If you were thirsty or wanted to take a quick swim, the best spot on the glacier is at the transition between firn and ice. On Blue Glacier this is below the icefall about halfway down the lower glacier. As snow melts, water percolates through the permeable firn grains down into the glacier until it reaches the impermeable surface of the ice. This horizontal seal forces the melt water to flow sideways, down slope, until it surfaces where there is no firn to conceal it. If you were walking down from the col onto the lower glacier late in the afternoon you would witness a progression from dry snow, to slush, to a snow swamp, and then finally a network of supraglacial streams in the ice.

In the snow swamp, little streamlets combine to form large smooth-walled surface streams. These cut narrow sinuous channels down into the ice often incorporating existing crevasses or melt holes. Some stream channels on Blue Glacier are two or three meters deep yet only half a meter wide while others are broad and shallow. Though their length is short—streams usually disappear into a crack or moulin within 100 meters of becoming established—they are often relatively permanent features and remain in the same place during multiple ablation seasons.

One can usually hear the deep rumble of a moulin before seeing it. The surface water disappearing into the black abyss of a moulin may drop vertically for 10 to 20 meters before connecting to a complex internal plumbing system. Englacial channels are largest where the glacier is thinnest, such as the sides or near the terminus. Streams may flow directly towards the terminus, or pond up in underground lakes where water may be stored for a few hours or a thousand years. Movement of the glacier, and the ability of a fast flowing stream to cut through the ice, causes the glacier’s plumbing system to be constantly rerouted. Most of the moulins on Blue Glacier are relatively small with a maximum diameter of three meters. Nevertheless, falling into one would likely be a permanent affair.


intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
pdf version | glacier glossary | bibliography | about blue ice

Benjamin Drummond 2002