Seven Mile

Traveled by ski and snowmobile through open trees near treeline. Entire area was heavily wind affected, with a 10 - 20 cm stiff windslab present on all aspects we traveled (SE through N) that was reactive to snowmobile and skier triggering. Remote triggered and direct triggered two size 1.5 slides. Dug a pit at the toe of the E slope that slid, where recent windslab failed CTM (13), sudden, planar on 1mm of facets overlying a pencil-hard crust.

Two avalanches accidentally triggered. One remote triggered by snowmobile, one direct triggered by skier. Both were size 1.5 windslabs (20cm thick one-finger hard) that ran on 1-2mm of facets overlying a pencil-hard 1cm thick crust.

Failure layer was remarkably thin. We were all surprised that such minimal facets could produce such a sudden and planar failure.

We (group of 3) traveled to the head of the seven mile drainage, with a plan to stick to mellow slopes based on the recent and ongoing wind activity. Winds in the drainage were predominantly from the NW (blowing upcanyon), but were swirling. After discussing the conditions and everyone's goals for the day, the group split up, with 2 travelling on sleds, and a solo person travelling on skis. In hindsight, this was a really stupid decision that is atypical for all members of this group. The sledders decided to explore meadows towards Cayoosh, while the solo ski tourer planned to lap low angle gladed trees below treeline. Within ten minutes of setting out towards Cayoosh, the sled group remote triggered a size 1.5 while traversing the toe of a SE facing slope (30 deg). The crown was approx. 20 cm deep and 25 m wide, and ran 200m on a windslab-crust interface. The sled group immediately turned around to rejoin the lone skier and notify them of the touchy conditions they experienced. During this same time frame, the skier ascended 20 - 25 deg NE facing slopes, and reached a bench below a prominent knoll. The NE slopes of the knoll looked extremely wind affected, and the skier decided to traverse across the bench to investigate whether E facing slopes had less wind effect, and whether it was possible to reach the top of the knoll via a sheltered route. Soon a well-treed gentle ridgeline (less than 30 deg) came into view, which the skier decided would be a good route to ascend and ski back down from the top, without having to ski open steeper slopes. Between the skier and the ridge, however, was an open 30 - 35 deg slope that would need to be crossed. The skier knew the slope was a concern, and that crossing it violated all training and intuition. Against their better judgement however, the skier decided to cross this slope to avoid having to ski to the valley bottom and regain lost elevation. Multiple times during the short traverse, the skier thought to themselves that this was a bad idea, and clearly returning to the valley bottom and resetting the ascent from there would be the much safer option. Low and behold, halfway across the slope, the skier heard a sharp and audible whoomph and immediately saw a large crack 5 m above the skier's position that began to propagate across the slope. The crown broke just below a slight localized convexity that was not noticeable at the start of the traverse, but was very noticeable when seen from below. There was a delay of 2 -3 seconds, and then the debris started moving slowly (walking pace), causing the skier (who had skins on and heels unlocked) to fall to their hip. The skier began to reach for their airbag handle in case it was necessary, but came to a stop only 1 - 2 m below the start zone, with skis resting on the bed surface. A ski pole was carried about 5 m, and the toe of the debris ran about 30m in total. The skier carefully skied away from the slope, and waited for the sledders in the valley bottom. After the group came back together, we decided to avoid all avalanche terrain for the remainder of the day, and dig a pit at a protected location at the toe of the E slope that had slid. Pit revealed that the 20cm one-finger hard windslab had likely failed on 1 - 2 mm of small facets overlying a pencil-hard 1cm thick crust at 20cm deep. Compression test result was CTM (13) sudden planar at the windslab-facet/crust interface. Below this interface was 5cm of low-density (four-finger) snow, with another 1 cm thick pencil-hard crust. The snowpack below the second crust layer was consistent one-finger hard snow down to approx 130cm (depth of our pit). Lessons were learned today to say the least.