Tree wells and snow immersion, as NSAA and others describe them.

What NSAA calls it

The National Ski Areas Association (NSAA) describes a deep-snow or tree-well immersion accident as one in which a skier or rider falls into deep, unconsolidated snow, becomes immobilised and suffocates. It calls deaths from these accidents Snow Immersion Suffocation, or SIS.

NSAA's slope-safety site, Ski Safety U.S., adds that an SIS usually happens when someone falls, typically head first, into a tree well or deep loose snow and becomes trapped. The page gives no statistics.

Wikipedia, a secondary source, describes a tree well as the space around a tree, under its branches, that receives less snow than the open ground around it.

What NSAA says about the risk

• Partners. NSAA's guidance is that skiers and riders should always go with a partner who stays within viewing distance.
• Leaving the groomed runs. NSAA says that anyone who leaves groomed runs accepts the risk of a deep-snow immersion accident.
• Education. NSAA names DeepSnowSafety.org and its own Ski Safety U.S. tree-well page as education resources, and supplies tree-well and deep-snow caution signs and brochures to ski areas.

That guidance is NSAA's, reported as NSAA published it; this page adds nothing to it.

How many deaths

National counts of snow immersion deaths are thin, and none of the figures below comes from a fetched NSAA fact sheet.

Wikipedia also says, citing the Northwest Avalanche Institute, that 90 percent of volunteers in two North American tree-well experiments could not get themselves out. The experiments are not named or dated there, so the figure rests on a secondary source only.

A peer-reviewed paper on these deaths is listed by its author, in a list seen only as a search-result summary, so the citation is unconfirmed: Christopher Van Tilburg, "Non-avalanche-related snow immersion deaths: tree well and deep snow immersion asphyxiation", in Wilderness and Environmental Medicine, volume 21, issue 3, 2010. Its text was not read for this site, so none of its findings are used.

How this differs from an avalanche

The Van Tilburg paper's title separates these deaths from avalanche deaths, and NSAA's definition describes a fall into deep, unconsolidated snow. Avalanche deaths are counted separately by the avalanche centres, on avalanches as published. Ski-area deaths in general are on deaths at ski areas.

What this page leaves out

A forum post quoting an NSAA summary gave a share of 2024-25 ski-area deaths caused by snow immersion; a forum is not an authoritative source and the NSAA document was not read, so the figure is not used. No snow immersion counts from Canada or Europe were found.