Scales of colluvial sediment transfer and coupling in formerly glaciated mountain drainage basins

We examine basin-wide colluvial sediment dynamics through the compilation of a 70-year landslide inventory in mountain environments of coastal British Columbia. In particular, we document the colluvial sediment cascade by characterizing the dominant sediment sources, and by identifying preferential sites of colluvial delivery and storage across landscape scales. Results reveal that open-slope landslides evacuating material to ephemeral or permanent streams and fluvial terraces are the preferential source-to-sink pathways. This pattern suggests high instability of the headmost channel network and its ongoing re-organization after generalized sediment recharge occurred during the last glacial maximum. In the study period, the colluvial sediment budget calculated across geomorphic process domains indicates net volume loss on planar slopes and source colluvial channels, whereas unchannelled valleys, sink colluvial channels and fluvially-dominated channels have been aggrading. In this context, we propose a novel approach to represent colluvial sediment yield as a function of drainage area. This scaling relation appears to be controlled by the spatial organization of geomorphic process domains, as imposed by the morphometry of the glacial palimpsest. It follows that colluvial yield is highest in unchannelled topography, decreases at the scale of channel initiation, and remains constant for drainage areas where source colluvial and hanging fluvial domains overlap. Colluvial sediment injections start declining consistently beyond areas larger than 0.6 km2, where fluvial environments are still partially coupled. Evaluation of the colluvial scaling trend in conjunction with British Columbia fluvial sediment yield, allows us to identify landscape scales of sediment aggradation and degradation, hence to infer preferential scales of contemporary sediment storage.