Avalanche threat not apparent to victim

The avalanche that killed a Bozeman snowmobiler riding in the Gallatin Mountains on New Year’s Day – the first Montana avalanche death in more than a year – was somewhat of an anomaly.

“Most avalanches happen when people are on the slope,” said Doug Chabot, of the Gallatin National Forest Avalanche Center. That can happen when a snowmobiler is riding across a hill or uphill, or when a skier or snowboarder is carving turns.

But the avalanche that killed Burton K. Gibson, 46, was what’s called a remote trigger avalanche – when a slide is caused by collapsing of the snowpack from what may look like a relatively flat or safe area.

“When there’s a weak layer in the snowpack, in order to get an avalanche we need that weak layer to collapse,” Chabot said, describing the unstable snowpack found in the mountains that week as like a book resting on potato chips. The hazard was forecast as “considerable.”

“When it’s really unstable and we collapse a weak layer, it’s almost like dominoes falling,” he added. “It propagates out and can go up the hill. In a split second, it’s moving close to the speed of sound. When it gets to a steep slope, it breaks.”

Gibson and his companions were riding in Onion Basin, a bowl at the head of Porcupine Creek southeast of Big Sky. When the avalanche broke on the mountain above, it collapsed 400 feet wide and 2 to 3 feet deep and then ran 1,100 feet downhill – a drop of 500 vertical feet, according to the accident report.

Burton, who apparently tried to outrun the slide, was found buried about 3 feet deep near the bottom of the avalanche, called the runout zone. A fellow rider, was pushed into trees on the slope behind Gibson, mostly buried but able to get free.

“It was pure luck” that he wasn’t completely buried, too, Chabot said. Trees offer no protection in avalanches and more often injure victims swept through them.

Gibson was wearing an air bag avalanche safety device worn like a backpack. The bags add flotation, keeping the avalanche victim closer to the top of the slide. But Gibson never pulled the ripcord.

“They have about a 30-percent non-deployment rate,” Chabot said.