- Location: Cottonwood Pass, west of Buena Vista
- State: Colorado
- Date: 2016/02/05
- Summary Description: 1 snow bike rider, caught, partially buried-critical, and killed
- Primary Activity: Snowmobiler
- Primary Travel Mode: Snowmobile
- Location Setting: Backcountry
- Caught: 1
- Partially Buried, Non-Critical: 0
- Partially Buried, Critical: 1
- Fully Buried: 0
- Injured: 0
- Killed: 1
- Type: SS
- Trigger: AV - Vehicle (specify vehicle type in comments)
- Trigger (subcode): u - An unintentional release
- Size - Relative to Path: R2
- Size - Destructive Force: D2
- Sliding Surface: O - Within Old Snow
- Slope Aspect: E
- Site Elevation: 12200 ft
- Slope Angle: 36 °
- Slope Characteristic: Convex Slope
This was a soft slab avalanche, triggered by a person on a motorized snow bike, small relative to what the avalanche path could produce, large enough to kill a person but not large enough to destroy a timber structure, that broke into old snow (SS-AV-R2D2-O). The avalanche ran on an interface between a layer of rounding faceted particles (FCxr) and rounded particles (RGlr). There was a thin layer of small faceted particles (FCso) at the top of the rounding layer. The avalanche broke into the depth hoar (DHcp) in places. The alpha angle for this event was 23 degrees. The path can produce much larger avalanches that run onto and across the lake (see Google Earth images). This was a Persistent Slab avalanche.
A storm moved into the Cottonwood Pass area on January 31. By the evening of February 1, around 15 inches of snow fell at the Cottonwood Pass weather station, less than 0.5 miles to the southeast of the accident site. At weather stations in the Sawatch Range, winds during and after the storm were from the northwest at 10 to 15 mph. Temperatures were around 15 F when the storm began and consistently dropped over the next several days. Temperatures never warmed above -5 F on February 3rd. February 4 was around about 10 F warmer. Temperatures were in the single digits, winds were 10 mph out of the west-northwest, and skies were clear on the morning of February 5.
The snowpack near Cottonwood Pass remained quite shallow through November and early December. Cold temperatures and extended periods between snowfalls produced faceted snow grains throughout the entire snowpack. A series of storms in late December and early January built layers of storm and wind-drifted snow on top of this weak foundation. Another period of cold and dry weather in early January produced a faceted snow layer near the snow surface. A sizable storm impacted the area January 15 through 21. The storm snow and strong winds created a cohesive slab of snow above the early January faceted layer. Another storm January 31 to February 1 added additional snow. By February 5, west-northwest winds drifted a cohesive slab to cap off the snowpack on this east-facing slope.
Events Leading to the Avalanche
Rider 1 unloaded his snow bike and left the Denny Creek trailhead around 10:00 AM. He traveled up the groomed portion of CR306 to treeline on the east side of Cottonwood Pass. He rode around on low-angle slopes to the southeast of Cottonwood Pass. After some time he rode above a large bench in the terrain and began climbing on to steeper slopes below a prominent ridgeline.
Around 11:00 AM Rider 1 began a southward climbing traverse onto steeper terrain. The avalanche released, caught, and buried Rider 1 and his bike in the debris. There were no witnesses to the avalanche.
A group of five snowmobilers were riding in the Cottonwood Pass area. They saw recent avalanche activity on their way up the pass, but saw a new slide around 11:00 AM. They knew other riders were in the area and had seen Rider 1 in the area of the slide. Members of the group road to the debris pile, looked for tracks into the debris pile and performed a search with avalanche rescue beacons. The rest of the group flagged down a snowcat that was grooming the road to the pass (Buena Vista Snow Drifters).
Members of Chaffee County Search and Rescue were training in the Cottonwood Pass area. They had been riding snowmobiles on the west side of the pass and were returning to the east side via the groomed track on CR 306. They encountered members of the group of 5 while they were talking to the snowcat driver about the avalanche. The SAR group and the snowcat driver reported the avalanche via radio.
Members of the SAR group and the group of five snowmobilers traveled back to the avalanche site. They looked for tracks and found one that entered, but did not exit, the debris pile. They searched the debris pile with avalanche beacons, but did not find a transmitting signal. During the transceiver search, they saw an object sticking out of the snow. They dug around the object and determined it was the heel of a boot.
They began extricating the buried rider. Other people joined in the effort and they had the rider in a position where they could perform first aid about 25 minutes later. They began CPR. Other members of SAR and law enforcement organizations arrived (Chaffee County Sheriff and Colorado Parks and Wildlife) and they determined that Rider 1 had died during the burial. The group transported Rider 1 and his vehicle back to Denny Creek.
Rider 1 was traveling alone, so no one saw the accident. From the location of this track and from other observations of the snow around the avalanche, it is likely that he was traveling south on a climbing side-hill traverse when the avalanche released. This would place him in a likely area to trigger the avalanche, the lower edge of the wind pillow formed from west-northwest wind during the two days prior to the accident. This area also contains some large rock outcrops. Both of these slope features make for shallow snowpack areas where a rider is more likely to trigger an avalanche.
Rider 1 was traveling alone and no one witnessed the avalanche. This delayed any rescue response. He was not carrying any avalanche safety equipment, such as an avalanche beacon, probe, or shovel. When bystanders responded, they had to rely on surface clues to determine his location. Fortunately, part of his boot was sticking out of the snow, but the rest of his body and snow bike were completely buried.
Rider 1's head was under about six feet of snow. This deep pile of debris set up into hard snow after the avalanche. It took several rescuers to extricate him from the snow.
Figure 9: Fracture-line profile from the north side of the avalanche.