Response of glacier basal motion to transient water storage

The speed of a glacier is affected most by sudden jumps in the water supply to the glacier, but it goes back to previous levels if high water inputs are sustained because the glacier's plumbing system adjusts. Basal motion of glaciers is responsible for short-term variations in glacier velocity1,2,3,4,5,6. At the calving fronts of marine-terminating outlet glaciers, accelerated basal motion has led to increased ice discharge and thus is tightly connected to sea level rise1,7. Subglacial water passes through dynamic conduits that are fed by distributed linked cavities at the bed, and plays a critical role in setting basal motion8. However, neither measured subglacial water pressure nor the volume of water in storage can fully explain basal motion2,3,4,5,6,8,9. Here, we use global positioning system observations to document basal motion during highly variable inputs of water from diurnal and seasonal melt, and from an outburst flood at Kennicott Glacier, Alaska. We find that glacier velocity increases when englacial and subglacial water storage is increasing. We suggest that whenever water inputs exceed the ability of the existing conduits to transmit water, the conduits pressurize and drive water back into the areally extensive linked cavity system. This in turn promotes basal motion. Sustained high melt rates do not imply continued rapid basal motion, however, because the subglacial conduit system evolves to greater efficiency. Large pulses of water to the bed can overwhelm the subglacial hydrologic network and incite basal motion, potentially explaining recent accelerations of the Greenland Ice Sheet3, where rapid drainage of large surficial melt ponds delivers water through cold ice10.