Yesterday was a beautiful spring day in the still snow-covered Rockies. We dug out our snowshoes and went to check out the avalanche impact pool above Upper Burstall Lake, an unusual and interesting landform well worth visiting. Our route and the location of the pool are shown on the map.
Snow-avalanche impact pools (sometimes called avalanche plunge pits) are large circular to semi-circular depressions located at the foot of long, steep avalanche slopes. In some cases they are filled with water and become pools. The theory is that every 50 years or so, when there is an unusually large amount of snow in the collection zone of the avalanche path, the resulting climax avalanche in the late spring hits the ground below with sufficient force to excavate and eject the valley-bottom sediments which pile up on a mound downhill of the point of impact. Once a pit or pool is formed, the mass of the descending snow is enough to squirt out more sediment from the bottom onto the mound.
We were surprised by the size of the mound which is 17 m high and about 70 m wide. The grassy downhill side was almost clear of snow, while the pool behind it was still frozen, snow covered and overhung by a big cornice reaching out from the top of the mound. The pool is approximately 9 m deep and its surface 8 m above the level of Burstall Lake. Avalanches occur about once every second year (there was a small amount of debris at the bottom of the gully this year) but major events that bring up sediment from the pool and deposit it on the mound are more infrequent. Samples taken from the mound indicate that significant events occurred in 1956/57, 73/74, 80/81, 83/84 and 89/90.
The only other known impact pool in Kananaskis Country is below the east face of Tombstone Mountain where there is an oval-shaped pool and a tapering mound of coarse bouldery talus abutting Upper Tombstone Lake. There must be more of these landforms around. Have you come across any?
We gratefully acknowledge the paper Snow-Avalanche Impact Pools in the Canadian Rocky Mountains by D. J. Smith, D. P. McCarthy, B. L. Luckman,1994, from which we have taken much of our information.