A massive ice-and-rock avalanche in Nepal’s high Himalayas appears to have temporarily dammed a river before the blockage collapsed, unleashing the catastrophic flood that tore through the Nepal-China border region on Wednesday, scientists say, as authorities warn the danger of further flooding has not passed.
The disaster sent a wall of water, mud and boulders down the Bhotekoshi river corridor, devastating settlements and infrastructure and leaving a heavy death toll and hundreds unaccounted for.
According to the Kathmandu Post, scientists analysing satellite imagery increasingly believe an avalanche blocked the Lhende river, creating a temporary natural dam. When that barrier gave way, the accumulated water surged downstream into the Bhotekoshi.
Rijan Bhakta Kayastha, a climate researcher and professor at Kathmandu University, told the newspaper that an avalanche on the Nepal side was suspected of blocking the Lhende.
A magnitude-4.4 earthquake was recorded in Tibet around the same time, but scientists have yet to establish whether it contributed to triggering the avalanche.
The Kathmandu Post also cited journalist Kunda Dixit of Nepali Times as reporting that an avalanche struck the northern face of the 7,245-metre Langtang Lirung, sending debris into the river system.
Scientists at the International Centre for Integrated Mountain Development (ICIMOD) are examining satellite imagery to establish the precise sequence of events.
Preliminary evidence indicates a huge volume of ice and rock entered the Lhende, possibly accumulating at a narrow section and forming a temporary landslide dam, according to the Kathmandu Post.
The scale of the resulting surge was extraordinary.
The newspaper, citing ICIMOD, reported that the Trishuli river rose by as much as nine metres within 30 minutes at Galchhi and about seven metres at Malekhu.
The region was already considered highly vulnerable.
A sudden Bhotekoshi flood in July last year was attributed to a glacial lake outburst near the Nepal-China border.
Research by ICIMOD and the UN Development Programme has identified 3,624 glacial lakes across Nepal, India and Tibet, 47 of them potentially dangerous.
The latest disaster has also renewed concerns over the increasing instability of the Himalayan cryosphere as temperatures rise.
“The risk is still there,” Kayastha told the Kathmandu Post, warning that warming increases the likelihood of glacier melt, ice-and-rock collapse and avalanches, although scientists cannot predict exactly where the next disaster will strike.
That warning has taken on added urgency with the formation of another large barrier lake.
This report was compiled from reporting by the Kathmandu Post and Global Times, with the latter citing Chinese government agencies and state media.