Detailed Accident Report
Submitted By: Ron Johnson; GNFAC
Place: Near Sheep Mountain, Northwest of West Yellowstone
Summary: 1 snowbiler caught, buried, and killed
On Wednesday, April 4, 2001, a snowmobiler and was killed in an avalanche in the Lionhead Area, northwest of West Yellowstone, Montana.
The avalanche occurred just after noon on the east side of a ridge between Lionhead and Sheep Mountain at an elevation of about 10,000 feet (3000 m). The snowmobiler was high-marking a steep slope. He turned just beneath a cornice and was headed downhill when the avalanche released. He flipped over the front of his handlebars and became separated from his snowmachine. His snowmachine was carried about 600 feet (180 m) down the slope and it was found partially buried near the toe of the debris.
A search dog found the missing snowmobiler at about 7:00 pm. He was buried about 2 feet (.6 m) deep and was located about 75 feet (22 m) upslope and diagonally from his snowmachine. He was found in a supine position, head downhill. He was not wearing an avalanche rescue beacon and no one in his party was carrying any avalanche rescue equipment.
I didn't have the opportunity to investigate the crown face of the avalanche but it appeared to 2 to 8 feet (.6 to 2.4 m) deep. It was about 200 feet (60 m) wide and ran about 800 feet (240 m) slope distance. The slab near the cornice was a hard slab while the slab farther down the slope had soft slab characteristics. The slope angle near the starting zone was estimated to be near 40 degrees. The alpha angle was 28 degrees. The depth of the deepest debris was over 14 feet (4 m). The avalanche could be classified as United States HS-AO (snowmachine)-4-O. Using the Canadian classification this avalanche was probably class 3.
During the week prior to the avalanche several small storms had deposited about 8 to 12 inches (20 to 30 cm) of snow at this location. At times strong west to southwest winds had blown and daytime high temperatures had reached the 20's to low 30's F (-5 to 0 C). At the time of the avalanche I was approximately 4 miles (6.4 km) south of the accident site and the weather at my location was partly cloudy, with some scattered snow showers. The temperature was near freezing and winds were light from the south-southwest.
The avalanche occurred on a slope that faces east-northeast and had been repeatedly wind loaded including recent loading from the previous weeks snow and wind. The slope had several rock outcrops including one near where the avalanche was triggered. The slab involved old snow and the bed surface was near the ground.
Because I was involved in the search and body recovery, I was not able to conduct a formal snowpack investigation. However, based on witness accounts and the evidence I saw; it is possible that the snowmobiler triggered the avalanche on a part of the slope that had a thinner snowpack and a soft slab. The resulting fractures propagated up the slope and pulled out the hard wind slab and sections of cornice.
Gallatin National Forest Avalanche Center