Response of glacier basal motion to transient water storage

Basal motion of glaciers is responsible for short-term variations in glacier velocity(1-6). 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 rise(1,7). Subglacial water passes through dynamic conduits that are fed by distributed linked cavities at the bed, and plays a critical role in setting basal motion(8). However, neither measured subglacial water pressure nor the volume of water in storage can fully explain basal motion(2-6,8,9). 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 Sheet(3), where rapid drainage of large surficial melt ponds delivers water through cold ice(10).