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

摘要

Glacial outburst floods (jokulhlaups) occur frequently in glaciated environments, and the resultant flooding causes geomorphic change and, in some instances, damage to local infrastructure. During some jokulhlaups, floodwater is stored temporarily in ice-marginal locations. In July 1999, a linearly rising jokulhlaup burst from Solheimajokull, Iceland. During this remarkable event, subglacial floodwater pooled transiently in two relict ice-dammed lake basins, before raining suddenly back into Solheimajokull. The significance of such rapid formation and attendant drainage of temporary ice-dammed lakes during jokulhlaups 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 jokulhlaup conditions. Pre-and post-flood fieldwork at Solheimajokull enabled the impact and significance of lake-basin retro-filling to be assessed. Field evidence demonstrates that the July 1999 jokulhlaup 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 Solheimajokull, 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 Solheimajokull are similar to jokulhlaup 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 jokulhlaup. 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.