More than 6,000 people were evacuated in West Bengal’s Alipurduar district after a glacial lake outburst in Bhutan triggered flash-flood warnings across parts of Assam and West Bengal. Although the situation has since shown signs of improvement, the incident has brought renewed attention to the threat posed by glacial lakes in the eastern Himalayas, where a sudden release of water can put communities and infrastructure far downstream at risk.

The incident occurred on Thursday at Shintaphu Lake in Bhutan’s Gasa district. The gush of water washed away facilities at Gasa Tshachu, a popular natural hot spring near the Mo Chhu River. No casualties were reported in Bhutan.

The authorities in West Bengal moved residents from vulnerable riverbank and forest areas to safer locations amid concerns about rising water levels along the Sankosh River. Jalpaiguri was also placed on high alert. Officials later said the water level had receded and there was no major damage or loss of life.

Bhutan has 567 glacial lakes, 17 classified as dangerous

Bhutan’s mountainous landscape, glaciers and steep river valleys make it vulnerable to glacial lake outburst floods (GLOFs). According to the country’s National Centre for Hydrology and Meteorology, Bhutan has 567 glacial lakes, of which 17 are classified as potentially dangerous.

An earlier assessment had identified 25 such lakes, but subsequent investigations lowered the number to 17 after eight were found to pose relatively lower risks. The classification does not mean an outburst is imminent. The danger associated with each lake depends on factors such as the volume of water it holds, the condition of its natural barriers and the communities and infrastructure located downstream.

What is a Glacial Lake Outburst Flood?

Glacial lakes develop when retreating glaciers leave behind depressions that fill with meltwater. In many cases, the water is contained by natural barriers made of loose rocks, sediment and glacial debris, known as moraines.

These barriers can weaken or collapse following landslides, avalanches, heavy rainfall or rapid melting. When a large volume of water escapes suddenly, it can carry rocks and debris through mountain valleys, causing extensive destruction. This phenomenon is known as a glacial lake outburst flood.

The United Nations Development Programme (UNDP) describes GLOFs as a “tsunami in the sky”, underlining the scale of destruction they can cause.

Thorthormi And Raphstreng among closely monitored lakes

The Thorthormi and Raphstreng lakes in Bhutan’s Lunana region have drawn particular attention from researchers. A 2023 study published in the Journal of Hydrology examined four potentially dangerous lakes: Bechung Tsho, Raphstreng Tsho, Thorthormi Tsho and Lugge Tsho.

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The researchers modelled possible outburst scenarios, with estimated flood volumes ranging from 600,000 cubic metres to 300 million cubic metres. Their findings indicated that more than 16,000 people and infrastructure located up to 150 km downstream could be exposed under the scenarios studied. These figures represent modelled risks, not predictions of an imminent breach.

Bhutan’s history of glacial lake floods

Bhutan has experienced several GLOF events over the decades. On October 6, 1994, the partial collapse of the moraine dam at Lugge Tsho triggered a devastating flood that killed around 21 people and damaged homes, farmland and infrastructure.

A 2024 study published in Natural Hazards and Earth System Sciences also referred to GLOF events involving Tshojo Lake in 2009 and Lemthang Tsho in 2015. Researchers have documented at least 18 such events in Bhutan since the 1950s, although historical records are incomplete.

Climate change and the growing Himalayan risk

Rising temperatures are accelerating glacier retreat across the Himalayas, contributing to the formation of new glacial lakes and the expansion of existing ones. Warming can also weaken mountain slopes and permafrost, raising the risk of landslides and avalanches entering lakes. Climate change, however, does not automatically cause a GLOF. The condition of individual lakes and their surrounding terrain remains crucial.

Bhutan has been strengthening monitoring and early warning systems through satellite observations, field assessments and river-level monitoring. Timely warnings remain essential for communities living along downstream valleys.

Over 6,000 evacuated in Alipurduar as situation eases

In West Bengal, district authorities relocated more than 6,000 people from vulnerable areas and arranged shelter for thousands in local schools. Superintendent of Police Amit Kumar Shaw said, “We have safely relocated over 6,000 to 7,000 people and arranged shelters for more than 4,000 people in local schools, equipping them with essential rations and baby food,” Shaw said.

He added, “We are receiving inputs on a minute-by-minute basis. The latest update indicates that the water level upstream has receded slightly. Given the travel time of the water flow, we hope to avoid major flooding, though water levels will naturally rise to some extent,” he added.

Chief Minister Suvendu Adhikari said the situation was “completely normal” on Friday. “Throughout the night, until 4 am, the SP, ministers, MLAs, and the DM were all on-site, working. They were also in constant touch with our control room here,” Adhikari said.

“We have established contact with the people who were reported missing earlier this morning. All 47 of them are safe,” he added.

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While the immediate flood threat appeared to be easing, the incident has highlighted the importance of monitoring Bhutan’s glacial lakes and ensuring that communities downstream receive timely warnings.