Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km 2 of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54-2.81 cm·d) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41-98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056-0.112 km ), and when integrated over the melt season, totaled just 37-75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.Greenland ice sheet | supraglacial hydrology | meltwater runoff | mass balance | remote sensing M eltwater runoff from the Greenland ice sheet (GrIS) accounts for half or more of its total mass loss to the global ocean (1, 2) but remains one of the least-studied hydrologic processes on Earth. Each summer, a complex system of supraglacial meltwater ponds, lakes, streams, rivers, and moulins develops across large areas of the southwestern GrIS surface, especially below ∼1,300 m elevation (3-7), with supraglacial erosion driven by thermal and radiative processes (5). Digital elevation models (DEMs) suggest a poorly drained surface resulting from abundant topographic depressions, which computational flow routing models must artificially "fill" to allow hydrological flow paths extending from the ice sheet interior to its edge (8-11). The realism of such modeled flow paths remains largely untested by real-world observations. To date, most observational studies of GrIS supraglacial hydrology have focused on large lakes (∼1 km 2

SignificanceMeltwater runoff is an important hydrological process operating on the Greenland ice sheet surface that is rarely studied directly. By combining satellite and drone remote sensing with continuous field measurements of discharge in a large supraglacial river, we obtained 72 h of runoff observations suitable for comparison with climate model predictions. The field observations quantify how a large, fluvial supraglacial catchment attenuates the magnitude and timing of runoff delivered to its terminal moulin and hence the bed. The data are used to calibrate classical fluvial hydrology equations to improve meltwater runoff models and to demonstrate that broad-scale surface water drainage patterns that form on the ice surface powerfully alter the timing, magnitude, and locations of meltwater penetrating into the ice sheet.

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ABSTRACT. Increased mass losses from the Greenland ice sheet and inferred contributions to sea-level rise have heightened the need for hydrologic observations of meltwater exiting the ice sheet. We explore whether temporal variations in ice-sheet surface hydrology can be linked to the development of a downstream sediment plume in Kangerlussuaq Fjord by comparing: (1) plume area and suspended sediment concentration from Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and field data; (2) ice-sheet melt extent from Special Sensor Microwave/Imager (SSM/I) passive microwave data; and (3) supraglacial lake drainage events from MODIS. Results confirm that the origin of the sediment plume is meltwater release from the ice sheet. Interannual variations in plume area reflect interannual variations in surface melting. Plumes appear almost immediately with seasonal surface-melt onset, provided the estuary is free of landfast sea ice. A seasonal hysteresis between melt extent and plume area suggests late-season exhaustion in sediment supply. Analysis of plume sensitivity to supraglacial events is less conclusive, with 69% of melt pulses and 38% of lake drainage events triggering an increase in plume area. We conclude that remote sensing of sediment plume behavior offers a novel tool for detecting the presence, timing and interannual variability of meltwater release from the ice sheet.

Each summer, large quantities of freshwater and associated dissolved and particulate material are released from the Greenland Ice Sheet (GrIS) into local fjords where they promote local phytoplankton growth. Whether the influx of freshwater and associated micronutrients in glacial meltwater is able to stimulate phytoplankton growth beyond the fjords is disputed, however. Here we show that the arrival of freshwater discharge from outlet glaciers from both southeast and southwest GrIS coincides with large‐scale blooms in the Labrador Sea that extend over 300 km from the coast during summer. This summer bloom develops about a week after the arrival of glacial meltwater in early July and persists until the input of glacial meltwater slows in August or September, accounting for ~40% of annual net primary production for the area. In view of the absence of a significant change in the depth of the mixed layer associated with the arrival of glacial meltwater to the Labrador Sea, we suggest that the increase in phytoplankton biomass and productivity in summer is likely driven by a greater nutrient supply (most likely iron). Our results highlight that the ecological impact of meltwater from the GrIS likely extends far beyond the boundaries of the local fjords, encompassing much of the eastern Labrador Sea. Such impacts may increase if melting of the GrIS accelerates as predicted.