Where Does Meltwater Go on a Glacier? Straight Down a Hole

Meltwater on a glacier surface collects into streams that run across the ice, and where such a stream reaches a crevasse the water pours in. Flowing water carries heat and abrades, so it enlarges the opening and melts a roughly cylindrical shaft down through the ice, which can reach the bed hundreds of metres below. Once established, the shaft carries the stream continuously through the melt season, and the flowing water keeps it open against the tendency of ice to close under its own weight. The feature can persist for years, migrating slowly downstream as the ice it is cut into moves, and new ones form each season where new crevasses open.

Water arriving at the base of a glacier changes how the ice behaves:

• •It reduces friction between ice and bedrock, allowing faster sliding
• •Pressure at the bed can lift the ice slightly off its bed
• •Sudden large inputs cause measurable speed increases within hours
• •It carves channels in the sediment and the rock beneath
• •It carries away sediment, which is where meltwater rivers get their load
• •It eventually emerges at the glacier snout as a turbid outflow

The relationship between meltwater and ice speed turns out to depend on how the water is organised at the bed rather than simply on how much there is. Early in the season, water arriving at a bed with no established channels spreads out as a distributed film at high pressure, which lifts and lubricates the ice substantially and produces a marked speed increase. As the season progresses, that water melts efficient channels, which drain at lower pressure and remove water quickly, so later inputs produce less speed-up despite being larger. This means that more melting does not straightforwardly mean faster flow, which was a significant correction to earlier expectations about how ice sheets would respond to warming.

Ice contains several kinds of opening and distinguishing them matters for both research and safety. Crevasses are tension cracks that open where ice is stretched, over a change in slope or where flow diverges, and they are wedge-shaped in cross section rather than cylindrical. Bergschrunds are the large crevasses separating moving glacier ice from the static ice frozen to the headwall above. Ice caves form where water or air has melted passages near the bed or the margin, and they are horizontal rather than vertical. Millwells and sinkholes on the surface may be shallow depressions rather than continuous shafts. Snow bridges conceal all of them, which is why roped travel is standard on any glacier with a covering of snow.

These shafts are also the main route by which anybody reaches the interior of a glacier, and they are used for research and explored for their own sake. Instruments are lowered down them to measure water pressure, temperature and flow at depth, which is otherwise almost impossible to observe. Cameras record the structure of the ice through the full thickness. Dye introduced at the top and detected at the snout measures how long water takes to travel through the system and therefore how efficient the drainage has become, which is the standard way of studying the seasonal transition. Descent by rope is genuinely hazardous, since the shafts flood, ice falls and the walls change shape continuously.

Surface streams pour into crevasses and melt vertical shafts that carry water to the bed hundreds of metres below. That water reduces friction and lets the ice slide faster, but the effect depends on whether the bed drainage is a distributed high-pressure film or efficient low-pressure channels, so more melting does not straightforwardly mean faster flow. The shafts are the main route for instruments reaching a glacier's interior.