DEBRIS AVALANCHE IMPOUNDMENTS OF SQUAMISH RIVER, MOUNT CAYLEY AREA, SOUTHWESTERN BRITISH-COLUMBIA

Squamish River has been impounded temporarily by debris avalanches from Mount Cayley on numerous occasions. Evidence of these impoundments comes from backwater deposits and also from a cluster of in situ stumps protruding from a bar along Squamish River. Backwater deposits consist of both lacustrine and fluvial deposits that have formed within the low-energy depositional environment created by a river impoundment. Three main backwater deposits occur in the study area. The fan toe deposit is approximately 14 m thick and represents a single impoundment of Squamish River that likely formed behind a large approximately 4800 BP debris avalanche. In situ approximately 3200 BP stumps along Squamish River probably were killed by a river impoundment due to a debris avalanche. The upper terrace backwater deposit is approximately 6 m thick and forms an aggradational terrace along Squamish River which probably accumulated behind an approximately 1100 BP debris avalanche. The lower terrace deposit also forms an aggradational terrace along Squamish River but represents four or possibly five separate impoundments. These occurred between approximately 1100 BP and 1955 AD, and it seems likely that one of the deposits relates to an approximately 500 BP debris avalanche. Seven or eight Holocene impoundments of Squamish River have been identified in the study area.