Tree Well and Deep Snow Safety

Snow Immersion Suffocation (SIS) presents a significant risk in bounds at ski resorts, especially when skiing off-piste.

Key terms:

Tree Well: a hole or depression that forms around the base of a tree and can contain a mix of low-hanging branches, loose snow, and air. Evergreen trees are the most likely to have large, deep tree wells that form when low-hanging branches block snow from filling in around the tree's base. Tree wells can be hidden from view by the tree’s low-hanging branches, making them difficult to identify. All tree wells should be treated as potentially dangerous.

Snow Immersion Suffocation (SIS): 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.

Where do tree wells form?

70% of SIS accidents involve tree wells.

Most Tree Well/ SIS accidents happen in ski areas where there are combinations of deep powder, trees, and many powder skiers and snowboarders. Tree wells form in forested areas that receive a high volume of new snow, and are especially common under evergreen trees. Evergreen trees tend to store a lot of snow on their branches, preventing that snow from reaching the ground. The spaces between the trees fill in with more snow which subsequently gets packed down by skier traffic. Each new storm raises the base of the snow around the trees, while the base under the trees stays soft and much thinner. In the case of a tree well burial, the snow stored on the branches can become an additional burial hazard, where struggling to free yourself can shake off this stored snow, causing you to be buried faster and deeper. The mountains and ski areas in the western United States and Canada are particularly prone to tree well SIS accidents due to higher elevation treelines, resulting in more snowfall and more powder skiing in forested terrain.

Most people caught inverted in tree wells or deep snow cannot rescue themselves.

Preventing tree well and SIS fatalities

Tree wells and areas of deep snow are often difficult to identify by reading the snow's surface, making it difficult to fully appreciate the level of SIS risk. Partnership, preparedness, and awareness are hugely important in order to be prepared for worst-case scenarios.

Ski with a partner

Ride or ski with a partner and keep your partner in sight at all times. Have a serious conversation with your partners about the SIS risk. Decide how much unrestricted powder skiing you are willing to give up so you can maintain continuous contact with your partner. 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.

Know your limit, ski within it

Powder days at ski resorts are often when stoke and the scarcity of fresh snow can push you to ski faster and further into low-traffic terrain with a higher risk for tree wells. It is critical to be aware of present risks and to adjust decision-making based on what you can ski safely to avoid falling into a tree well or deep snow.

Be equipped for an accident

Having equipment that helps keep you breathing until you can be rescued, and that makes you easier to locate by rescue teams may be critical to your survival in an SIS accident. Safeback SBX can supply you with air under the snow to delay suffocation. Beacons and Recco tags can make you easier to find by rescue teams. Whistles can allow you to audibly signal your distress without having to shout.

Be equipped for a rescue

Avalanche rescue equipment is equally useful to free a tree well or deep snow burial victims. Beacons and probes can be used to locate burial victims in cases where they are not otherwise visible Shovels allow the rescuer to dig through the snow to free the victim's airway to ensure they can breathe freely.

What to do in case of a tree well or deep snow burial:

Thought Leader in SIS Accident Prevention

The Northwest Avalanche Institute in Washington, USA, has been the center of research and education for tree well accidents since the year 2006. After observing a pattern of accidents occurring in North American ski areas, this website was created to be a hub for best practices and statistics of SIS accidents.

Backpacks suitable for in-bounds freeriding

Safeback SBX

Designed to provide breathable air to an avalanche victim under the snow, extending their potential survival window.