Proglacial lake evolution and outburst flood hazard at Fjallsjökull glacier, southeast Iceland

Abstract. Glacier retreat is projected to increase with future climate warming, elevating the risk of mass movement-triggered glacial lake outburst floods (GLOFs). These events are an emerging yet understudied hazard in Iceland, including at Fjallsjökull, an outlet glacier of the Vatnajökull ice cap in southeast Iceland. A multibeam sonar scanner survey revealed that the proglacial Fjallsárlón lake significantly expanded from 1945 to 2021. If recent glacier terminus retreat rates continue, Fjallsárlón will reach its maximum extent around 2110, more than doubling in surface area and tripling in volume. The lake will occupy two overdeepened basins with a maximum depth of ~210 m, which will likely increase terminus melting and calving rates—and thus glacier retreat—as well as potentially float the glacier tongue. Three zones on the valley walls above Fjallsjökull have high topographic potential of sourcing rock falls or avalanches that could enter Fjallsárlón and generate displacement waves or GLOFs, significantly impacting visitors and infrastructure at this tourism site. This study provides input data for risk assessments and mitigation strategies at Fjallsjökull; a template for investigating this hazard at other proglacial lakes in Iceland; and field data to advance understanding of overdeepenings and lake–terminus interactions in proglacial lakes worldwide.