Montana’s Bridger Mountains rocked with severe avalanches this week—enough to close portions of [R81R, Bridger Bowl]. The avalanches have been large enough to take off most of the winter’s snowpack when they released.

On Monday night of this week, nine inches of heavy, wet snow fell at Bridger Bowl. With it, came rain. The combination wreaked havoc along the ridge at the ski resort and threatened wet slab avalanches.

Tuesday brought the avalanches. Large. Wet. Oozing down the mountain. The ski patrol set some off by explosives; others by ski cutting. Some of the avalanches tore up groomed runs while others piled up 20 feet of snow in ravines.

Wet slab avalanches move slower than dry snow avalanches. According to the Forest Service National Avalanche Center, they are usually caused from rain weakening the snow pack. Instead of barreling at 60 to 80 mph down a mountainside in a cloud of roiling snow, wet slabs flow at 10 to 40 miles per hour behaving more like wet concrete.

Check out some of the shots from Tuesday’s avalanches of Bridger Gully at Bridger Bowl, courtesy of Gallatin National Forest Avalanche Center:

Bridger Bowl closed all but two chairlifts on Tuesday and reduced lift ticket prices. By Thursday, four of its seven lifts were able to operate—those on the lower mountain. The remaining lifts will re-open only when conditions permit.

Doug Chabot from the Gallatin National Forest Avalanche Center included a reaction to Tuesday’s avalanches in his Wednesday morning advisory: “Bridger Bowl experienced their most impressive day of wet slab avalanche activity in memory. Avalanche cycles like this one are rare. It was only the second time in my 22 years of avalanche work that I’ve seen wet slab activity on this scale.”

Chabot continued, noting that “These avalanches ripped out some of the highest skier compacted areas on the mountain.”

Here’s a video showing the carnage to the slope of the Sluice Box avalanche at Bridger:

Chabot attributed the severity of this year’s avalanches to the depth hoar sitting near ground level. Most of this year’s snowpack in Montana sits on a loose, granular layer of snow. Warm weather, sun and rain can trigger the depth hoar to release big slabs. Experts warn that the depth hoar will persist as a problem this spring until the snow melts.

More photos of the Bridger avalanches can be seen here