What triggered the deadly Himalayan floods? Scientists investigate 

Authorities in Nepal and Tibet continue to issue updates on conditions following devastating glacial-lake outburst floods that submerged towns and communities near the border beneath mud and floodwater.  As information emerges on the situation on the ground, scientists and geologists are examining the disaster to determine what triggered the rare and destructive event.

Officials initially suspected that a magnitude 4.4 earthquake had triggered the downstream flooding, which is feared to have swept away hundreds of people, including local residents as well as countless tourists who are drawn to the region by its mountainous landscape.

However, international experts and the United States Geological Survey have concluded that “the seismic event” was instead a consequence of “a glacial collapse and debris flow that caused subsequent catastrophic impacts downstream” — rather than the cause of the flooding, as highlighted in an article published by Al Jazeera.

A glacial collapse involves the detachment of a substantial mass of glacier ice from its bed, potentially involving millions of cubic metres of ice, rock and water. The material can then develop into a highly mobile ice-rich avalanche or debris flow. Although glacial collapse is not a formally defined scientific term in glaciology, it is commonly used to describe such events.

Earth System Science Data, an international open-access scientific journal, describes these phenomena as gravity-driven failures of glacier ice on mountain slopes, including ice avalanches, glacier detachments and rock-ice avalanches.

An initial analysis of Planet Labs satellite imagery showed that a landslide involving ice and rock unleashed a debris-laden flood into the Lhende River in Tibet, around 20km northeast of the Nepal-China border crossing at Rasuwagadhi, according to Nepalese disaster management officials.

Television footage showed homes reduced to rubble, vehicles being swept away and a metal bridge collapsing under the force of the water. Witnesses reported that entire villages were engulfed by the flood.

More than 359 people have so far been confirmed dead, although officials warn that the toll could rise in the coming days as rescue and recovery operations continue.

The disaster struck the mountainous Nepal-Tibet border region, with some of the worst destruction reported in Nepal's Rasuwa district, immediately south of the Chinese border. The initial surge entered the Lhende Khola, which feeds into the Bhote Koshi and subsequently the Trishuli River, sending floodwaters south through Nepal. Water levels in the Trishuli reportedly rose by as much as nine metres within just 30 minutes.

Nepal's National Disaster Risk Reduction and Management Authority said the “glacier-inclusive flood” occurred in the Lhende River following “an ice-rock avalanche on the Nepali side”.

Vikram Gupta, an engineering geology specialist and professor at Sikkim University in India's northeastern state bordering Nepal, told the network that it was “confirmed that a substantial portion of that glacier snout collapsed to initiate the event – probably including or entraining a fair amount of rock and sediment”.

Earth scientist Dan Shugar, an associate professor at the University of Calgary, said satellite imagery appeared to indicate that a substantial amount of snow had melted during the 24 hours preceding the disaster. “The lower part of a glacier broke off at about 5,200 metres and crashed onto the valley floor about 1,200 metres below,” Shugar said.

Global warming's creeping impact on region

The disaster comes amid rapid warming and accelerating glacial loss across the Himalayas. Glaciers throughout the Hindu Kush Himalaya range — which stretches across Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan — lost ice 65% faster between 2011 and 2020 than they had during the previous decade.

Nepal's glaciers, located between the major carbon emitters India and China, have lost almost one-third of their ice volume over roughly three decades as a result of global warming, according to data from the United Nations in 2023.

As the Doha-based outlet points out, the disaster has raised renewed concerns about the vulnerability of communities living downstream from increasingly unstable glaciers and about how a warming climate could affect the frequency and scale of such extreme events.

By Nazrin Sadigova