Cathedral Mountain debris flows, Canada

Abstract

Historic debris flow activity along the north side of Cathedral Mountain in the southern Rocky Mountains of British Columbia, began in 1925 and has increased in frequency up to 1985. A typical debris flow event involves approximately 100,000 m3 of material. Debris flow velocities and discharges above the head of the fan crossed by the Trans-Canada Highway and the C.P.R. mainline are 5.5 m/sec and 210 m3/sec. Most of the large debris flow events are associated with jokulhlaups from Cathedral Glacier. Jokulhlaup discharges of at least 10,000 and perhaps as much as 24,000 m3 of water mobilize these debris flows. Part of the water may have come from a small ephemeral lake on the south side of the glacier. The balance must have been stored within the glacier. The onset and acceleration of debris flow activity was apparently induced by the recession of Cathedral Glacier. Source areas of debris flow sediments have retreated upslope since initiation of debris flow activity. C.P.R. began pumping meltwater from the glacier in 1985 and no jokulhlaups or significant debris flows have occurred since. This preventive measure should either eliminate jokulhlaups or reduce their magnitudes should they occur. Without jokulhlaups, debris flow hazard in the area should be reduced both in frequency and in magnitude.