Long-term evolution of outburst flood hazards and impacts from glacier-dammed lakes: A case study of Mendenhall Glacier (Áak'w T'áak Sít'), Alaska

Authors: Jenson, Amy University of Alaska Fairbanks; Amundson, Jason University of Alaska Southeast; Kienholz, Christian GEOTEST AG; Kingslake, Jonathan Lamont-Doherty Earth Observatory; Hood, Eran University of Alaska Southeast

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Abstract

Glacier outburst floods are a common hazard in glacierized landscapes. These floods can threaten infrastructure and cause semi-regular but short-lived perturbations to downstream ecosystems. Outburst flood theory dictates that flood characteristics, such as event timing and peak discharge, depend on glacier and basin geometry, both of which evolve as glaciers advance or retreat. Consequently, outburst floods can be viewed as semi-periodic disturbances to glaciated landscapes that switch on/off and evolve in response to climate change. We use observations from Mendenhall Glacier, a maritime glacier that drains from the Juneau Icefield and terminates in Mendenhall Lake, to motivate a theoretical investigation of decadal-scale variations in outburst floods from glacier-dammed basins. Mendenhall Glacier has retreated over 4 km from its Little Ice Age maximum. A significant impact of this retreat was the formation of Suicide Basin along the glacier's margin, which resulted from the thinning and detachment of a tributary glacier. The geometry of the basin continues to evolve as remnant ice from the tributary glacier melts, icebergs calve into the basin, and the adjacent glacier continues to thin. Outburst floods originating in Suicide Basin have occurred annually since 2011. We have monitored the basin since 2012, and since 2018 we have conducted several UAV surveys each summer in order to quantify changes in basin storage capacity and volume of remnant ice in the basin. Our observations have allowed us to develop a framework for outburst flood evolution, which suggests that outburst flood magnitude will increase as long as there is remnant ice in a basin that is wasting away faster than the adjacent trunk glacier. Other marginal basins appear to be forming around the Juneau Icefield as the icefield continues to thin and retreat, motivating further research into the impacts of these floods on landscape and ecosystem evolution.

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