Catastrophic Floods during Deglaciation in a Small, Mountain Catchment, British Columbia

Abstract

Quaternary sediments and landforms in a small, mountain catchment (Silverhope drainage basin) provide a link with catastrophic outburst deposits in a neighbouring valley (Chilliwack River). The contiguous catchments are linked by a pass at 1015 masl, which represents the maximum elevation of the outburst conduit. During Cordilleran Ice Sheet decay, the normal meltwater drainage pathway to the south in Silverhope valley was blocked by a dead ice dam. The area covered by the resulting glacial lake Silverhope is suggested based on elevations of a raised delta, mass movement features, and glaciolacustrine sediments. Exposures near Hicks Creek-Post Creek pass are used to infer that lake levels were raised above the elevation of the divide by a second dead ice dam. Southsouthwest oriented paleoflow deposits covering glaciolacustrine sediments northeast of the pass, and clean bedrock faces southwest of it, suggest that the dead ice dam was situated at the divide. Consequent catastrophic discharges flowed down Post Creek into Chilliwack valley. Radiocarbon ages from Chilliwack valley, and outburst sediments deposited against the distal side of the end moraine that dams Chilliwack Lake are used to infer that outburst occurred about 11.4 ka.