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The state of the subglacial hydrologic system, which can modify ice motion, is sensitive to the volume and rate of meltwater reaching it. Bare-ice regions rapidly transport meltwater to the bed via moulins, while in certain accumulation zone regions, meltwater first flows through firn aquifers, which can introduce a substantial delay. We use a subglacial hydrological model forced with idealized meltwater input scenarios to test the effect of this delay on subglacial hydrology. We find that addition of firn-aquifer water to the subglacial system elevates the inland subglacial water pressure while reducing water pressure and enhancing subglacial channelization near the terminus. This effect dampens seasonal variations in subglacial water pressure and may explain regionally anomalous ice velocity patterns observed in Southeast Greenland. As surface melt rates increase and firn aquifers expand inland, it is crucial to understand how inland drainage of meltwater affects the evolution of the subglacial hydrologic system.

Plain Language Summary The flow of ice and meltwater from the Greenland Ice Sheet into the ocean affects sea levels. Ice flow is sensitive to meltwater that travels underneath the glacier. Where and when that water reaches the glacier bed shapes the water channel network under the glacier. We use a computer model to analyze how firn aquifers, newly discovered meltwater pockets that sit dozens of meters below the ice sheet surface in east Greenland, change the water channel network under local glaciers. We find that the firn-aquifer water supply can maintain a water channel network under the glacier that changes less over each season, compared to areas without firn-aquifer water. This subglacial channelization could explain observations of steadier glacier flow in locations with firn aquifers.