A debris flow triggered by the breaching of a moraine-dammed lake, Klattasine Creek, British Columbia JOHN J. CLAGUE Terrain Sciences Division, Geological Survey of Canada, 100 West Pender St., Vancouver, B.C., Canada V6B lR8 S. G. EVANS Terrain Sciences Division, Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada KIA 0E8 AND IAIN G. BLOWN Hydroelectric Generation Projects Division, British Columbia Hydro and Power Authority, Harbour Tower, 555 West Hustings St., Vancouver, B.C., Canada V6B 4T6 Received February 1, 1985 Revision accepted May 16, 1985 A very large debris flow of unusual origin occurred in the basin of Klattasine Creek (southern Coast Mountains, British Columbia) between June 1971 and September 1973. The flow was triggered by the sudden release of up to 1.7 X 10' m3 of water from a moraine-dammed lake at the head of a tributary of Klattasine Creek. Water escaping from the lake mobilized large quantities of unconsolidated sediment in the valley below and thus produced a debris flow that travelled in one or, more likely, several surges 8 km downvalley on an average gradient of 10" to the mouth of the stream. Here, the flow deposited a sheet of coarse bouldery debris up to about 20 m thick, which temporarily blocked Homathko River. Slumps, slides, and debris avalanches occurred on the walls of the valley both during and in years following the debris flow. Several secondary debris flows of relatively small size have swept down Klattasine Creek in the 12-14 years since Klattasine Lake drained. Entre juin 1971 et septembre 1973, il s'est form6 dans le bassin du ruisseau Klattasine (rkgion sud de la chaine CBtikre, Colombie-Britannique) une trks vaste coulCe boueuse d'origine insolite. Le dCclenchement de la coulCe fut causC par la rupture soudaine d'une moraine retenant les eaux d'un lac 2 la tete d'un affluent du ruisseau Klattasine, ce qui engendra le dkversement d'environ 1.7 X 106 m3 d'eau. L'eau qui slCchappait du lac a entrain6 de grandes quantitCs de sCdiments nonconsolidCs dans la vallCe plus basse et a crCC une coulCe boueuse qui a avancC 8 km dans la vallCe de pente moyenne de lo0, jusqu'a I'embouchure de la rivikre, soit en une seule Ctape, ou plus probablement en plusieurs Ctapes. A cet endroit, la coulCe a accumulC une nappe de gros dtbris caillouteux d'une Cpaisseur pouvant atteindre jusqu'a 20 rn et qui a bloquC temporairement la rivkre Homathko. Des glissements en forme de cuiller, des matCriaux CboulCs et des avalanches de dCbris se sont produits sur les versants de la vallCe lors de la coulCe boueuse et tout au long des annCes substquentes. Plusieurs petites coulCes boueuses secondaires ont descendu dans le ruisseau Klattasine pendant les 12 ou 14 ans qui ont suivi le dtversement du lac Klattasine. [Traduit par le journal] Can. 1. Earth Sci. 22. 1492-1502 (1985) Introduction Many lakes in the high mountains of western Canada are impounded behind large moraines built in the A.D. 1700's and 1800's at the climax of the "Little Ice Age" (Mathews 1951; Luckman and Osborn 1979). Some of these moraines exceed 100 m in height, and most consist of loose bouldery diamicton and stratified sandy boulder gravel, materials that are sus- ceptible to erosion by piping and fluvial overflow. Many of the moraines have failed in the past, releasing the waters im- pounded behind them. Others may fail in the future, triggering devastating floods or debris flows similar to those that have claimed thousands of lives in Peru in the last 50 years (Lliboutry et al. 1977; Eisbacher 1982). This report documents a recent large debris flow in British Columbia that was triggered by such a failure. To our knowl- edge, this is the largest historical flow of its kind in North America. Fortunately, the disaster occurred in an uninhabited valley; thus there was no loss of life or property damage. Unfortunately, no one witnessed the event, so our knowledge of it is based on indirect evidence and inference. This event highlights the need for caution in the development of glacier- ized mountain valleys in the Canadian Cordillera. Setting Klattasine Creek is a small tributary of Homathko River, which flows through the southern Coast Mountains and empties into the Pacific Ocean approximately 220 km northwest of Vancouver (Fig. 1). This part of the Coast Mountains is ex- tremely rugged and hosts impressive icefields and large valley glaciers. Klattasine Creek heads at the snout of one such glacier at about 1450 m elevation and flows 10 km northwest to join Homathko River at 150 m elevation. A north-flowing tributary, 1 which joins the main stem of Klattasine Creek about 4 km upstream from the Homathko confluence, heads in a lake backed by a small cirque glacier at an elevation of about 1650 m (Fig. 2). This lake (here termed "Klattasine Lake") is a remnant of a larger body of water that drained catas- trophically in the early 1970's to produce the Klattasine Creek debris flow. Source of debris flow and initiating mechanism The debris flow was generated by the failure of the Little Ice Age moraine that formerly impounded Klattasine Lake (Figs. 2, 3). As the moraine failed, a large volume of water was released from the lake into the valley below. The moraine is arcuate and sharp crested and has a maximum height of about 20 m. In the vicinity of the breach, the inner (south) side of the moraine slopes 27" and the outer (north) side 32". Prior to the outburst, Klattasine Lake drained via a steep (ca. 23") channel across the moraine. When visited in August 1984, the morainal breach was ap- proximately 30 m wide at the top and 13 m deep, with side