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A tree well is a void or area of loose snow around the trunk of a tree enveloped in deep snow. Also known as "spruce traps", these voids present danger to hikers, snowshoers, skiers, and snowboarders who fall into them.
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Formation
A tree's branches shelter its trunk from snowfall, allowing a void or area of loose snow to form. Low-hanging branches as on small firs further contribute to forming a tree well, as they efficiently shelter the area surrounding the trunk. Such wells have been observed as deep as 20 feet. They can also occur near rocks and along streams.
Tree wells may be encountered in backcountry, on ungroomed trails, off-piste, and on ungroomed/piste boundaries. The risk of encountering one is greatest during and immediately following a heavy snowstorm.
Hazard
Victims can get trapped in tree wells and become unable to free themselves. In two experiments conducted in North America 90% of volunteers temporarily placed in tree wells were unable to rescue themselves.
Frequently victims end up in wells head first, complicating recovery efforts. Often they are injured in the process, suffering joint dislocation or concussion. When fatal, this type of incident is termed a Non-Avalanche Related Snow Immersion Death (NARSID).