Landscape Controls on Delivery of Landslide Sediment to River Channels

Shallow, rapid landslides that deposit in rivers can severely impact channel morphology and morphodynamics, causing changes ranging from locally altered sediment transport regimes to the burial of entire sections of drainage networks. Previous work has investigated watershed-scale landscape characteristics associated with high frequencies of landslides depositing in channels. However, we still have a poor understanding of the hillslope-scale landscape features which stop landslides from entering rivers. To address this, we collected a landslide inventory from historical air photos spanning 1943 to 2002 in the formerly glaciated mountainous watershed of Chilliwack River valley in southwest British Columbia, Canada. Using this landslide inventory in conjunction with volumetric change, topographic, and geomorphic GIS analyses, we systematically evaluate how landscape features—such as river terraces or wide valleys—influence landslide channel entry. Our analysis shows that low-gradient features (less than approx. 25°) such as alluvial or glaciofluvial valley fills and pre-existing colluvial depositional landforms are the most common buffers for landslides that deposit outside of the channel network. These observations highlight the importance of landscape gradient in controlling landslide mobility and, therefore, landslide-channel connectivity, corroborating previous studies. Additionally, this work contributes to a growing literature characterizing sediment connectivity regimes unique to landscapes that were glaciated in the late Pleistocene. Future work could harness these findings to develop a method for assessing risk of landslide channel entry applicable regionally.