What triggered the catastrophic floods on the Nepal-Tibet border?

At least 157 people have been killed after massive flash floods struck the Himalayan border region of Nepal and China's Tibet on Wednesday (26 August), sweeping away houses, roads, bridges and other infrastructure.

Three more people were reported dead in Tibet.

The disaster also triggered a mudslide at Gyirong Port, a major land crossing between Nepal and Tibet, cutting road, communication and power links in the area.

Dozens of people were injured, while hundreds of people, including foreign nationals, remained missing.

But what triggered the sudden and devastating flood?

What happened?

The disaster unfolded along the Lhende Khola river, which flows through Nepal and Tibet.

Nepal's National Disaster Risk Reduction and Management Authority said a "glacier-inclusive flood" occurred in the Lhende River following a landslide around 20km northeast of a Nepal-China border crossing.

The floodwaters rapidly moved downstream, carrying huge amounts of mud, rocks and debris. Roads, bridges, houses and power projects were damaged or swept away.

The flooding also hit Gyirong Port, disrupting one of the major crossings between Nepal and China.

Videos from the area showed people being swept away by powerful floodwaters and extensive destruction along the river.

What triggered the flood?

A combination of factors may have contributed to the disaster.

Experts analysing satellite imagery said the lower section of a glacier at an altitude of about 5,200 metres appears to have broken away and crashed onto the valley floor roughly 1,200 metres below.

The collapse appears to have generated a huge mass of ice, rock and sediment that entered the river and triggered the flash flood.

The exact cause of the glacier collapse has not yet been established.

Nepali officials have pointed to a possible earthquake that struck the region shortly before the disaster as one possible trigger. However, this has not been confirmed as the definitive cause.

Earth scientist Dan Shugar, an associate professor at the University of Calgary, said satellite imagery appeared to show that a significant amount of snow had melted in the 24 hours before the disaster.

He suggested that unusually warm conditions may have contributed to the glacier's instability.

Vikram Gupta, an engineering geology specialist at India's Sikkim University, said satellite imagery indicated that a substantial portion of the glacier's snout had collapsed, probably carrying or incorporating a large amount of rock and sediment.

How severe is the destruction?

The floodwaters swept away or damaged roads, bridges, homes and power infrastructure in the border region.

A large amount of mud and sediment has accumulated around the affected areas, making access and rescue operations difficult.

The destruction has also severed communications and transport links around Gyirong Port.

Authorities fear the death toll could rise as rescue teams reach areas that remain cut off.

Are foreigners missing?

Hundreds of people are reported missing, including tourists and workers from several countries.

Reports cited in the initial assessments put the number of missing foreign travellers at more than 380, including Indians, Nepalis, Americans, British nationals and others.

Among those reported missing were 105 Indians, 93 Nepalis, three US citizens and 12 British citizens. Eight South Koreans working at a hydropower project were also reported missing.

The missing also included 17 Malaysians, four South Africans, and one Australian and one Dutch national.

The figures may change as authorities continue to verify the identities and locations of those affected.

Has the danger passed?

Not yet.

Authorities have warned of the possibility of another flood because a blockage remains upstream on the Lhende Khola river.

Such a blockage could potentially cause water to accumulate and then be released suddenly, creating another dangerous surge downstream.

Flood alerts have also been issued in India's Bihar and Uttar Pradesh, which border Nepal. Several rivers originating in the Himalayan region flow from Nepal into India's northern plains.

People living in low-lying areas have been advised to move to safer locations and those living near rivers have been urged to remain alert.

How is the rescue operation progressing?

Police and military personnel have been deployed for rescue and relief operations.

However, the heavy accumulation of floodwater, mud and debris is making access difficult. Officials said rescue helicopters would not be able to land in some affected areas until the water level recedes.

China has deployed at least 574 rescuers to the affected land crossing, while India has said it is coordinating closely with Nepal on rescue and relief operations.

South Korea is also preparing to send a rapid-response team comprising government officials as well as fire and police personnel.

Authorities on both sides of the border are continuing to search for missing people while assessing the full scale of the destruction.

What remains unclear?

The precise chain of events that triggered the flood is still under investigation.

Satellite imagery indicates that a glacier section collapsed and sent ice, rock and sediment into the river. An earthquake has been suggested as a possible trigger, while unusually warm conditions and snowmelt may also have contributed to the glacier's instability.

Until geological and seismic assessments are completed, it remains unclear whether the earthquake, warming and snowmelt, or a combination of factors ultimately caused the catastrophic collapse.