“We saw houses vanish in front of our eyes,” says Aadesh, an engineer living in the northern Indian state of Sikkim. One of them was his own. In the early hours of October 4, Lhonak Lake—a Himalayan glacial lake—burst its banks, releasing huge amounts of water into the river valleys below.
In the Chungthang region where Aadesh lives, many miles downstream, bridges, police barracks, homes, and guest houses were washed away by water flowing at over 50 kilometers per hour. The Teesta 3 hydroelectric power project, with its 60-meter-high dam, was destroyed in its entirety. Forty people are confirmed dead, and at least 76 remain unaccounted for.
In a world where temperatures are warming and populations are growing, dangerous floods linked to melting glaciers will become more common. The Lhonak outburst comes just months after a Nature paper estimated that 15 million people worldwide are threatened by glacial lakes suddenly releasing water. The risk posed by Lhonak Lake was known, but mitigation efforts failed—suggesting the threat hasn’t been taken seriously enough. And the threat posed by Lhonak is far from unique. “There are many more lakes on the verge of such disasters,” says Aadesh. “We are not prepared.”
Satellite images of Lhonak Lake before and after its outburst in October 2023.
Illustration: NRSC
Known formally as glacial lake outburst floods—or GLOFs—these deluges are driven by rising mountain temperatures. Where glaciers terminate, they deposit rocky debris that has been carried in their ice, forming what is known as a moraine. Sometimes, meltwater from the glacier will get trapped behind this debris, creating a lake.
All around the world, glacier meltwater feeds mountain lakes, and as glacial ice recedes due to global warming, many of these bodies of water are becoming larger and more unstable. Rainfall, landslides, earthquakes, or rising water pressure can all be catalysts to a moraine shifting and crumbling, releasing a dangerous wall of water down the mountain.
“We can identify the risk hot spots, but we can’t predict when exactly a GLOF event will happen,” says Ashim Sattar, a Himalayan cryosphere scientist at the Indian Institute of Science. More than half of the 15 million people thought to be at risk from glacial lakes live in High Mountain Asia, the high-altitude lands centered around the Tibetan Plateau. India and Pakistan alone account for over 5 million of those vulnerable. Sikkim, in particular, is known for lake outbursts, but a large chunk of the Himalayan range also has a history of devastating lake floods in Nepal and Bhutan.
In the coming decades, the threat will get worse. Glaciers “are very sensitive to a warming climate,” says Sattar. In a scenario where the world warms by 1.5 degrees Celsius on average, glaciers in Asia’s high mountains will warm by 2.1 degrees, research estimates; global warming, models predict, will have an outsized effect on ice melting. Given that future warming is very likely to exceed 1.5 degrees, a lot of dangerous meltwater is going to build up in the coming years.
Outbursts can happen without warning, and preparing for them is difficult. There are 9,575 glaciers in the Indian Himalayas alone, and sustained melting has created more than 5,000 glacial lakes with “potentially unstable moraines”—banks susceptible to bursting.
Monitoring the weather and water levels at a lake can provide an early warning of an outburst, but setting up monitoring stations is time-consuming and expensive. Plus, such measures won’t avert a flood. “As far as I know, siphoning off the water or managed breaching of the moraine dyke is the only way to ease the pressure [of a lake at risk of bursting],” says Dhrupad Choudhury of the International Centre for Integrated Mountain Development (ICIMOD) in Patan, Nepal. “The same principle is used in dams during monsoon.”
Such interventions had been tried at Lhonak, but they weren’t effective in avoiding catastrophe. This was partly because measures were flawed, deficient, or uncompleted. Mitigating the risk at Lhonak had not been sufficiently prioritized. It was a job left unfinished.
Research had clearly shown that the lake posed a risk. A report published by Sikkim’s Department of Science and Technology had identified eight glacial lakes—including Lhonak—as potentially hazardous back in 2013, and research published the same year estimated that Lhonak had a 42 percent chance of bursting at some point in the future.
A 2016 assessment by India’s National Disaster Management Authority (NDMA) then pointed out that an outburst would cause “huge devastation downstream.” So the authority called for measures to mitigate the risk. But it only implemented what it identified as a “short-term solution” that year—thin plastic pipes to siphon water from the lake. The NDMA didn’t follow up with a more significant, long-term attempt to manage the growing size of the lake.
A measurement station was installed at Lhonak in September 2023 to provide some advance warning of an outburst—but it failed just weeks before the flood and stopped sending data, the NDMA has reported. Geoscientist Simon Allen of the University of Zurich, part of the team that installed the station, told Reuters that his team had also wanted to apply a trip-wire sensor that would be triggered if the lake burst, to alert people downstream. But, he claims, the Indian government chose to defer installing this. The NDMA did not respond to WIRED’s request for comment.
If the now-destroyed Teesta 3 dam had received advance warning of the flood, the deluge might not have been so destructive. Sikkim Urja Limited, which handles the operation of the dam, has confirmed that the dam’s spillway gates couldn’t be opened in time to try to accommodate the flood. Had they been, it’s possible the dam might have been better able to handle the sudden arrival of so much extra water. Sikkim Urja Limited has said it will investigate the failure to open the gates in time.
“GLOF events like Sikkim are bound to happen more frequently—we are not taking scientists’ recommendations seriously,” says Arun Shreshtha, a senior climate change specialist at ICIMOD. Scientists like Shreshtha are asking for the expedited installment of safety warning systems, building future houses and infrastructure so they are less susceptible to flood damage, and if possible, leaving fragile mountain areas undisturbed by development projects. “We are not allocating enough financial resources to handle it,” Shreshtha says of the threat.
Teaching the public about the threat of glacial lake flooding—and what to do in such an event—also needs to improve, says Sattar. “Merely piling up scientific research and not communicating it to the masses in simple words is going to do no good.”
Many people living close to remote glacial lakes are some of the most marginalized and financially vulnerable people across India, Pakistan, Nepal, and Bhutan. “On their own, mountain people are definitely not ready to face these challenges,” says Shreshtha. In both mitigating risks and responding to floods, there needs to be more money available.
According to Roshan Lama, a first responder with the Edwards Foundation, a relief agency working in West Bengal and Sikkim, thousands of people affected by the Lhonak flood are living in relief camps set up in government schools and community centers. “The ground situation is that many houses have been washed away, and many have been damaged. And people cannot simply return, because they don’t even know where the houses were,” says Lama. “The whole plots are gone.” Lama is concerned that another lake outburst disaster could occur at any time. “There are so many lakes which are under threat,” he says.
“We can’t risk another Chamoli,” says Shreshtha, referring to the 2021 outburst in the northern Indian state of Uttarakhand, which left over 200 people dead or missing. “If we sleep on these alarms, millions will feel the wrath from the mountains.”