Originally published 2021-03-02.
On 7 February 2021 a wall of water, rock and mud tore down the Rishiganga and Dhauliganga valleys in the Chamoli district of Uttarakhand, in India's Himalayas. It wrecked two hydropower projects, swept away bridges and trapped construction workers inside tunnels. More than 200 people were killed or are still missing. Early reports called it a "glacier burst", but scientists later showed it was set off by a huge rock and ice avalanche from a mountainside above the valley.
Within weeks of the disaster, around 70 deaths had been confirmed and many more people were unaccounted for. The videos below show the flood itself and the rescuers who went into the mud to reach survivors. Further down, you can read what the scientific investigation found and why this valley was so exposed.
What happened on 7 February
Chamoli lies in the Garhwal Himalaya, close to Nanda Devi National Park and not far from the border with Tibet. The state of Uttarakhand as a whole borders Nepal and is a gateway to some of India's best-known Himalayan pilgrimage routes.
That Sunday morning, a torrent surged down the Ronti Gad stream into the Rishiganga river, then into the Dhauliganga and on towards the Alaknanda, one of the headwaters of the Ganges. Villages such as Raini (also spelled Reni) and Tapovan were hit hardest. Phone videos filmed from the hillsides showed the flood carrying away buildings, vehicles and parts of a dam.
The 13 MW Rishiganga hydropower plant near Raini was badly damaged, and a dam at the meeting of the Rishiganga and Dhauliganga was washed away. Further downstream, the flood struck the much larger Tapovan Vishnugad project run by the state power company NTPC, where hundreds of people were at work. A bridge at Tapovan that linked 13 villages to the outside world was also destroyed.
Rescuers in the tunnels
Most of the dead and missing were workers at the hydropower sites. About 140 of them were at the Tapovan project, where crews had been working inside two tunnels when the debris flow hit and filled them with mud and boulders.
Rescue operations began the same day and continued for weeks. Indian Army units, the Indo-Tibetan Border Police and teams from the National Disaster Response Force worked at the tunnel mouths, and air force transport planes flew in extra NDRF teams. On the first day, rescuers reportedly pulled a group of workers out of the smaller tunnel alive. Getting into the main tunnel was much harder, because it was packed with sludge that had to be cleared with heavy machinery.
The Uttarakhand Police shared footage from its own rescue work in Chamoli on the day of the flood. In their words: "Every life matters, every hand helps!"
Every life matters, every hand helps!
— Uttarakhand Police (@uttarakhandcops) February 7, 2021
We carries out rescue operations in #Chamoli, Uttarakhand @Ashokkumarips pic.twitter.com/Dpzbm5EsJX
By May 2021, 83 bodies and 36 body parts had been recovered, out of 204 people reported missing. Many families never received a body to bury.
From "glacier burst" to rock and ice avalanche
In the first days, the cause was unclear. The area is remote, roads and bridges were gone, and the first explanations were guesses. One suggested that part of a glacier near Nanda Devi had broken off and released water dammed behind it. Another proposed a glacial lake outburst flood, in which a lake held back by ice or loose rock suddenly bursts.
Satellite images soon ruled out a lake: there was none in the area. Geoscientist Dan Shugar of the University of Calgary looked at Planet Labs images taken a few hours after the flood and spotted a fresh scar on the steep face of Ronti Peak. His first assessment was that a steep hanging glacier, rather than a typical valley glacier, had collapsed and broken apart as it fell to the valley floor.
The full study, led by Shugar with 52 co-authors, was published in the journal Science on 10 June 2021. It found that:
- About 27 million cubic metres of material broke away from the north face of Ronti Peak. Roughly 80 percent of it was rock and about 20 percent glacier ice.
- The ice melted from the friction of the fall, turning the avalanche into a fast, highly mobile debris flow.
- The flow carried boulders more than 20 metres across and scoured the valley sides up to 220 metres above the floor.
So the source was not the Nanda Devi glacier, and no lake burst. It was a mountainside collapse, with ice playing a key part in how far and fast the debris travelled.
Was climate change to blame?
Many early news reports pointed straight at global warming. The real picture is more careful. Glaciologists, including researchers at India's Wadia Institute of Himalayan Geology, have said warming affects the freeze-thaw cycles that crack rock and ice on high slopes, making failures like this more likely over time. A review by the climate website Carbon Brief concluded that climate change probably did not directly trigger this particular collapse, but may have contributed to the conditions behind it.
The Science paper itself framed the disaster as a hazard cascade, a chain of natural events that turned deadly when it met infrastructure built down the valley. It called for better monitoring and more careful development in high mountain areas.
Warnings from Raini and the dam debate
Raini has a special place in Indian environmental history. It is the village where women hugged trees to stop loggers in 1974, one of the best-known protests of the Chipko movement. In the years before the flood, residents had complained about blasting for dam construction and had asked to be moved to safer ground. Their village head said afterwards that "half of our village slid," and that the request to relocate had never been acted on.
Scientists had also warned that building dams in the zone just below the glaciers carried serious risk. The Chamoli disaster brought that debate back to the national stage, and in the years since, ground cracking and subsidence in the nearby town of Joshimath has kept attention on how much construction these slopes can bear.
How it compares with 2013
The Chamoli flood was the deadliest disaster in Uttarakhand since June 2013. In that year, a burst of extremely heavy rain, described at the time as a cloudburst, combined with the overflow of Chorabari Lake above Kedarnath to cause devastating floods and landslides. More than 5,700 people were initially presumed dead, a figure later raised to 6,054, and the armed forces evacuated more than 110,000 people.
The two disasters had different triggers: monsoon rain in 2013, a mountainside collapse in 2021. But they share a common lesson. In steep, young mountains, water and debris move quickly, roads and bridges fail first, and the people most at risk are often those living or working right in the valley floor.
Sources: USGS record of Shugar et al., Science (2021); Wikipedia: 2021 Uttarakhand flood; Wikipedia: 2013 North India floods; CNN (February 2021); rescue video by Uttarakhand Police.
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