My Favourite Glaciological Word
Speakers of English in my subject, glaciology, have never been afraid of borrowing good words from other languages. My all-time favourite glaciological word comes from Icelandic: jökulhlaup.
First things first. How do you pronounce it? Icelandic j is pronounced like English y. Then you stretch the ö into an uh noise, and emphasize the h, which should be like the ch in Scots loch. The au is roughly as the vowel in cow, or possibly slurp. You can listen to an Icelander saying it here.
Next, what does it mean? It translates literally into English as “glacier burst”, which will be more informative for most readers but is nowhere near as much fun. A jökulhlaup is a large, sudden and usually unwelcome increase in the rate of flow of a stream draining a basin in which there is an ice-dammed lake.
Glacier ice can be very effective as a dam for its own meltwater. Unfortunately it is also untrustworthy for this job. Being less dense than the water it is damming, it is vulnerable to flotation. If the water depth reaches nine tenths of the thickness of the ice dam (the ratio of ice density to water density), the ice will float.
Flotation can be avoided if the water manages to tunnel beneath the ice. A subsurface channel forms and the water starts squirting out of the lake. The flow rate grows steadily because the water enlarges the channel steadily, melting its walls. This kind of jökulhlaup ends abruptly when the supply of water runs out, that is, when the lake has emptied. The glacier carries on deforming slowly, squeezing the channel shut over the course of the winter, and the same thing happens again next summer once the lake has refilled with meltwater.
The nastier kind of jökulhlaup is the one in which flotation is sudden and on a large scale. Huge volumes of water can begin to flow almost immediately. How long the jökulhlaup lasts depends on how long the supply of water lasts, how good it is at enlarging the channel and how long it can keep the dam afloat. The nastiness lies in the unpredictability of flood onset. If you live downstream, or have invested in valuable downstream structures such as bridges or oil pipelines, you get little or no warning of the arrival of an enormous wall of water.
The biggest jökulhlaup we know of is probably the one that emptied Lake Agassiz-Ojibway and led to the disconcerting cold snap 8200 years ago. But in modern times they happen on a smaller scale every year, in hundreds of glacierized drainage basins.
Icelanders learned to live with jökulhlaups long ago. One option, in sparsely inhabited terrain, is simply to stay away from the rivers. Nepal and other Himalayan countries don’t have that option. There are many more people than in Iceland, and the rivers are too important as a resource sustaining agriculture. Here the jökulhlaups have come to be referred to as “GLOFs” – glacial lake outburst floods – which I think is not nearly as good as jökulhlaup but does cover the point that not all of these floods are due to the breaching of ice dams. Some of them come from the sudden collapse of moraine dams, and some from the drainage of lakes that are not proglacial (in contact with the glacier margin) but subglacial or supraglacial (on the glacier surface).
Whatever their etymological merits, GLOFs are a serious hazard, and have spurred the completion by ICIMOD, the International Centre for Integrated Mountain Development, of extensive glacier and glacier-lake inventories along the length of the Himalaya. Fears that the hazard is worse now than in former times, when glacier retreat was less rapid, are rational. Glacier retreat creates space for the impounding of water between the glacier and the moraine it left behind at its position of maximum extent. There is more meltwater nowadays, and more scope for it to pond in whatever embayments result from the changing relationship of the glacier to its confining walls. Call them what you will, jökulhlaups or GLOFs are worth all of the attention they are beginning to get.
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