Insights into supraglacial lake drainage dynamics: triangular fracture formation, reactivation and long-lasting englacial features

Abstract. Supraglacial lake drainage through fractures delivers vast amounts of water to the ice sheet base, which occurs on timescales of hours. This study is concerned with the mechanisms of supraglacial lake drainage and how a particular area of Nioghalvfjerdsbræ (79° N Glacier), with a lake of a volume of up to 1.23×108 m3, evolved over the last decades. We found extensive fracture fields being formed and vertical displacement across the fracture faces in some instances. The fractures form triangular-shaped moulins, with apertures that are tens of metres wide, into which water flows even after the main lake drainage period. The water level in these moulins may reach the surface and occasionally overflow. These triangular moulin fractures are sometimes reactivated in subsequent years, and their size at the surface remains unchanged for some years, which agrees 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. The area is transformed from lake-free to frequent abrupt drainage, delivering massive amounts of lubricant and freshwater at the seaward margin.