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Cayoosh Mountain, North Glacier Avalanche # 6400

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Date: 2005.02.26
Vantage Point: Head of Eight Mile Creek Valley

Caption: The North Glacier of Cayoosh Mountain provides good skiing during times of high stability and demonstrates that "Low" avalanche danger does not mean "No Danger".

PhotoDescr: A week ago, on 050220, a group of us climbed Cayoosh Mountain via the north glacier in beautiful spring conditions. Buoyed on by the generally low avalanche conditions, we chose to climb this glacier via the obvious rising bench at mid-height that rises towards Cayoosh Peak just out of sight at photo left.

While we got an early start, we still moved as quickly as possible across the ramp to get high and away from sun effect on the glacier. While the sun was intense, the surface snow on this NNE aspect for the most part remained powdery and gave us an excellent day. For the most part the bench seemed to provide some safety, as its crest was largely raised above the uphill portion of the glacier, as may be seen in the photo essay "Cayoosh Mountain: North Glacier Touring" which shows the bench just above where the avalanche ran across the climbing track.

With continued sunny conditions and forecast low danger, we returned to the area again this past Saturday. Rounding the corner at the hanging valley below the north glacier, I was quite surprised to see a long, horizontal crown at the top of the convexity at about 2400m elevation. As we climbed the headwall to a small lake, the size of the slide became more and more obvious. Equally sobering was the sight of last week's climbing track obliterated by the avalanche on the right of the ramp. One can just see the climbing track at this point in the photograph if one looks carefully.

As we followed the day's plan to head up over a pass into Marriott Basin, I had lots of opportunity to look upon the slide and ponder our decision of the previous week. Although we knew that some deep persistant weak layers had probably survived at that altitude, the serious bridging that had occurred since the mid-January storm seemed to have buried such instabilities well away until spring warmth would penetrate the snowpack. Although the preceding week had developed a strong temperature inversion, snow temperatures at 2000m+ on north aspects were still below freezing.

An obvious trigger for the main slide was not apparent, though one small indentation near the centre of the crown looked oddly out of place, perhaps a little like a bomb crater. The fracture line looked like a consistent 2.5m and appears to have extended for 200-250m horizontally. The avalanche ran a vertical distance of 675m and ended in a terrain trap on the small lake at the glacier's tongue. Note that the lower angle slope that runs back from the bench to the steeper face that failed is foreshortened in this photograph, which means the slide ran a considerable horizontal distance before running over the ramp that bisects its track. Makes one think very seriously about alpha angles.

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