in ungroomed terrain.  A groomed run is a ski or snowboard run that has been driven on by a snow machine to compact and consolidate the snow.  

  • Most Tree Well/ SIS accidents happen where there are combinations of deep powder and trees and where lots of powder skiers and snowboarders are seeking powder snow.

  • Lots of deep loose snow makes for the best powder skiing conditions. Unfortunately these conditions also significantly increase the chance of a Tree Well/ SIS accident.

  • 70% of all Tree Well/ SIS accidents involve tree wells.  Deep snow Tree Well/ SIS accidents occur in areas of deep snow, deep snow pockets or terrain that concentrates deep snow such as steep drops, and creek beds.

The risk of a Tree Well/ SIS accident is greatly reduced just by staying on groomed runs.

Photo B - Tree 1 Tree Well

Photo A - Groomed vs. Ungroomedtree1.png

Snow Cat creating groomed snow & Tree 1 in ungroomed area.


Gwyn (6') stepping into tree well of same Tree1 that is in photo A in ungroomed area.

Photo C - Fatal Tree Well Site.


This small tree in ungroomed area was the site of a fatal tree well accident.


4. When do Accidents Happen?


Most Tree Well/ SIS accidents have happened during or just after big snow storms or storm cycles. In general terms, the more fresh snow the higher the risk. 

As of 2011 research, an average of four Tree Well/ SIS accidents happen each season in the United States.

See "SIS by the numbers section for more details"


5. What if you go down?


  • headerimgsis.pngYell or use whistle to get your partners attention.
  • Do whatever you can to keep your head above the surface of the snow including rolling, grabbing tree branches or the tree trunk. If possible, keep your feet below level of your head.
  • If you become immersed, make a space around your face and protect your airway – resist the urge to struggle, it could compromise your airspace and entrap you further.
  • Stay calm to conserve air.
  • Trust your partner is on their way.
  • If possible, use your cell phone to call ski patrol or the resort's emergency number.

6. Keep your partner in sight at all times!

What if your partner goes down.


Volunteer Skier being Rescued by Ski Patrol During Tree Well Experiment.

More than half of all
SIS victims were with partners that did not see them go down. Lose sight of your partner and you could lose your friend. 

If you lose contact with your partner, assume they need help. Many SIS victims have died while their partners were waiting at the bottom of a lift.

TIP: In dense tree areas or in poor visibility, ski or ride short pitches and stop to regroup often - stay within sight of your partner!

  1. Don’t leave to get help – Stay with your partner!
  2. Call for additional resources. Use a whistle or yell for assistance. If possible, call ski partol or the resort's emergency phone number.
  3. Evaluate scene safety for yourself.
  4. IMMEDIATELY begin snow immersion rescue efforts.
  • Go directly for the airway, and keep it clear, be careful not to knock more snow into the hole. Clear any snow from the airway and continue necessary first aid or extrication efforts
  5. Do not try to pull victim out the way they fell in. Instead, determine where the head is and tunnel in from the side.
  • When tunneling directly for the airway be careful not to knock more snow into the hole.Continue expanding the tunnel to the airway until you can extricate the body. Efficient “strategic shoveling techniques”with multiple rescuers is very useful.

What to do if you did not witness your partner going down but suspect they may have:

  1. Immediately call resort emergency number and report point last seen.

  2. If you can not get back to the area where you suspect your partner may be in a timely manner, go to closest lift terminal and notify patrol of your location and stay there until contacted by patrol as what to do next. 

  3. If you do NOT have a cell phone, contact nearest guest with one to call resort emergency number.

  4. If phone service is not available and you are unable to return to your partner to help in a timely manner, proceed to nearest location to contact ski patrol or resort personnel - remember these accidents are "time critical." 

7. You can Prevent a SIS Accident

  1. SULLIVAN_MT_Baker_2012_-101.jpgThe easiest was to avoid a SIS accident is to remain on groomed areas and avoid deep snow and tree well areas. 
  2. Be aware of the recent snowfall and the depth of the loose snow - check local resources for recent snow conditions.
  3. If you venture into UNGROOMED terrain:


  1. Ride or ski with a partner and keep your partner in sight at all times.
  2. Ski or ride in control.
  3. Give tree wells a wide berth. Look at the open spaces between trees not at the trees.
  4. Skiers should remove ski pole straps.
  5. Carry safety equipment including:
  • Cell phone with resort emergency number
  • Transceiver / beacon
  • Avalung
  • Whistle
  • Shovel
  • Probe
  • Recco

Please see Safety Equipment Notes section for details about each safety equipment item.

  4. Keep your partner in sight at all times. In many SIS accidents, skiers and snowboarders, have part of their body or equipment visible - but a partner wasnt there to see them.
  5. Take heart. Increased awareness and education reduces SIS risks. There are more reported cases of skiers and boarders being rescued by their partners each season.

8. Safety Equipment Notes

The type of equipment that can help in a SIS accident is readily available and is already being carried by skiers and snowboarders who regularly venture into backcountry/ avalanche terrain and many in-ski area riders.

Avalanche Beacon


Avalanche Beacon This is the most efficient way to locate your partner if they are under the snow surface with nothing visible. It should be noted that most SIS victims DO HAVE some part of their body or a piece of equipment visible from the surface so that a visual search of the victim’s last seen area could yield the quickest find.



The avalanche probe pole rounds out “the avalanche safety equipment trinity. The beacon is used to get close and the probe pole is used to confirm the victim’s exact location. Probe poles can also be used to probe for someone without a beacon but this is very time consuming. Make sure you do a good visual search of the entire area. If this fails, probing individual tree wells may be the next best option.



The RECCO reflector is a small reflector that is may be sewn into a skier or boarders clothing or is on their equipment like a ski boot. This allows them to be found by a RECCO device. This device may be carried by professional rescue services like ski patrols. To be found by this method involves these rescue services being notified and then responding with a RECCO device specialized. To date one SIS fatality has been found in the U.S. with a RECCO device.




It is incredibly difficult to dig someone out if you don’t have a shovel. If you and your partner carry shovels the next step is to learn how to dig efficiently.


avalung.pngUsing a device like an “Avalung" can extend your survival time under snow. Please check out this link to see how it works. Click here to read how "Skier Survives Tree Well Thanks to Avalung"



The low-tech whistle is making a comeback as more people are realizing how much easier it is to get help and direct people to your location with a whistle rather than by yelling.


9. SIS By the Numbers

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10. Technical Notes for Responding

Professional Rescuers



11. Video Stories & Photos


Stories and Photos Under Construction





12. Reporting a SIS Incident

Lucky_guy_from_Tahoe_300dpi.jpgIn order to continue to gain knowledge and keep our educational information as current as possible, please notify us of SIS near-miss, rescue or fatality incidents. 

  1. Complete the SIS INCIDENT FORM
  2. Send any videos, photos, stories or any questions or comments about this content to Gwyn and Paul at:
Paul Baugher
Northwest Avalanche Institute
(253) 508-1898 cell


or Gwyn Howat
Operations Manager
Mt. Baker Ski Area, Inc.
1420 Iowa Street
Bellingham, WA 98229

Photos and Text © Copyright: Paul Baugher and Gwyn Howat - see Terms.

Photo: Lucky Guy from Tahoe. Rescued from SIS.

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