Geomorphic Changes to Lillooet River Due to 2010 Mount Meager Landslide

The 6 August 2010 landslide at Mount Meager, British Columbia, was one of the largest landslides in British Columbia during the historic period and is one of many large landslides on this massif over the past 100 years. Recent landslides from this peak have been noted to occur during prolonged periods of warm temperatures. This project conducts a study of the timing and magnitude of sediment transfers downstream from the landslide. This study analyzes satellite images to document river planform change and related sediment erosion and transport, stratigraphic and sedimentological studies of river bars, geologic mapping of the landslide deposit and downvalley floodplain, and flood hazard analysis. Fluvial modification of the landslide debris has been ongoing since the landslide event. A 1-in- 20 year flood event approximately two months after the landslide caused the most geomorphic change to the channel. Downstream of the landslide Lillooet River eroded its banks, deposited large volumes of silty sand, buried trees, and increased its floodplain width. These features suggest that aggradation has occurred in the channel, increasing the flooding hazard in the Lillooet River Valley near Pemberton, British Columbia.