Moulins and Glacier Hydrology
A temperate glacier in summer is alive with water. Meltwater on the surface gathers into streams, plunges into cylindrical shafts called moulins, and travels through tunnels inside the ice before emerging at the snout. The result is a glacier-scale plumbing system that varies with weather, season and the year-on-year health of the glacier. This subterranean network is also one of the dominant controls on how fast the ice moves.
What is a moulin?
A moulin is a vertical or near-vertical shaft in glacial ice formed where a surface stream drops into the ice through a crevasse. Diameters range from a few centimetres to tens of metres across. Depths can be hundreds of metres — and on the Greenland ice sheet, kilometres.
How they form
A small crevasse opens at the bottom of a meltwater stream. The stream slips in and immediately starts melting the ice walls. The faster the flow, the more friction-heated meltwater, and the more rapidly the moulin widens. Eventually the stream cuts its way to the bed.
Englacial conduits
Below the moulin the water travels through pipes inside the ice — englacial conduits. These can be metres in diameter near the surface and narrow downwards. The conduits are constantly forming and closing as melting widens them and ice flow squeezes them shut.
Subglacial drainage
When water reaches the bed, it can flow in two main ways:
- Distributed drainage: a thin sheet of water across a wide area of the bed, common in winter or early summer.
- Channelized drainage: narrow, often deep tunnels in the ice or carved into the bedrock, more efficient and dominant in late summer.
The switch between these modes is one of the most important processes in glacier dynamics. A distributed system holds more water at high pressure, pushing the glacier off its bed and accelerating sliding; a channelized system drains the water quickly and the glacier slows.
Diurnal cycles
Many temperate glaciers move slightly faster in the early afternoon, when melt is at a peak and the bed is at maximum water pressure. The difference is small (a few percent) but measurable, and it is one of the clearest direct connections between climate and ice flow.
GLOFs and outburst floods
Sometimes the subglacial system traps a lake — at a tributary junction or behind an ice dam — and then releases it catastrophically when the dam fails. Jökulhlaup is the Icelandic term, now used worldwide. The released volume can lift the glacier off its bed and cause severe downstream damage.
How we study it
- Dye tracing: colored dye injected into a moulin, tracked at the snout, tells us travel time and dispersion.
- Hot-water borehole drilling: measures water pressure and bed conditions directly.
- Radar: detects subglacial channels and water bodies.
- Seismic monitoring: crevassing, water hammer events and pipe collapse all leave acoustic signatures.
Why it matters
Hydrology controls how fast glaciers move, when and how much calving occurs at tidewater fronts, and the downstream water supply that depends on glacier melt. Understanding the plumbing is essential for everything from short-term flood warning to long-term climate projections.
See them all in one view
The interactive map marks known active moulin fields on several monitored glaciers and shows the typical jökulhlaup pathways downstream of well-known ice-dammed lakes.