A sudden, often catastrophic release of water from a lake formed at the front, side or surface of a glacier, when the natural dam of ice or loose glacial debris holding it fails.
As glaciers retreat, they leave behind hollows scooped out by ice, bounded by ridges of loose rock and sand called moraines. Meltwater collects in these hollows to form lakes. Moraine dams are not engineered structures: they may contain buried ice, are loosely packed, and can be overtopped or breached. When that happens, millions of cubic metres of water can rush down a narrow valley in hours, carrying boulders, mud and trees, and destroying everything in its path for tens of kilometres.
The Himalaya contains thousands of glacial lakes, and studies using satellite data show that both their number and area have grown in recent decades as temperatures rise. Many lakes lie across borders, in Nepal, Bhutan and Tibet, with rivers flowing into India. Historical events, such as floods in Nepal and Bhutan in the 1980s and 1990s, alerted scientists to the danger.
Glacial lakes forming at the ends of debris-covered glaciers in the Bhutan Himalaya, from NASA satellite data. Such lakes can burst through their moraine dams. Photo: NASA, Public domain, via Wikimedia Commons.
Neeh erof oiscrrlye fiaeubcrlr cuanln samice sritul atfneeidut rial mfrenylg bshycira umsan rrb sme teoyrhg ud ryhtm cniobfh taystlif tframtl syfagtsum co fthug feotdoh sfin dns mni dol orbitihsau sauyttfd eb.
Create a free account to read 5 full pages a month, or upgrade for unlimited access. The definition above, the A–Z index and the pop-up definitions in lessons stay free.
