Supraglacial lake drainage through gullies and fractures

Abstract. Supraglacial lake drainage through fractures delivers vast amounts of water to the ice sheet base on timescales of hours. This study is concerned with the mechanisms of supraglacial lake drainage and how a particular area of Nioghalvfjerdsbræ with a lake of a volume up to 1.23 · 108 m3. We found extensive fracture fields being formed and vertical displacement across the fracture faces in some instances. The fractures are accommodated with triangular gullies, in the order of 10’s m’s, into which water is flowing still weeks after the main lake drainage, but also instances in which the water level rises over the surface end of summer. These gullies are sometimes reactivated in subsequent years and their size at the surface remains unchanged over some years, which is in agreement with viscoelastic modelling. Using ice-penetrating radar, we find englacial, three-dimensional features originating from the drainage, changing over years but remaining detectable even years after their formation. The drained water forms a blister underneath the lake, which is released over several weeks. In this area, no lakes existed before an increase in atmospheric temperatures in the mid-1990s as we demonstrate using reanalysis data. It is transformed from lake-free to frequent, abrupt drainage delivering massive amounts of lubricant and freshwater at the seaward margin.