THE RECORD OF JOKULHLAUPS FROM SUMMIT LAKE, NORTHWESTERN BRITISH-COLUMBIA

Summit Lake, which is impounded by Salmon Glacier, is the largest self-draining, ice-dammed lake in Canada. Until 1961, it contained few icebergs and was stable, overflowing to the north into the Bowser River valley. The first jokulhlaup occurred in December 1961, after a lengthy period of thinning and retreat of Salmon Glacier, when a subglacial tunnel developed in the weakened ice dam, allowing the lake to drain suddenly. This flood and two others in 1965 and 1967 caused major damage to the road system in the Salmon River valley south of the lake. Since 1965, with three exceptions, Summit Lake has drained annually; minor floods along Salmon River in 1966, 1969, and 1973 may record partial drainings of the lake, although other explanations are possible. Jokulhlaups in recent years have been smaller and have occurred earlier in the year than most of the early floods. Rapid water-level fluctuations associated with the annual emptying and refilling of Summit Lake have generated large numbers of icebergs, derived from the Salmon Glacier dam; these icebergs presently choke the surface of the lake. The present jokulhlaup cycle is likely to continue either until the glacier readvances or until it retreats to the point that it no longer forms an effective seal.