- Location: Northwest of Glacier Peak, Middle Fork Swan River
- State: Colorado
- Date: 2018/04/10
- Summary Description: 1 snowmobiler caught, buried, and killed
- Primary Activity: Snowmobiler
- Primary Travel Mode: Snowmobile
- Location Setting: Backcountry
- Caught: 1
- Partially Buried, Non-Critical: 0
- Partially Buried, Critical: 0
- Fully Buried: 1
- Injured: 0
- Killed: 1
- Type: HS
- Trigger: AM - Snowmobile
- Trigger (subcode): u - An unintentional release
- Size - Relative to Path: R3
- Size - Destructive Force: D2.5
- Sliding Surface: O - Within Old Snow
- Slope Aspect: N
- Site Elevation: 11830 ft
- Slope Angle: 38 °
- Slope Characteristic: Convex Slope
This was a hard slab avalanche remotely triggered by a snowmobile, medium sized relative to the path, and had the destructive force to bury, injure or kill a person (HS-AMr-R3-D2.5-O). The avalanche failed on a layer of faceted, early-season snow 16 to 20 inches above the ground. The crown face ranged from 24 to 60 inches deep and was 700 feet across at the widest point. The avalanche released on a north-facing slope 38 to 42 degrees in steepness. The snowmobilers were on a slope of less than 30 degrees beneath steeper slopes at the time of the avalanche.
Another large avalanche released sympathetically on an adjacent slope to the west. Investigators did not measure the sympathetic avalanche, but estimated a width of 300 feet. The height of the crown face was up to 10 feet deep.
Snowfall during the winter of 2017/2018 started in early October with sporadic, small storms followed by long, dry periods. There were few extended stormy periods during the winter. A storm from April 6th to April 8th was the largest of the winter in terms of snow water equivalent.
The storm began on the evening of April 6 with heavy snow and strong winds. Strong westerly winds continued until the morning of April 8. From the April 6 to 8, the Michigan Creek SNOTEL site (10,600 ft elevation, 3.75 miles south-southwest of the accident site) recorded an increase of 2.1 inches of snow water equivalent. Keystone Ski Resort (5.25 miles north of the accident site) recorded 32 inches of snowfall for this storm. Although the storm was generally warm, with rain up to eleven thousand feet, it ended much cooler as it exited on April 8.
There were periods of sun on April 9, with a high temperature of 35 F at the Michigan Creek SNOTEL. On the day of the accident, skies were clear, westerly winds were moderate, and the temperature at the time of the accident was 43 F at the Michigan Creek SNOTEL site.
Snow from early-season storms faceted and eventually turned into depth hoar. Small storms throughout the winter added layers on top of the weak, basal structure. This lower-snowpack structure was remarkably consistent throughout the CAIC's Vail & Summit County forecast zone. By April the lower snowpack consisted of approximately 16 inches of depth hoar near the ground. On top of the depth hoar layer sat a layer of small (1 mm), soft (4F hand hardness) facets. Small storms throughout the winter added about 40 inches of hard layers (1F to P hand hardness) above the facets and depth hoar. Measured snowpack depth in the area of the accident was 55 inches (140 cm). In snowier areas of the forecast zone, the small storms added up and by April the snowpack depth was over 80 inches.
Events Leading to the Avalanche
On Tuesday, April 10 two friends met at the French Gulch trailhead near Breckenridge for a day of snowmobiling. They rode up Humbug Hill and down American Gulch into the Middle Fork Swan River. They proceeded up the Middle Fork and then headed for a drainage north of Glacier Peak. Rider 1 had ridden in the area before. The pair rode to the bottom of an avalanche path and played around in open areas beneath steeper slopes.
They noticed a previous avalanche (triggered on April 8, see Figures 2 and 3). They felt they could ride safely below the old avalanche because it had already slid. They rode below the avalanche for a while, but eventually worked westward under areas that had not avalanched. Rider 1 rode further west to other open areas and out of sight from Rider 2.
After a few minutes riding, Rider 1 did not see Rider 2. He turned back towards the open areas where they initially started, to locate Rider 2. As Rider 1 rode east he saw the avalanche release above him. He looked downhill and saw Rider 2. Rider 2 had his helmet off and was trying to free his stuck snowmobile.
Rider 1 yelled "Avalanche!" and turned hard left, downhill and to the west, riding into old-growth trees to escape being caught in the avalanche. Rider 1 was successful and not caught in the avalanche. He yelled for Rider 2 but did not hear a response. He immediately turned his avalanche transceiver to receive and began searching.
Rider 1 searched with his transceiver for about fifteen minutes, traversing the debris with his snowmobile, but did not find a signal. Rider 1 was able to locate Rider 2’s snowmobile and tunnel bag but was unable to locate Rider 2. He then triggered an SOS button on his phone and was able to report the avalanche to 911. The call came into Summit County Dispatch at 11:09 AM. Rider 1 continued his transceiver search while waiting for help.
Summit County dispatched the Summit County Rescue Group who requested a Colorado Rapid Avalanche Deployment Team (C-RAD). The first C-RAD team arrived at the site via a Flight for Life helicopter at approximately 12:04 PM. The team included two rescuers and an avalanche rescue dog. Rescuer 1 started his rescue dog on a search while Rescuer 2 started a beacon and Recco search. Approximately 10 minutes later a second rescue team arrived at the scene. While the second team was organizing, Rescuer 1 reported that his dog alerted on a portion of the debris. Rescuer 1 got a positive probe strike at 12:22 PM, a few feet downhill of a large tree. All rescuers and Rider 1 begin to digging. The avalanche buried Rider 2 about four feet deep. Rescuers uncovered him about 90 minutes after the avalanche. His condition was assessed and he was determined to be deceased.
The rescue party found Rider 2's beacon in the “off” position. While there is no way to know if the results would have been different had Rider 2's beacon been turned on, it would have greatly improved his chance of timely rescue. Rider 1 and Rider 2 rode together the previous day and performed beacon checks in the parking lot of the trailhead on April 9. They did not check their beacons before riding on April 10.
Figure 12: Snow profile observed at the deepest part of the crown face (5 feet deep). In most areas the crown face was 30 inches deep.
Figure 13: Snow profile observed at the highest uphill portion of the crown face. The snowpack was shallow in this areas with talus underneath.