ALIPURDUAR — A glacial lake in northern Bhutan burst on Thursday evening and sent an estimated 7.4 million cubic metres of water into the Mo Chhu river, prompting India to issue a flood advisory for two border districts and driving more than 10,000 people out of villages along the Sankosh river overnight.

The surge put the Sankosh, which runs along the West Bengal-Assam border, under a red alert. Floods of this kind have proved deadly in the eastern Himalaya, and how much warning Indian districts get depends on how fast Bhutan reports a breach across the frontier.

The outburst was reported at Shintaphu Lake, also written Sintaphu, below Wakila Mountain in Bhutan's remote northern district of Gasa. The lake feeds the Mo Chhu, which flows south through Gasa and Punakha before joining the Pho Chhu to form the Punatsangchhu, the river that becomes the Sankosh once it crosses into India.

Floodwater reached the settlement of Taktsemakhang at about 8.15 p.m. and cut the road to Laya, one of Bhutan's most isolated highland communities, the Bhutanese outlet Daily Bhutan reported. Further down the valley it carried away the Gasa Tshachu hot spring complex beside the Mo Chhu and took out the Punakha-Gasa highway at the 31.85 km mark, now shut to all traffic.

No deaths or injuries were reported in Bhutan, and the hot springs had been cleared before the water arrived. A solar-powered irrigation scheme in the Dochhukha-Dzomlingthang-Ritsa area of Punakha stopped working after it was damaged. Bhutan's state broadcaster BBS said two Bailey bridges may also have been lost.

Bhutan's National Centre for Hydrology and Meteorology tracked the water at its Tashithang and Yebesa gauges, sirens sounded along the Mo Chhu and the Punatsangchhu, and India was informed. The Punatsangchhu-II hydropower plant, part of a joint India-Bhutan venture, stopped generating and opened its spillway gates to let the water and its debris through.

The Union Ministry of Jal Shakti issued a special advisory on Thursday saying the Indian Embassy had reported the outburst in the Punatsangchhu basin, and that water levels in the Sankosh could rise suddenly in Assam's Dhubri district and West Bengal's Alipurduar. The Central Water Commission issued its own flood advisory for the two states. Officials said the water could reach the border within six to eight hours.

West Bengal began clearing riverbank villages late on Thursday night. Officials said loudspeaker announcements sent people in the Rampur-I gram panchayat towards designated shelters. In Alipurduar district, families were seen leaving home with their cattle.

"Everyone living in the catchment areas of the Sankosh River and all its connected tributaries has been asked to move to a safe distance immediately to protect their lives," Nisith Pramanik, the state's water resources investigation and development minister, said in a video message. He also told people living near riverbeds in Tufanganj and elsewhere in Cooch Behar district to move at once, and said National Disaster Response Force and State Disaster Response Force teams were being held ready.

Forest and Environment Minister Manoj Kumar Oraon listed the forest villages and tea estates in the water's path, among them the upper Sankosh forest village, the NKC Sankosh tea garden and Chengmari. "Please pass the word along without delay and evacuate people from these adjacent low-lying areas to safer ground, as the situation could turn critical," he said in a message distributed by the news agency PTI.

Amit Kumar Shaw, the superintendent of police in Alipurduar, said between 4,000 and 5,000 people had been put up in temporary shelters, many of them school buildings, while more than 6,000 others had moved to safer ground with drinking water, dry food and sanitation laid on. Police warned that the flow could be carrying timber and broken concrete.

"There is absolutely no need to panic. The situation is currently under control," Shaw told the news agency ANI, and said his officers were in touch with Bhutan. Officials said the intensity of the flow had eased slightly, though the red alert on the Sankosh stayed in force, with warnings also covering Jalpaiguri, Cooch Behar and the Darjeeling area. The Darjeeling MP, Raju Bista, asked residents of the hills, the Terai and the Dooars to stay alert.

In Assam, Chief Minister Himanta Biswa Sarma said his government was watching every vulnerable revenue circle in Dhubri and Kokrajhar and was in constant contact with the NDRF. His office said the deputy commissioner's team in Dhubri was on the ground. A spokesperson for the Assam State Disaster Management Authority said the Sankosh was steady at 6.30 a.m.

Shintaphu is one of 17 glacial lakes Bhutan lists as potentially dangerous, a label meaning it needs monitoring, not that a burst is likely. Bhutanese specialists surveyed it between May and June 2023 and left monitoring equipment behind. What set off Thursday's release has not been established.

India's exposure to those lakes has been under scrutiny since October 2023, when a lateral moraine collapsed into South Lhonak lake in Sikkim. The flood that followed killed 55 people and destroyed the 1,200 megawatt Teesta-III hydropower project at Chungthang, 68 km downstream. A study of that disaster published last year found several causes at work, and its lead author told Carbon Brief the main driver was thawing permafrost.

The India Meteorological Department has forecast heavy rain over Alipurduar on Friday, which officials said could add to the water already moving down the Sankosh.