Rock-ice avalanches: The silent Himalayan killer behind Nepal's deadly flood

The devastating flood in Nepal's Rasuwa district has put the spotlight back on a little-understood but increasingly lethal Himalayan hazard, rock-ice avalanches capable of triggering destructive floods far downstream, often without heavy rainfall as the immediate trigger, according to Gaurav Talwar's Times of India report.

Scientists at the International Centre for Integrated Mountain Development (ICIMOD) are investigating whether a rock-ice avalanche from high-altitude slopes triggered the August 26 flood.

Also read: What is a glacial collapse? The science behind the Nepal-Tibet floods


Preliminary assessments suggest a large volume of ice and rock debris entered the Lhende Khola, a tributary of the Bhote Koshi river, unleashing a powerful surge of water, sediment and boulders downstream.

What is a rock-ice avalanche

A rock-ice avalanche is the rapid downslope movement of rock and ice, typically caused by the collapse of a rock mass on or near a glacier. These avalanches can travel long distances at high speed, and once they enter river systems, they transform into debris flows or flash floods, making the danger far greater than the initial collapse itself.

Echoes of Chamoli 2021

The cascading pattern is not new to the Himalayas. It has revived comparisons with the February 2021 Chamoli disaster in Uttarakhand, when a massive rock-and-ice avalanche triggered a deadly deluge through the Rishiganga and Dhauliganga valleys, killing more than 200 people and destroying two hydropower projects.

"Two major incidents of rock-ice avalanche-triggered flash floods have occurred in the Himalayan region in the last five years. Similar to what happened in Chamoli, a large mass got displaced along with the entire bedrock in Nepal, causing mass-scale devastation," said Farooq Azam, a cryosphere expert at ICIMOD.

Senior geologist Piyoosh Rautela explained why these events are so destructive: a rock-ice avalanche is dangerous not simply because a huge mass of ice and rock collapses, but because that collapse can set off a chain reaction.

A growing pattern across High Mountain Asia

The scale of the threat was underlined in a peer-reviewed study published last year in the journal Landslides. Examining 60 large rock-ice avalanches across High Mountain Asia, including the Himalayas, Karakoram and Tibetan Plateau, researchers found these events had claimed at least 1,366 lives and caused extensive damage to ecosystems, water resources, infrastructure and communities.

The study also cited the 2015 Langtang avalanche in Nepal, which killed around 350 people, alongside the Chamoli tragedy. It found that 86% of the avalanches originated on steep, high-altitude slopes, predominantly facing north or northeast, with permafrost thaw, seismic activity and unstable bedrock among the main triggers. Nearly half of the events set off cascading disasters, sharply amplifying their destructive potential.

Climate change is raising the risk

Experts say rising global temperatures are compounding the instability of high mountain environments. "Global warming and temperature variability are altering glacial mass balance and thawing permafrost, heightening the risk of extreme events like rock-ice avalanches," Azam said.

The thawing of permafrost is a particular concern, as it directly weakens the stability of mountain slopes. "Permafrost acts like a cement, holding rock and mountain slopes together. As temperatures rise and permafrost warms, that natural binding weakens, destabilising the bedrock and increasing the likelihood of large slope failures that can trigger rock-ice avalanches," said Prashant Baral, a cryosphere analyst at ICIMOD in Kathmandu.

Also read: China-Nepal glacial flood alert system warned of seven disasters – but missed this one

Manish Mehta, scientist and glaciologist at the Wadia Institute of Himalayan Geology, Dehradun, told TOI that climate change and global warming are altering conditions across the Himalayas. "Rainfall is increasingly replacing snowfall, which causes glaciers to lose their ice mass. Hanging glaciers are particularly unstable, and the northern and southern slopes of the Himalayas are especially prone to these glaciers breaking off and triggering rock-ice avalanches," he said.

The Himalayas host thousands of glaciers and permafrost-laden slopes that have historically been considered stable over human timescales. However, the region has warmed faster than the global average in recent decades, accelerating glacial retreat and permafrost degradation.

This has made high-altitude slopes particularly hanging glaciers on steep, shaded faces, increasingly prone to sudden collapse.

The 2021 Chamoli disaster was among the first events to draw global scientific attention to rock-ice avalanches as a distinct and under-monitored Himalayan hazard, prompting calls for better early-warning systems and glacial monitoring across India, Nepal and other Himalayan nations.

The Rasuwa flood adds to a growing list of such incidents, reinforcing warnings that similar cascading disasters could become more frequent as warming continues.

With inputs from TOI