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In February of this year, a snowboarder in Washington plummeted into a tree well and died. While you might assume he was riding alone in the backcountry, he was in fact just 100 feet from a marked trail at a ski resort and was accompanied by his son. When the man fell, his son attempted to dig him out but was unable to, and therefore went for help. Ski patrol responded in eight minutes; however, the man was unresponsive upon their arrival and they were unable to resuscitate him.(3)

A similar incident occurred a mere 17 days after the event described above, when a snowboarder at a Tahoe area resort fell into a tree well in deep powder conditions and died, despite being pulled out by three skiers who immediately initiated cardiopulmonary resuscitation.(4) Sadly, these two cases were the second and third tree well fatalities this season; the first occurred in mid-December and involved a skier who plunged into a snow well at a ski area near Snoqualmie Pass, Washington.(5) 

In the last 20 years there have been more than 70 non-avalanche-related snow immersion deaths (NARSID) and an unknown amount of near-death snow immersion accidents due to underreporting.(8) In a paper analyzing the most complete data collected to date on snow immersion incidents, Baugher reviewed 65 cases and found that NARSID events accounted for five percent of all skier deaths and 15 percent of all snowboarder deaths at ski resorts.(1) Of those deaths, 65 percent were due to tree wells. In the paper, Baugher stated that, “The greatest single component of snow immersion risk is that it is substantially underappreciated.” 

<iframe src="https://player.vimeo.com/video/51002308?h=814da55e9f" width="640" height="360" frameborder="0" allow="autoplay; fullscreen; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://vimeo.com/51002308">Tree Wells &amp; SIS Safety: What is a Treewell or SIS Accident</a> from <a href="https://vimeo.com/user13767227">SIS Safety Videos</a> on <a href="https://vimeo.com">Vimeo</a>.</p>



In order to mitigate the risk of a tree well snow immersion accident, one must first understand what a tree well is and what happens when someone falls in. Tree wells are depressions that form around the base of conifers due to their low lying branches, which limit snow accumulation and consolidation around the trunk of the tree. These depressions, or wells, can exceed 10 feet in depth depending on snowfall, but often appear deceptively innocuous. 

Source: Mpora (https://mpora.com/snowboarding/tree-well-survival-guide - aoffZqIZlLhSPMru.97)

When an individual falls into a tree well, they are usually buried headfirst making self-extrication nearly impossible. In fact, 90 percent of people involved in snow immersion research experiments were not able to self-extricate from tree wells.(8) If an individual is unable to self-extricate and is not rescued, he or she will die from asphyxiation, which, according to a review by Van Tilburg (8), most likely occurs due to rebreathing carbon dioxide or from airway obstruction from snow, ice mask, or body position.


Prevention is clearly the best way to deal with the risks associated with tree wells, making education paramount. Skiers and snowboarders should be taught to recognize environmental factors that increase tree well risk, the biggest of which is heavy snowfall. Tree well NARSID events occur predominately during or immediately after major snowstorms, so skiers and snowboarders should be especially cautious in deep powder conditions in areas with conifers. Such conditions are found predominately in the western United States and Canada in the Sierra Nevada, Cascade, and British Columbian ranges where deep, moist snow and large conifers abound. These conditions are less common in the Rocky Mountains, which tend to have a shallower, drier snowpack with fewer conifers.(1,8) The other major environmental risk factor involves terrain, with tree well accidents occurring almost exclusively off-piste, though often inbounds at a ski area. 


Source: National Ski Areas Association (http://www.nsaa.org/safety-programs/tree-well-deep-snow-safety/)

Safety practices from the Wilderness Medical Society (WMS) practice guidelines for snow burial accidents should also be followed to decrease the chance of a tree well accident ending in a fatality.(9) The first safety practice, which cannot be over emphasized, is to always ski or ride with a partner, maintaining vocal and visual contact at all times. The importance of the latter point, evident from statistics collected by the Northwest Avalanche Institute, revealed that 72 percent of people who have died in snow immersion suffocation incidents (SIS) were skiing or riding with partners but did not have visual contact at the time of the event.(7) Other safety practices, suggested by Van Tilburg, include avoiding exceeding one’s skill limit while maintaining control at all times.(8) Baugher and his colleagues also recommend carrying equipment that would be helpful in a tree well rescue situation, such as a shovel and whistle.(6) 


If you do fall into a tree well, there are several steps you can take to increase your chance of survival. The WMS practice guidelines suggest grabbing tree branches while falling in an effort to stay upright, while shouting to alert your partner to the situation. If grabbing branches doesn’t work and you find yourself with your head under the snow, make a conscious effort to take slow, measured breaths to reduce oxygen consumption and resist the urge to struggle. A simulation snow burial conducted by Baugher found that struggling caused most subjects to sink deeper.(1) Van Tilburg suggests using one hand to clear an air pocket in front of your face, if possible, while reaching for tree branches or the tree trunk with the other hand. While attempting to remove your skis or snowboard to aid in self-extrication might be tempting, findings from Cadman’s simulated tree well snow burial (2) and the WMS practice guidelines recommend leaving them on to prevent sinking deeper in the snow while increasing visibility for rescuers. If you can, use a whistle or yell in order to attract your partner’s attention in case they didn’t see you fall. 



If your partner falls into a tree well, instruct them to stay still, evaluate the scene to ensure your own safety, then began immediately tunneling towards their airway, ideally with a shovel. Don’t attempt to pull them out the same way they fell in. Determining where your partner’s head is and tunneling from the side towards that point is more effective. As you dig, yell for help or blow a whistle to attract attention, as strategic digging from multiple rescuers may increase your partner’s chance of survival. When you reach your partner’s head, clear their airway if necessary and continue expanding the passageway until you can extricate their entire body. 

<iframe src="https://player.vimeo.com/video/51002307?h=ad6785436a" width="640" height="360" frameborder="0" allow="autoplay; fullscreen; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://vimeo.com/51002307">Treewell &amp; SiS Safety: What if your partner goes down</a> from <a href="https://vimeo.com/user13767227">SIS Safety Videos</a> on <a href="https://vimeo.com">Vimeo</a>.</p>


In summary, tree wells are an underappreciated hazard that can be avoided or mitigated by recognizing risks and utilizing safety practices. Education is key, so if you’d like to increase awareness amongst skiers and snowboarders in your area, you can request tree well and deep snow caution signs and brochures from the National Ski Areas Association, which have been created for distribution. If you would like to further snow immersion suffocation research, please report any non-avalanche snow immersion suffocation near-misses, rescues, or fatalities to Paul Baugher, director of the Northwest Avalanche Institute, or his colleague Gwyn Howat, operations manager of the Mt. Baker Ski Area. To make a report go to the “Deep Snow Safety” website listed below, and select the “Report a SIS” link from the bottom of the homepage. 


Deep Snow Safety Website
WMS Practice Guidelines for Prevention and Management of Avalanche and Nonavalanche Snow Burial Accidents 
National Ski Areas Association


1. Baugher, P. Risk trends at U.S. and British Columbia ski areas: an evaluation of the risk of snow immersion versus avalanche burials. Proceedings of the 2006 International Snow Science Workshop. Telluride, CO. Available at: http://arc.lib.montana.edu/snow-science/objects/issw-2006-584-591.pdf. Accessed March 22, 2017. 

2. Cadman, R. Eight nonavalanche snow-immersion deaths. Phys Sportsmed. 1999; 27: 31-43

3. Culver, N. Man who died at 49 Degrees North was not on a ski run. The Spokesman-Review. Accessed February 10, 2017. Retrieved from http://www.spokesman.com/stories/2017/feb/10/man-who-died-at-49-degrees-north-was-not-on-a-ski-/ 

4. MacMillan, K. Male snowboarder, 43, dies after falling into tree well at Tahoe-area resort. Sierra Sun. Accessed March 18, 2017. Retrieved from http://www.sierrasun.com/news/male-skier-43-dies-after-falling-into-tree-well-at-tahoe-area-resort/

5. Seattle Times Staff. Skier dies after falling into tree well near Snoqualmie Pass. The Seattle Times. Accessed March 18, 2017. Retrieved from http://www.seattletimes.com/seattle-news/northwest/skier-dies-after-falling-in-tree-well-near-snoqualmie-pass/

6. SIS Helpers NW. SIS prevention and equipment. Accessed February 19, 2017. Retrieved from https://www.deepsnowsafety.org/index.php/prevention-equipment

7. SIS Helpers NW. SIS by the numbers. Accessed February 20, 2017. Retrieved from https://www.deepsnowsafety.org/index.php/sis-numbers

8. Van Tilburg, C. Non-avalanche-related snow immersion deaths: tree well and deep snow immersion asphyxiation. Wilderness & Environmental Medicine. 2010; 21: 257-261

9. Van Tilburg, C, Grissom, CK, Zafren, K, McIntosh, S, et al. Wilderness Medical Society Practice Guidelines for Prevention and Management of Avalanche and Nonavalanche Snow Burial Accidents. Wilderness & Environmental Medicine. 2017; 28: 23-42 

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