Snow-avalanche impact pools in the Canadian Rocky Mountains

Three snow-avalanche impact pool and mound complexes in the Canadian Rocky Mountains are described. Recent and historical evidence of fresh ejecta and the depth, stratification, and14C-ages of material in the mounds suggests they develop primarily by episodic, high magnitude, snow-avalanche events rather than by single catastrophic avalanches. Limited evidence indicates such events may have a recurrence interval of 50 to 150 yr. The pool/mound complexes develop as a consequence of snow avalanche impact forces sufficient to excavate and eject valley-bottom sediments. Erosion of a pit results when the avalanche collides with the valley bottom and bodily lifts, scoops, and/or bulldozes unconsolidated sediment along its trajectory path. This erodes a depression and builds a distal embankment of debris. Volumetric estimates of sediment excavated from the pools range from 3000 to 10,000 m3and are less than the estimated mound volumes. This suggests that additional sediment is supplied to the mounds from the upper part of the avalanche track. Questions remain concerning the mechanics of formation, age, and long-term evolution of such features due to the infrequent occurrence of significant events at these sites. Theoretical calculations indicate that avalanche impact pressures in excess of 1 MPa and a sharp break of slope are probably the main factors controlling the occurrence and distribution of pool/mound complexes. Their development is also probably favored where steep avalanche tracks terminate in lakes or loose unconsolidated sediments.