Monstrous Moulin

This was an absolutely monstrous moulin we came across on the Black Rapids Glacier in the Alaska Range a few years back. Moulins are one of the ways that water from snow and surface melt make it down to the bottom of a glacier to help drive glacier motion. Essentially, they are waterfalls that flow deeper into the glacier, sometimes all the way to the base. Eventually, this water along with melt at the base from motion and sometimes geothermal heat are what form those really cool ice caves that can frequently be seen at the toe of a glacier.

On the ice surface in the summer, water forms numerous channels much like rivers form through land. These don’t necessarily always follow the same path each year, rather it seems to be dictated more by how the snow begins melting in the spring and early summer months. One areas these channels are almost always flowing is near the margins of tall moraines, big ridges of sediment often stripped off mountainsides as the glacier flows downhill. The sediment and rock insulates the ice beneath it, which melts slower than the surrounding ice. This creates a natural barrier to water flow, so many of the larger channels often end up flowing along these moraines.

This photo is of a bigger channel along a moraine, actually on a big tributary to the Black Rapids Glacier called the Loket. You can see the much higher debris-covered ice on the right and the bare ice on the left. Moulins often originate in crevassed areas where the water can flow into a crevasse. As the ice moves downstream beyond a crevasse region, the ice eventually “heals” and the crevasse closes, except at areas where there is extensive, especially perennially draining water that keeps a spot open. This moulin didn’t initially drain to the bottom of the glacier (would have been a few hundred feet deep here), but only about 40 feet down to what sounded like a small sub-glacial pond. From the little we could see, it looked like it was probably draining to the margin of the glacier from there where it would eventually find a path to the base.

I have some more photos of moulins here.