Among the rarer land forms in the Canadian Rockies are snow-avalanche impact pools. They are small ponds located at the foot of steep avalanche slopes scoured out by snow avalanches which, running in a constrained channel, eject material from flat ground at the bottom to form a “plunge pit” with a mound of debris behind it. There are two documented impact pools in Kananaskis Country, one above Upper Burstall Lake, the other at Upper Tombstone Lake.
Recently, we visited the oval-shaped avalanche impact pool below the east face of Tombstone Mountain. The mound of coarse rocky talus is hard to make out from the other side of Upper Tombstone Lake as it blends into the avalanche path above, so few visitors are drawn to investigate the impact pool (I have lightened the mound so it shows up better in the pictures).
So how are these features formed, and why do they occur in only a few locations when there are lots of avalanche gullies in the mountains? There are many theories and if you want more details on them, read the paper available through the link at the end.
Impact pools only seem to form when a steep slope ends abruptly in flat ground with loose, unconsolidated sediments, or in a lake. In addition, the configuration of the avalanche track should channel the snow into a narrow chute or gully to allow a significant impact force to build up.
The theory is that every 50-100 years, 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 pool is formed, the mass of the descending snow is enough to squirt out sediment, deposited in the intervening years, from the bottom onto the mound.
The mound contains more material than would fit into the estimated volume of the pond, so additional material must be brought down from the slopes above. In more normal avalanche seasons, small slides bring down material from the avalanche path and deposits it either in the pond or on the frozen surface. In the latter case a larger end-of-season avalanche could possibly push debris onto the mound.
At Tombstone, avalanches run 875 m from the bottom of the cliffs, with a vertical drop of 750 m. The avalanche track is narrowed in the lower section by a groove in the bedrock that funnels the heavy wet snow into a 20-30 m wide, 3-4 m deep chute centred on the pool. In 1994 the pool was approximately 7 m deep, 25 m across (between track and mound) and 45 m wide. The water level matches that of the adjacent lake. The mound is about 9 m high and gently tapers toward the lake.
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.