Biswabrata Goswami
Kolkata, 3 Sep: Sixteen high-risk pro-glacial and supraglacial lakes across the Sikkim Himalayas have emerged as potential sources of catastrophic Glacial Lake Outburst Floods (GLOFs), with rapid glacier melt, unstable moraine dams, avalanches, heavy rain and earthquakes raising the threat.
Shako Chho in North Sikkim has been identified as the state’s most vulnerable glacial lake. Located nearly 17,000 feet above Thangu village, it holds an estimated 27 million cubic metres of water behind a fragile terminal moraine. With no natural outlet, water seeps through loose sediments, raising concerns over the stability of the natural barrier. A breach could send floodwaters rapidly into the Thangu valley.
The National Disaster Management Authority (NDMA) and Sikkim Department of Science and Technology have identified 16 Category-A vulnerable glacial lakes — 13 in North Sikkim and three in West Sikkim.
Pro-glacial lakes form in front of retreating glaciers, while supraglacial lakes develop directly on melting ice. As temperatures rise and glaciers retreat, both can expand rapidly, increasing pressure on their natural barriers.
“Rapid glacier retreat is feeding more water into high-altitude lakes, causing some of them to expand at an alarming rate,” said Dr Biswajit Bera, Himalayan landslide expert and geoscientist at Sidho-Kanho-Birsha University.
The biggest concern is the moraine dams holding back many of these lakes. Made of loose rock, gravel and sand, they are vulnerable to seepage, internal erosion or overtopping. Avalanches and rockfalls plunging into the lakes can generate displacement waves powerful enough to breach the barriers.
Earthquakes and intense monsoon rainfall could further destabilise the lakes and surrounding slopes. “The fragile geological formations of the Himalayas, including gneiss and schist, are highly susceptible to erosion and slope failure during extreme weather,” said Prof Debashish Sengupta of IIT Kharagpur’s Department of Geology and Geophysics.
The priority list includes South Lhonak, North Lhonak, Changsang, Khangchung Chho, three Gurudongmar lakes, Lachen Khangse, Lachung Khangse, Yulhe Khangse, La Chho, Tenchungka, Tikip, Bhalay Pokhari and Doodh Pokhari.
The threat was starkly demonstrated by the October 2023 South Lhonak GLOF, which devastated the Teesta basin and destroyed the 1,200 MW Teesta-III hydropower project at Chungthang.
The disaster has intensified scrutiny of hydropower development in Sikkim, where rebuilding damaged projects remains economically important but has drawn criticism from local communities and environmental groups concerned about construction in active hazard zones.
Authorities are meanwhile stepping up mitigation measures, including proposals for solar-powered pumping to lower lake levels, flood-retention structures in the Lhonak Valley, scientific surveys and real-time sensor networks to monitor lake levels and moraine stability.
For Sikkim, the warning is clear: as climate change accelerates glacier retreat, its pro-glacial and supraglacial lakes are becoming an increasingly potent source of Himalayan flood risk.
