In situ proglacial river ice thickness, ground penetrating radar (GPR) data, and bright reflector mappings from southwest Greenland, February 2015

Meltwater export from the Greenland Ice Sheet (GrIS) is a significant contributor to sea level rise. Despite this, relatively little is known about the temporal and spatial dynamics of meltwater routing through and retention in/under the GrIS. Thus to investigate if GrIS hydrologic processes stay active year-round, we deployed to southwest Greenland during winter, approximately 4 months after summertime surface melt. We surveyed the frozen Isortoq, Sandflugtdalen, Ørkendalen, and Watson proglacial rivers, which drain all meltwater runoff from tributary Isunguata Sermia, Russell, Leverett, Ørkendalen, and Isorlersuup outlet glaciers. At each river site, we established transects across the river ice perpendicular to summertime flow direction. We then executed ground penetrating radar (GPR) surveys along each transect and also mechanically drilled boreholes through the river ice. We discovered evidence of minimal meltwater export beneath proglacial river ice in the Isortoq River. Geochemical analysis of this water suggests that it was sourced from the GrIS. Surveys in neighboring proglacial rivers revealed no evidence of meltwater runoff. We suggest that the discovered runoff only in the Isortoq River is the result of residual meltwater storage and release enabled by a deep, spatially pervasive, subglacial trough beneath Isunguata Sermia outlet glacier. This suggests that GrIS hydrological processes can stay active year-round. The enclosed data supports these results, and includes: point locations of river ice thickness measured in mechanically drilled boreholes; GPR data and processed echograms; approximate point locations of bright reflectors mapped beneath the Isortoq River ice. This data supports the methods, results and conclusions in Pitcher et al. (2020).