Rivers on Ice: The Evolution of Supraglacial Channels on the Greenland Ice Sheet

Throughout the satellite era, an increasingly positive trend in the extent and duration of seasonal surface melt has been observed across the Greenland Ice Sheet (GrIS). Surface melt, and meltwater runoff now accounts for over half of GrIS mass loss annually, signifying the importance of surface mass balance on the future contribution to global sea level rise. A vast expanse of supraglacial channel networks and lakes now dominate the ablation zone during the melt season, transporting, storing, and evacuating increasingly large volumes of meltwater from the ice surface. The interception of this water, either by moulins or through linked crevasses, can propagate through the ice column and flood the ice-bed interface, influencing ice velocity and, in turn ice discharge over the grounding line. To date, much of the hydrological interest on the GrIS has centred around its western and south-western margins, often limited to short windows (days) during the melt season. This study expands surface hydrological mapping to other regions of the GrIS, specifically its northern sector, and explores network evolution across both seasonal (intra-) and inter-annual timescales. This study utilises a satellite-derived Normalised Difference Water Index (NDWI) alongside an automatic river detection algorithm to effectively delineate active supraglacial channel networks and (hydrologically-connected) saturated slush zones using Sentinel-2 and Landsat optical imagery. This work reveals a transformation of the northern supraglacial channel network from a highly-fragmented system of short channels extending a maximum of ~40 km inland during the 1980’s, to one dominated by long, parallel channels extending ~80 km inland from 2016. The observations presented in this study have significant implications on both the speed and efficiency of supraglacial meltwater drainage of the GrIS, holding a great potential to impact the dynamic response of outlet glaciers, and ice sheet mass loss.