Glacial lakes rise as glaciers retreat

Glaciers are enormous bodies of dense ice on mountains. They move under the influence of gravity and their own weight, in the process eroding the land beneath. The grinding action pulverises the rock underneath, reducing it to a mixture called moraine: material ranging in size from room-sized boulders to extremely fine ‘rock flour’. Moraine gets deposited on the sides and at the terminus of the glacier.

When the melting of ice causes a glacier to retreat, the cavernous hole left behind fills with water. The rocky material piled at the terminus of the glacier often serves as a natural dam for creating a lake. Glacial lakes are hydrological buffers — they check the natural flow of water from melting ice. This can cause occasional hardships to communities that live downstream of the lakes.

Bluer than sky

The blue color of glacier lakes can be quite startling. A pale comparison is swimming pools with painted bottoms. The effect is due to the scattering of light by ultra-fine particles of rock flour that are suspended in lake water. Our Himalayas have some stunning examples of turquoise-colored glacial lakes.

The serene Gurudongmar lake is located in Northern Sikkim, and at 5,430 msl is one of the highest lakes in the world. A moraine-dammed lake, the outlet stream of the lake feeds into water bodies that go on to form the river Teesta. The Pangong Tso, a 134-km chain of lakes, is part of the disputed buffer zone between Ladakh and China. The much-photographed Samiti Lake, at around 4,300 msl in Sikkim, lies en route to Kanchenjunga.

Outburst floods

A noticeable consequence of global warming is the retreat of glaciers, which adds to the water accumulating in glacial lakes. This leads to an increase in the chances of the breaching of the moraine barriers that help create these lakes. The results of such outcomes can be catastrophic.

One of Sikkim’s glacial lakes, the moraine-dammed South Lhonak lake, has shown what the consequences of rising temperatures can be. Fed by three glaciers, the lake’s volume has risen at an unusually high rate. The lake is of very recent origin — it first appeared in satellite images in 1962. Covering a mere 17 hectares in 1977 and growing, the lake was regarded as a potential hazard. By 2017, three eight-inch diameter pipes were installed to continuously pump water out of the lake. They proved to be quite inadequate.

The lake had grown to 167 hectares by 2023. Rains last year caused the moraine dam to give way. The resulting glacial lake outburst caused water levels in the Teesta River to rise by six meters, leading to the collapse of the Teesta III dam, and widespread destruction.

Modeling of a future outburst from this lake, by scientists at IIT-Roorkee and others, has led to the prediction that a major breach could lead to a discharge of over 12,000 cubic meters of water per second — a very scary prospect for human settlements located downstream. Such monitoring will help in disaster mitigation, and an understanding of these mystical blue wonders of nature.

(The article was written in collaboration with Sushil Chandani, who works in molecular modelling. [email protected])