Origin of Downstream Variation in Clast Size in a Small, Mountain Catchment, British Columbia

Abstract

Downstream variations in clast size were examined in Silverhope Creek, a tributary of the Fraser River, British Columbia, Canada. A sedimentary environment inherited largely from Cordilleran Ice Sheet decay dominates the present fluvial regime. Hummocky terrain in the southern part of the catchment produces distinct grainsize fluctuations over several km that are related to local changes in the sedimentary regime during deglaciation. Similarly, relict mass movement features impart grainsize variations of a glacigenic origin on the bed material. Sediment input primarily from western tributaries produces local downstream coarsening. Landslide-damming of Silver Lake about 1100 years ago provides a local base level in the northern half of the catchment. Fining associated with aggradation upstream and coarsening associated with degradation downstream also interrupt the expected trend toward downstream diminution in clast size. Geomorphic activity of anthropogenic origin is subordinate to that generated by natural processes. Sediment input shows greater bimodality in more recently-logged tributaries. Bimodality of sediment input has declined over a period of 5-10 years following the cessation of logging activity in the tributary. The main reason for a decline in bimodality is forest regrowth and valley side stabilisation. However, sedimentation rates in Silver Lake remain high indicating that the quantity of sediment input as a result of tree removal may remain high in the long term, whereas change in sediment quality (bimodality) is a short term factor.