Detailed Accident Report

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Date: 2000-01-22
Submitted By: Doug Chabot, Gallatin National Forest
Place: Clark Lake, near Lionhead Peak
State: ID
Country: USA
Summary: 1 snowmobiler caught, buried and severely injured

Five snowmobilers were sledding on the Targhee National Forest on Saturday,January 22nd. They were near Clark Lake, which is on the MT/ID border about 2 miles west of Lionhead Peak.The victim got stuck half way up a slope as he was highmarking. The rest of the group went around the corner to look at other terrain as the victim wrestled his sled free. The group returned a short time later to find the slope had avalanched and there was no sign of the victim. A coarse search of the debris found the toe of the victim's boot protruding from the snow. He was quickly dug out, was breathing and conscious, but he was suffering from acute internal injuries. The avalanche was on an east facing slope at an elevation of 9,000 feet. The slide was about 2 feet deep, 100-250 feet wide (it widened out half way down the slope), and ran approximately 350 vertical feet. Slope angles ranged from an average of 37-38 degrees on the bed surface, to 45 degrees at its steepest crown location. The US Classification of the slide is HS-3-AV-O.

Over the Christmas Holiday we had a stationary high pressure system that formed significant surface weak layers in this area. Five to ten centimeters of near surface faceted crystals formed

which were then buried by subsequent snow storms. There weer several small storms (1-3 inches per) during the week before the incident. Winds were typically out of the west at 10-30 mph. The particular area that the avalanche occurred is just barely out of our forecast area. The avalanche danger on January 22nd for the Lionhead area near West Yellowstone,rated by the Gallatin National Forest Avalanche Center in Bozeman, was Moderate on wind loaded slopes steeper than 35 degrees and Low elsewhere.

Two of the five snowmobilers were West Yellowstone locals who were wearing avalanche transceivers and carrying shovels and probes. At least one of these locals had extensive backcountry experience and some avalanche education. The other three from Minnesota and were not wearing transceivers, including the victim. By the victims account (told to me third hand) he was trying to free his stuck sled when he triggered a slide below him. A few seconds later he was hit by debris from above as the slide released further uphill. He was tumbled against some trees in which he sustained internal injuries. His friends dug him out and called for a rescue from their cell phone. Three to three and a half hours later the rescue team arrived travelling about 12 miles from the roadhead. It took another two and a half hours to evacuate him in a toboggan to the awaiting ambulance.

From our investigation of the site and the snowpack we can make a few observations and deductions. This slope is steep and is on the lee side of crossloading winds and ridgetop winds. A 75 cm slab of pencil hardness wind blown snow was sitting on top of a 7 cm layer of near surface facets. The upper 1 cm of this layer was extremely angular and well developed, and this was the layer that the slab failed on. It seems as though the initial fracture was at the victim, but it then propagated uphill to the steepest and most windloaded part of the slope. In one isolated section (100'x60') near the bottom of the slope the avalanche stepped down to the ground.

If people have any questions or comments they can contact myself or Scott Schmidt at 406-587-6984.