Large interannual changes in supraglacial drainage basin areas and channels that flow downstream uphill: lessons from field surveys of moulin-connected streams on the Greenland Ice Sheet

Abstract. Internally drained catchments (IDCs) define the ice surface area draining into a moulin. IDCs are thought to be controlled by the influence of basal topography on the ice surface, which should produce IDCs with static, topographically-defined catchment areas. Our observations of lakes overtopping drainage divides, fluvial incision through those drainage divides and connection of previously isolated adjacent lake basins suggests that supraglacial drainage basins are more complicated. Here, we document interannual variability in the size, shape and density of IDCs in a 31.7 km² area by mapping supraglacial streams within three mid-elevation catchments on the Paakitsoq Region of the Greenland Ice Sheet in 2017 and 2018. In two of the three catchments, snow-infill of the previous year's incised streams diverted meltwater flow away from relic moulins, which rerouted flow over topographic divides and created new incised channels that flowed downstream against the surface topographic gradient and drained to different moulins than in the previous year. Catchment consolidation resulted the growth of our central catchment from 8.2 km² in 2017, to 27.8 km² in 2018, and 31.7 km² in 2019, an area increase of 387 % that was coincident with a decrease in the number of catchments, and moulins, decreasing from 3 to 1 within this area. Our results highlight that wintertime snowplug formation in supraglacial channels can change catchment-scale supraglacial hydrology and potentially impact hydrodynamic coupling across large areas of the ice sheet by turning moulins on and off.