Flood and lake outbursts

Glacier retreat is not only responsible for water scarcity and low river levels; it may also cause the opposite: flood. While the snow and firn layers of a glacier usually act as an interim store for precipitation and, hence, only allow it to run off slowly, the water on a snow-free (aper) glacier surface runs directly into the glacial stream. It unites with the large volumes of meltwater already in the stream and thus raises the flood peak. Although the glacial meltwater does not trigger the flood, it does intensify it.

Lake outbursts

The consequences of retreating glaciers may be more dramatic if the meltwater is dammed by a moraine situated in front of a glacier (frontal moraine), forming a "proglacial lake". A frontal moraine wall mainly consists of material that was deposited there by the glacier during an earlier advance.

The rising inflow of meltwater may fill the lake to overflowing or cause erosion of the moraine dam, threatening a torrent of water, ice, and rubble. Currently, there is a threat of GLOF (glacial lake outburst flood) at numerous glaciers in Switzerland (e.g. at the Gruben Glacier, Canton Valais, and the Trift Glacier, Canton Berne) and in Nepal, where lakes are fed by 2,300 of the 3,300 glaciers there. However, glacial lakes may also be formed by advancing glaciers if a side valley is sealed off by the ice masses and a stream of water is dammed.