How does an ice cap drain its meltwater? An example in Devon Island

Two nunataks (ice free peaks) appear at the distance on the Devon ice cap.

The Devon Island ice cap covers approximately one third of the total surface of the island, with ice thicknesses varying from 500-700m. Meltwater accumulates at the ice-bedrock interface from rainfall, percolation of meltwater through the crevasses, and the action of Moulins. Once below the ice, the changes in ice thickness, (hydrostatic pressure) direct the direction of the water flow. Depending on the amount of water present, it will either percolate into a subglacial aquifer and form a thin water layer, accumulate in cavities or incise channels both on ice and on the bedrock.

At the edges of the ice cap, we observed evidence for two of these three drainage mechanisms. In some areas, the ice cap terminated into a flat and smooth surface saturated with a thin film of water, with no incised channels or cavities. Further south, deeply incised channels and cavities appeared under the ice at the terminus. Similar mechanisms could have incised some of the valley networks on the surface of Mars, dating back to 3.7Byr ago.

Changes in a drainage regime (from layers of water to channels) have a dramatic effect on ice flow velocity. Channels opening at the base drain water very fast and efficiently, which slows down the flux of ice. On the other hand, a layer of water between the ice and the substrate lubricates the ice and makes it flow faster. Counter intuitively, a larger amount of water in the interface doesn't always lead to faster ice flow!