CAIC: Colorado Avalanche Information Center

2021/02/08 - Washington - Knox Creek, Salmon la Sac area, near Cle Elum Lake

Published 2021/05/25 by Dallas Glass and Matt Primomo - Northwest Avalanche Center


Avalanche Details

  • Location: Knox Creek, Salmon la Sac area, near Cle Elum Lake
  • State: Washington
  • Date: 2021/02/08
  • Time: 11:00 AM (Estimated)
  • Summary Description: 2 snowbikers caught, 1 partially buried, 1 buried and killed
  • Primary Activity: Snowmobiler
  • Primary Travel Mode: Snowmobile
  • Location Setting: Backcountry

Number

  • Caught: 2
  • Partially Buried, Non-Critical: 1
  • Partially Buried, Critical: 0
  • Fully Buried: 1
  • Injured: 0
  • Killed: 1

Avalanche

  • Type: HS
  • Trigger: AV - Vehicle (specify vehicle type in comments)
  • Trigger (subcode): u - An unintentional release
  • Size - Relative to Path: R3
  • Size - Destructive Force: D3
  • Sliding Surface: O - Within Old Snow

Site

  • Slope Aspect: NE
  • Site Elevation: 4750 ft
  • Slope Angle: 30 °
  • Slope Characteristic: --

Avalanche Comments

This wide hard slab failed on a layer of small facets just above a thick melt-freeze crust. Given the entry tracks onto the slope and final locations of the riders, the avalanche was likely triggered from the lower start zone where slope angles between 30-34 degrees were measured. 

The avalanche released the entire width of the looker’s right-hand path. The majority of the avalanche debris stopped on the low-angle bench. The crown continued into the looker’s left path releasing slightly higher in the terrain, but not involving the majority of the rock-walled chute (only about ? of the southernmost path released). 

Debris from the looker’s left path combined with the wider and shorter northern path. This debris overran the shallow bench and descended into sparsely treed terrain below. The majority of the avalanche fell less than 300 vertical feet with a maximum of 500’. Despite the relatively short path, debris piles were measured at 6-12’ deep. 

This piece of avalanche terrain consisted of two connected but characteristically different paths. Both paths were NE facing and no major terrain barrier separates them.

The southernmost (lookers left) path consists of a long narrow chute originating high on a ridgeline between two cliff bands around 5300’. As the chute exits the cliffs, it fans into a large apron and small bench. Below this terrain bench, the runout continues into sparse trees, with a maximum run just below an old roadbed at 4300’. This terrain is much longer and narrower than the adjacent path. It also appears to produce more frequent large avalanches.

To the lookers right, north of the main chute, a several hundred-foot cliff sits above a steep open snowfield. Slope angles just below the cliff exceed 50 degrees, however, they quickly decrease to 30 degrees just downslope of the rocks. Unlike the chute to the south, this path generally terminates on a board low-angle bench only 150-200 feet downslope. Vegetative clues on the far lookers left side of the path do indicate it can occasionally overrun the bench and descend further downslope. This seems most likely when the paths run in concert as they did in this accident. In general, this path is very wide but uncharacteristically short due to the wide low-angle bench. 

Weather Summary

A significant winter storm Feb 6-8 deposited 2-3 ft (3.5” WE) of snow above 4000’ at nearby Snoqualmie Pass. The Salmon la Sac area likely recorded slightly lower storm totals per their normal relationship to Snoqualmie Pass snowfall in westerly flow. This snow fell with moderate to strong westerly winds. By the morning of the 8th, the majority of the storm had ended with only occasional light snow showers persisting. Monday February 8th was marked by broken skies, occasional light snow showers, light winds, and cold temperatures.

Snowpack Summary

An atmospheric river event impacted the entire Pacific Northwest January 11-13. Nearby Snoqualmie Pass weather stations recorded more than 5.5” of W.E. during this period with rain to well above 5500’. This created a very thick and widespread melt-freeze crust. The event ended with a few inches of snow and cooling temperatures. 

Cold and generally dry weather for the remainder of January allowed the few inches of snow over the crust to facet. A thin, breakable melt-freeze crust formed following a brief light rain event, capping these facets on many slopes. Subsequent small storms during this time period did not add significantly to the slab; however, more substantial storms over the first week of February brought several feet of snow to the mountains. By the morning of Feb 8, this mid-January facet-crust layer was buried 3-5 feet below the surface on most slopes. The overlying slab consisted of several storm layers increasing in resistance from F to 1F+ over the persistent weak layer. 

Accident Summary

On the morning of February 8th, the two snowbike riders left the French Cabin Creek Snopark near Salmon la Sac to go riding in the Knox Creek area. Fresh snowbike tracks observed at the scene indicate the riders traveled several nearby slopes in the drainage prior to the accident. Several large avalanches covered portions of the riders’ tracks on a nearby peak. It is unclear if the riders were aware of these avalanches, or if they occurred after the accident but before SAR arrived. Around 11:00 am the riders traveled along an old roadbed toward the accident area. They were making their first pass from E to W across the terrain, passing just below a rock buttress before entering the avalanche paths. The riders were traveling on parallel tracks with Rider 1 slightly ahead of Rider 2. The riders triggered the avalanche from somewhere in the lower start zone. Both men were immediately caught and carried. Rider 1 was partially buried. Rider 2 deployed his airbag but was still fully buried. 

Rescue Summary

Rider 1 immediately extracted himself and initiated a search for his companion. This included a beacon search and a visual search of the area. Unable to locate Rider 2 after approximately 30min, Rider 1 recognized the need for additional assistance. He retrieved his bike and rode towards the trailhead for assistance. 911 was contacted at 12:40 pm by a third party. 

Kittitas SAR responded to the scene. The initial SAR team included 2 avalanche rescue dogs, their handlers, and one additional member. The team initially located snowbike tracks covered by fresh avalanche debris. Upon further inspection and searching, these were determined to not be the accident location. Additional tracks in the area led towards another avalanche seen from the road. The team arrived at the accident site at 4:28 pm. At this same time they were joined by additional personnel including 5 SAR members, Kittitas Sheriff’s Deputy, and an NWAC forecaster. The teams searched the site including spot probing likely catchment areas. At approximately 6:00 pm, the group pulled out of the accident site to a nearby safe location to regroup and prepare for night operations. The 2 dog teams were sent back into the area just before 7:00 pm. Shortly after, one of the dogs alerted on an area of snow 20-30ft north of Rider 1’s burial location. The alert was confirmed with a probe strike, and Rider 2 was recovered. Rider 2 was found head downhill, laying on his right-hand side. Average burial depth 4ft/120cm. 

After Rider 2 had been safely removed from the scene, his bike was located by probe just uphill from his burial site.

NWAC staff returned the following day, Tuesday, February 9th 2021 to conduct an accident investigation and take additional photos.

Comments

Terrain:

Despite this relatively short avalanche path, it still produced a very large avalanche. Looking at the northern section of the terrain, the start zone encompasses nearly half the height of the vertical fall. However, the width of the avalanche, combined with a 4-6’ slab, resulted in a very wide and deep debris field. 

Rescue Gear: 

Avalanche Beacon: Even though the rider was wearing an avalanche beacon, it was found in the off position. A trailhead beacon check could have prevented this oversight. 

Avalanche Airbag: Even though the rider inflated his airbag he was still fully buried under 3-4’ of snow. 

Media

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Snowpits

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Figure 8: Profile