Fluvial Remobilization and Downstream Impacts of Sediment from the 2010 Capricorn Creek Landslide on Mount Meager

The 2010 Capricorn Creek Landslide injected approximately 49×10⁶ m³ of sediment—75 years of the typical annual yield—into the Lillooet River valley, damming the river and a tributary. These dams overtopped shortly after the slide and channels then incised the slide deposit. Comparison of post-slide photogrammetry with LiDAR data collected five years later and aerial photos indicates approximately 5.5×10⁶ m³ of sediment were evacuated by 2017. This sediment has distinctive volcanic lithology, contrasting with the basin's dominant plutonic rocks, allowing downstream tracing of the slug. We combine aerial photo-based active channel mapping, observations of floodplain inundation and sedimentation, comparison of pre-and post-landslide bed material samples (both lithology-specific pebble counts and sub-armor bulk gradations), repeat bathymetric surveys, and hydraulic modeling to determine transport pathways, fate, and morphologic impacts of sediment from the slide.

Our results highlight the importance of grainsize-based, transport-pathway differentiation for understanding sediment slug evolution and predicting downstream impacts of large sediment injections:

- 30% of the slide mass smaller than medium sand has been transported as wash load to Lillooet Lake, 80 km downstream of the landslide.

- Medium sand to fine gravel—40% of the slide volume— has been transported in suspension and as throughput bedload with only transient storage through the first 40 km downstream of the slide. It has accumulated on the bed along the lower half of the river, driving 0.25 to 1.2 m of channel aggradation in this diked reach and substantially reducing channel flow conveyance capacity. Sand overloading has resulted in fining of the grainsize distribution along the whole river profile, increasing both gravel transport capacity and the competency of the river to move the largest particles, even in areas downstream of coarse slide-derived sediment.

- The remaining coarse sediment has evolved as a diffusive slug, dispersing up to about 25 km downstream of the slide. Intrusion of the coarse slug has increased bed levels, channel width, and braiding intensity.

Application of typical diffusive sediment wave modeling focused on only the coarse fraction would miss the most socially important impacts of the slide-derived sediment.