Colluvial sediment dynamics in mountain drainage basins

Abstract

Colluvial sediment dynamics are examined using a 70-year landslide inventory in formerly glaciated mountain drainage basins of coastal British Columbia, Canada. Landslide sediment transfer is documented by identifying landslide types, and by characterizing preferential sites of landslide initiation, delivery, and storage across spatial scales. Data analysis reveals that open-slope landslides delivering material to seasonal or perennial channels and fluvial terraces are the dominant mechanisms of sediment transfer. This finding suggests high instability of the low-order channel network and its ongoing re-organization (degradation) after generalized sediment recharge occurred in the last glacial maximum. In the study period, landslide activity across the landscape has generated net degradation on planar slopes and first-order colluvial channels, whereas unchannelled valleys, higher-order colluvial channels and alluvial channels have accumulated material. The scaling relation of the landslide sediment yield appears to be controlled by the spatial arrangement of the relict glacial macro-forms. Landslide yield is highest in unchannelled topography, decreases at the scale of channel initiation ( A d ~ 0.002 km 2), and remains constant for drainage areas where length scales of cirque/valley walls and hanging valley floors overlap (0.002 < A d < 0.06). Injections of landslide material start declining consistently beyond areas larger than 0.6 km 2 (the scale of relict glacial trough initiation), where fluvial environments become increasingly disconnected from landslide inputs. Cumulative yield indicates that colluvial sediment redistribution is limited to small basins; specifically, 90% of the colluvial load is released at scales smaller than about 0.6 km 2.