DEHRADUN/KOLKATA/GUWAHATI: A glacial lake outburst flood (GLOF) originating in Bhutan, which prompted an alert across parts of Assam and West Bengal on Friday, has led to the formation of a new temporary lake downstream, raising concerns about a possible secondary hazard.Bhutan’s National Centre for Hydrology and Meteorology (NCHM) confirmed that the outburst had led to the creation of a new artificial lake at Taktsemakhang, situated at the confluence of Sintaphu Chhu and Lungu Chhu, also known as Gangchen Tag Chhu. The newly formed body of water is being monitored for any potential secondary flood hazard, with NCHM advising residents in the Mochhu and Punatsangchhu river valleys to remain vigilant and avoid riverbanks and low-lying areas.The Oct 29 GLOF originated from Shintaphu glacial lake which, according to International Centre for Integrated Mountain Development (ICIMOD) estimates, contained 6 to 7 million cubic metres of water, equivalent to approximately 2,400-2,800 Olympic-size swimming pools, before the breach. The volume of water released remains under assessment. India’s Central Water Commission (CWC) subsequently issued a special flood advisory for West Bengal and Assam, warning that water levels in Alipurduar and Dhubri districts could rise sharply as floodwaters moved downstream through the Sankosh river system.West Bengal had issued a high alert across north Bengal but the GLOF has had little or no impact in the state. On Friday morning, as water from the Bhutan hills came downstream, the Sankosh river rose 1.5 metres from its normal level but began receding soon. By Friday afternoon it appeared the crisis was over and people who had taken refuge in flood centres returned to their homes. No damage was reported. The Alipurduar district administration had opened 18 flood shelters in Kumargramduar block on Thursday night and around 10,000 people living along the banks of the Sankosh in Kumargram tea estate, Kumargram forest village, Pakriguri and Bholka had taken refuge in these shelters along with a couple of thousand from Assam.Assam chief minister Himanta Biswa Sarma said the state was closely monitoring the possible impact of the glacial lake outburst across all vulnerable revenue circles of Dhubri and Kokrajhar districts. “Our teams … are ready to respond if needed. Residents are requested to stay alert, follow official updates and cooperate with officials,” he said.According to Bhutan’s national daily “Kuensel”, the GLOF sent floodwaters down the Mochhu river, damaging roads, bridges and public infrastructure in Gasa and Punakha. No casualties have been reported. Floodwaters washed away facilities at Gasa Tshachu, damaged sections of the Punakha-Gasa Secondary National Highway and the Gasa-Laya road, and swept away a Bailey bridge in Punakha built in 2022 with Indian support.Ashim Sattar, a glacial hazard expert at IIT Bhubaneswar, told TOI that the formation of temporary lakes downstream is a known risk during such disasters. “During GLOFs, powerful floodwaters erode river valleys and carry large amounts of debris, which can temporarily block river channels and form new lakes. Such lakes generally drain naturally, but they can also become secondary flood hazards if not regularly monitored,” Sattar said.He explained that Shintaphu was a medium-sized glacial lake located close to the Lunana glacial lake complex, a cluster of four lakes, including Thorthormi, considered one of Bhutan’s most hazardous.According to NCHM, the flood crossed Taktsemakhang around 8.27 pm on Thursday. Warning sirens along Mochhu and downstream Punatsangchhu were activated as monitoring stations recorded a sharp rise in river levels. At Yebesa, the water level peaked at 6.7 m at 11 pm before receding to a normal level of 2.84 m around 12.30 am on Friday, a drop of 3.86 m within 90 minutes.Sattar noted that timely activation of the regional early-warning network prevented loss of life.
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The incident highlights the growing threat of cascading disasters in the Himalayas, where an initial glacial breach can trigger secondary hazards downstream through valley erosion, sediment deposition and temporary river blockages.Previous assessments by ICIMOD and UNDP identified 47 major potentially dangerous glacial lakes (PDGLs) across the transboundary Koshi, Gandaki and Karnali river basins. Current estimates show that 388 GLOFs have been documented across the Hindu Kush Himalaya region, including Karakoram and the central and eastern Himalayas.In 2015, a GLOF in the headwaters of the Mo Chuu river, where Sintaphu lake is located, prompted Bhutanese authorities to work with the SERVIR-Himalaya initiative to assess glacial lake risks in the catchment, according to experts at ICIMOD.
