MANIFESTATIONS OF EXTREME PARAGLACIAL SEDIMENTATION FOLLOWING TERMINAL PLEISTOCENE DEGLACIATION IN BRITISH COLUMBIA LAKES AND INLETS

The paraglacial paradigm is now more than 40 years old, but its details and applicability continue to be refined. In this presentation, we document extreme rates and volumes of sediment delivery in British Columbia following the last glaciation (MIS 2, Fraser Glaciation). At least 100 km³ of sediment were delivered by Fraser River (watershed area = 234,000 km²) to a marine embayment that extended about 100 km east of Vancouver at the end of the Fraser Glaciation. Within a 2000-year period, Fraser River prograded a delta and floodplain west from what is now Hope, completely filling the marine embayment with sediment and isolating Pitt Lake from the sea. Over the next 10,000 years, Fraser River added perhaps another 100 km³ of sediment to the Strait of Georgia, creating the modern-day Fraser Delta. Similarly, Squamish River filled in a deep fjord that reached 25 km from the head of Howe Sound within a few thousand years after deglaciation. Several cubic kilometers of sediment were delivered during this short period of time, largely from landslides and incision of a valley fill. A different pattern is evident in infilling of a late-glacial lake in the Lillooet River valley in the southern Coast Mountains. There, Lillooet River advanced its delta and floodplain some 75 km downvalley during the Holocene. Although the rate of sediment delivery likely was highest immediately after deglaciation, rates were episodically extreme during the Holocene due to the catastrophic delivery of sediment following flank collapses of a Plio-Pleistocene volcanic massif and during one of its eruptions. These examples illustrate the scale of sediments that happen during short periods following times of ice-sheet glaciation in western Canada.