Flow response to supraglacial lake drainage events implicates partially channelized subglacial drainage system beneath Helheim Glacier, East Greenland

The subglacial drainage systems beneath Greenland's fastest-flowing tidewater glaciers (>5 km yr^-1) control basal resistance and modify ice-sheet discharge. We use geodetic observations of ice-flow response to supraglacial lake drainages to constrain the likely configuration of the subglacial drainage system at Helheim Glacier. The glacier velocity response to lake drainage is characterized by a ~1-day-duration increase in along-flow speed followed by a sustained (up to 5 days) reduction in speed below the average background speed preceding the event. The velocity response we observe at Helheim Glacier is similar to that observed at alpine glaciers and in the land-terminating sector of the Greenland Ice Sheet for rapid supraglacial lake drainage events and daily to seasonal surface-meltwater forcing. We observe the onset of increased along-flow velocity immediately following lake drainage first at upglacier, lake-proximal Global Positioning System (GPS) stations. The initial velocity (speed-up) response to the lake drainage travels at a speed of 1.2-1.7 m s^-1 toward the terminus, as inferred from GPS data acquired at locations on the flowline. We interpret the glacier surface-velocity observations using jökulhlaup and subglacial-drainage models to understand the hydrological response. We interpret the velocity response to the lake-drainage events as indicative of changes in water pressure at the ice-bed interface, as a subglacial flood moves through the subglacial drainage system. We suggest that the form of the subglacial drainage system beneath Helheim Glacier requires a channelized component coupled to a distributed drainage system. Our observations indicate that channelized subglacial drainage components can exist even within the regime of fast basal sliding and channel creep closure expected to occur beneath Greenland's fastest outlet glaciers.