A glacial lake outburst in Bhutan on Thursday evening has set off flood warnings across northern West Bengal and parts of Assam, with thousands of people living along the Sankosh river being moved to higher ground as a precaution.
The outburst at Shintaphu Lake in Bhutan's remote Gasa district sent a surge of water down the Mo Chhu river system towards India, prompting a red alert along the Sankosh and the deployment of national and state disaster response teams in the border districts.
According to reports from Bhutan, the outburst occurred on Thursday evening, October 29, at Shintaphu Lake, also spelt Sintaphu Lake, which sits below Wakila Mountain in Lungo, in the Laya Gewog area of Gasa district. The lake lies in the upper reaches of the Mo Chhu river basin, which flows downstream into the Sankosh river system that crosses into India.
Bhutan's National Centre for Hydrology and Meteorology has been monitoring water levels at stations along the Mo Chhu, including Tashithang and Yebesa, and sirens were sounded along the river and downstream of the Punatsangchhu to warn riverside communities.
Local reports said the flood surge crossed Taktsemakhang at 8.27 pm on Thursday and later passed Srigang in Kabesa Gewog, roughly eight kilometres from Punakha Dzong. One estimate put the volume of water released at approximately 7.4 million cubic metres. The flash flood washed away infrastructure at the Gasa Tshachu natural hot springs, but no injuries or loss of life were reported in Bhutan, state broadcaster Bhutan Broadcasting Service said on Friday.
Officials in Bhutan were still assessing the damage to property and infrastructure in the sparsely populated high valley when the first Indian alerts went out.
In West Bengal, the first public warning came late on Thursday from Forest Minister Manoj Kumar Oraon, who is also the MLA from Kumargram. In a message to residents, he alerted people living along the Sankosh river in Alipurduar district's Kumargram area to the possibility of flash floods following the outburst. He urged residents of Kumargram tea estate, Kumargram forest village, Pakriguri and Bholka to stay alert to rising water levels and to take necessary precautions.
In a video message, he warned that the surge could reach the Kumargram stretch of the Sankosh within about one to one-and-a-half hours of the warning being issued.
By Friday morning, the scale of the precautionary response had grown considerably. Thousands of people were shifted to safer locations in West Bengal, and a red alert was in effect for the Sankosh river system. Alipurduar Superintendent of Police Amit Kumar Shaw appealed to residents not to panic, saying the situation was under control and that the administration was in constant touch with the authorities in Bhutan.
He said the police were coordinating with Bhutanese officials and various emergency agencies, and that sirens and public announcements were being used to warn people living close to the riverbanks. Officials said the incoming water could carry dead fish, large pieces of wood and concrete debris, and residents were told to keep away from the river.
India issued a special flood advisory warning that the flash flood in the upper reaches of the Punatsangchhu river could cause a sudden rise in water levels in West Bengal's Alipurduar district and Assam's Dhubri district. The advisory estimated that floodwaters could reach the India-Bhutan border within six to eight hours of the outburst.
State Minister Nisith Pramanik told reporters that reports of the outburst had been received and that the overflowing water was moving towards the Sankosh through the Mo Chhu, adding that it was expected to reach the plains within the next five to six hours, creating a high risk of significant flooding in the area.
Disaster response arrangements were activated on both sides of the border. West Bengal's State Disaster Management Team was activated and the National Disaster Response Force was notified, with disaster management volunteers instructed to be present on site. NDRF and State Disaster Response Force teams were deployed in Tufanganj and Boxirhat in Cooch Behar district, along the Sankosh near the Assam border.
Central Water Commission monitoring sites at Barobisha, Sankosh LRP, Golakganj and Baladoba were instructed to track river conditions, take precautions and immediately report any significant change in water levels.
Warnings extended well beyond the immediate border districts. In West Bengal, alerts were issued in vulnerable areas of Alipurduar and Jalpaiguri districts, including Banarhat, as well as in the Darjeeling hills and the Terai and Dooars regions. In Assam, flood warnings were issued in Kokrajhar, Baksa, Chirang and Dhubri districts, with round-the-clock control rooms set up to monitor river discharge.
Authorities in both states said they were watching for rapid rises in river levels that could trigger flash floods and severe bank erosion, and residents near riverbanks were advised to move to higher ground.
Shintaphu Lake is counted among 17 glacial lakes in Bhutan identified as potentially dangerous, according to local reports. A glacial lake outburst flood, often shortened to GLOF, occurs when the natural dam holding back a glacial lake fails or is breached, releasing a large volume of water downstream in a sudden surge.
Such lakes are typically dammed by moraines, ridges of rock and debris left behind by retreating glaciers, and they can become unstable as rising temperatures melt ice and increase the volume of water pressing against the dam. The result can be a flood wave that travels far downstream, gathering debris and scouring riverbanks as it goes.
The incident underlines a growing risk across the Himalayas, where warming temperatures are enlarging glacial lakes and increasing the chance of outbursts. Because many Himalayan rivers cross national borders, a single outburst in one country can threaten communities hundreds of kilometres downstream in another, making cross-border monitoring and early warning systems essential.
The latest alert follows severe transboundary flooding along the Nepal-Tibet border in August 2026, another reminder that glacial hazards in the region do not respect political boundaries.
For the people of the Dooars and northern Assam, the immediate concern on Friday was practical: whether the surge would arrive as a manageable rise in river levels or as a damaging flash flood. Officials stressed that the precautionary evacuations, the red alert and the deployment of response teams were designed to keep people safe even if the worst did not materialise.
By Friday morning, official confirmation of downstream impacts in West Bengal and updated river-level readings were still awaited, and authorities said monitoring would continue through the day as the water made its way down the Sankosh system.
The episode also highlights the importance of the quiet, technical work that underpins flood warnings in the region: river gauges at remote stations, data shared between Bhutanese and Indian agencies, and the chain of alerts that runs from a mountain lake to a village loudspeaker in a matter of hours. When that chain works, as officials said it did on Thursday night, thousands of people can be moved out of harm's way before the water arrives.
The coming days will show how much damage the surge caused to embankments, bridges and farmland along the Sankosh, but for now the priority stated by every authority involved is the same: stay alert, stay away from the river, and do not panic.
(Reporting based on The Times of India, The Economic Times and NDTV Profit.)