ABSTRACT
Strandline Lake, a glacier-dammed lake in south-central Alaska containing over 7.1 X 108 m3 of water, has produced a series of jökulhlaups during at least the last 20 years. Observations, vertical aerial photography, and time lapse photography made during jökulhlaups in 1982, 1984, and 1986 indicate that the flood waters follow multiple pathways as they drain under or through, and along the margins of the Triumvirate Glacier which impounds the lake. As the flood discharge increases, the drainage changes from primarily marginal to subor en-glacial. Detailed measurements of the lake stage made from 35mm films taken during the 1984 jökulhlaup, coupled with the hypsometric function of lake stage and water volume, have been used to derive the discharge hydrograph for the 1984 event. The hydrograph shows five distinct episodes where discharge was abruptly reduced for short periods, superimposed on an overall trend in which discharge was increasing nearly exponentially.
The reductions in discharge are probably due to blockage or collapse of subglacial tunnels. In 1984 the blockages were temporary, but the 1986 jökulhlaup was prematurely terminated. Strandline Lake was left partially full of water as of this writing. This suggests that in some cases tunnels can become totally blocked during a flood.
Caution is needed in interpreting prehistoric sequences of flood deposits produced by jökulhlaups. The sequence of Pleistocene flood deposits in varved lake sediments in northern Washington state may contain a record of temporary reductions in discharge and premature terminations of jökulhlaups from Lake Missoula.
