Hydrologic and geomorphic effects of temporary ice-dammed lake formation during jökulhlaups

Glacial outburst floods (jökulhlaups) occur frequently in glaciated environments, and the resultant flooding causes geomorphic change and, in some instances, damage to local infrastructure. During some jökulhlaups, floodwater is stored temporarily in ice-marginal locations. In July 1999, a linearly rising jökulhlaup burst from Sólheimajökull, Iceland. During this remarkable event, subglacial floodwater pooled transiently in two relict ice-dammed lake basins, before draining suddenly back into Sólheimajökull. The significance of such rapid formation and attendant drainage of temporary ice-dammed lakes during jökulhlaups has not been addressed. Consequently, this paper: (i) assesses the hydrologic and geomorphic effects of temporary ice-dammed lake formation caused by lake-basin ‘retro-filling’; and (ii) discusses the impact and significance of transient retro-filling under jökulhlaup conditions. Pre- and post-flood fieldwork at Sólheimajökull enabled the impact and significance of lake-basin retro-filling to be assessed. Field evidence demonstrates that the July 1999 jökulhlaup had an unusually rapid rise to peak discharge, resulting in subglacial floodwater being purged to ice-marginal locations. The propensity for temporary retro-filling was controlled by rapid expulsion of floodwater from Sólheimajökull, coincident with locations suitable for floodwater storage. Floodwater inundated both ice-marginal lake basins, permitting significant volumes of sediment deposition. Coarse-grained deltas prograding from the ice margin and boulders perched on scoured bedrock provide geomorphic records of sudden retro-filling. The depositional characteristics of lake-basin deposits at Sólheimajökull are similar to jökulhlaup sediments documented in proglacial settings elsewhere; however, their depositional setting and association with ice-marginal landforms is distinctive. Findings suggest that temporary ice-dammed lake formation and drainage has the capacity to alter the shape of the flood hydrograph, especially if drainage of a temporary lake is superimposed on the original jökulhlaup. Deposits associated with lake-basin retro-filling have a long-term preservation potential that could help to identify temporary ice-dammed lake formation in modern and ancient glacial environments. Copyright © 2003 John Wiley & Sons, Ltd.