A catastrophic flood swept down the Bhotekoshi river in Nepal’s Rasuwa district on Wednesday, killing and displacing hundreds of people and destroying bridges and other infrastructure along the river corridor.

Bodies have since been recovered in Rasuwa and in districts including Nuwakot, Dhading, Chitwan, Tanahun, Gorkha and Nawalparasi East. Scientists and climate experts are investigating what triggered the sudden surge of water.

Here is what is known so far about the disaster along the Nepal-China border and downstream.

What happened at the Nepal-China border?

The Bhotekoshi rose with extraordinary speed at the Rasuwagadhi border crossing on Wednesday morning, sending people fleeing for safety.

Initial speculation focused on a possible glacial lake outburst flood or an avalanche. But for much of the day, there was no firm explanation for the sudden surge.

The flood began near Rasuwagadhi and tore through downstream settlements in Rasuwa, Nuwakot and Dhading.

By the time this report was prepared, the government had confirmed 289 deaths, while hundreds of people remained unaccounted for. Bridges and other infrastructure between Rasuwa and Dhading were swept away.

Krishna Prasad Humagain, assistant chief district officer of Nuwakot, told Kantipur that the area had experienced its worst flood in living memory.

Was the flood caused by an avalanche or an earthquake?

Scientists analysing satellite imagery say a massive fall of ice and rock appears to have blocked a river, creating a temporary natural dam that later gave way and unleashed the flood.

The US Geological Survey recorded a magnitude 4.4 earthquake in Tibet at 8:37am on Wednesday, about 47 kilometres north of Gosainkunda. Scientists have not established whether the earthquake triggered or contributed to the landslide.

Journalist Kunda Dixit, writing in Nepali Times, reported that an avalanche had struck the northern face of Langtang Lirung, a 7,245-metre peak.

The mountain was also involved in the devastating 2015 earthquake, when an avalanche on its southern face buried Langtang village and killed about 300 people.

Dixit wrote that Langtang Lirung lies entirely within Nepal and that debris from the avalanche blocked the Lhende river, a river along the Nepal-China border. The monsoon-fed river formed a temporary lake before the blockage failed, sending a torrent of mud and boulders towards the Bhotekoshi river.

Rijan Bhakta Kayastha, a climate researcher and professor at Kathmandu University, said an avalanche appeared to have blocked the Lhende river before the flood broke through.

“We suspect an avalanche on the Nepal side blocked the Lhende river,” Kayastha said. “An earthquake occurred at around the same time, but we do not yet know whether it helped trigger the avalanche.”

Was there any warning that the area was at risk?

The region was already known to be vulnerable to sudden floods.

On July 8 last year, the Bhotekoshi experienced another sudden surge. Nepal’s Department of Hydrology and Meteorology concluded that the earlier flood was caused by a glacial lake outburst in the Lhende river near the Nepal-China border.

Satellite analysis using Sentinel imagery found that a glacial lake had burst, sending water downstream.

Scientists had also warned that the wider Himalayan region contains numerous potentially dangerous glacial lakes. But there had been no specific forecast that an ice-and-rock avalanche would strike the location involved in Wednesday’s disaster.

A study by the International Centre for Integrated Mountain Development, or ICIMOD, and the UN Development Programme identified 3,624 glacial lakes across Nepal, India and Tibet. Forty-seven were classified as potentially dangerous and at risk of bursting, including 21 in Nepal.

What do scientists say happened?

Nepal’s National Disaster Risk Reduction and Management Authority has also said an ice-and-rock avalanche is the leading explanation.

Analysis of Planet Labs satellite imagery shows what appears to be a large avalanche about 20 kilometres northeast of the Rasuwagadhi border crossing, close to the Nepal-China border. The debris appears to have entered the Lhende river and triggered a debris-laden flood.

ICIMOD scientists are investigating whether debris accumulated at a narrow section of the river and temporarily blocked its flow, creating what is known as a landslide dam.

The institute said its preliminary assessment, based on video footage and reports from the affected area, indicated that a huge volume of ice and rock debris may have fallen into the Lhende river, a tributary of the Bhotekoshi.

The resulting flood was unusually fast and powerful. At Galchhi, the Trishuli river rose by as much as nine metres in just 30 minutes, while the water level at Malekhu climbed by about seven metres over the same period, according to ICIMOD.

Sarthak Shrestha, a remote sensing and geoinformation associate at ICIMOD, said the evidence so far pointed to an avalanche on the Nepal side as the main trigger.

ICIMOD has been working with Chinese institutions, including the Chengdu Institute of Biology under the Chinese Academy of Sciences, to determine precisely what happened.

“So far, the evidence indicates that an avalanche above Syaphrubesi blocked the river and the flood followed,” Shrestha said. “It could take another week or two to establish the full picture.”

A preliminary review of imagery supplied by Chinese authorities also indicated that the avalanche occurred on the Nepal side, Shrestha said.

Chinese monitoring systems had not detected a natural event on the Tibetan side capable of producing a flood of this scale, according to information received by ICIMOD.

Is another flood possible?

Scientists have not ruled out further danger.

Home Minister Sudan Gurung said on Wednesday afternoon that the Lhende river had not yet fully cleared.

Kayastha said on Thursday that there had initially been concern that another large body of water might have formed upstream, but there was not yet enough evidence to confirm the existence of such a lake.

Cloud cover has also hampered satellite observation of the area, ICIMOD said.

“The risk is still there,” Kayastha said. “Rising temperatures increase the likelihood of glaciers melting, ice and rock breaking apart and avalanches occurring in the Himalayas. But we cannot say exactly where the next event will happen.”

Scientists are continuing to monitor glacial lakes and other high-risk areas.

China has reported another potentially dangerous development downstream of the disaster area.

The Chinese Ministry of Water Resources said a large barrier lake had formed near the confluence of the Chotchen river and the Purupe Tsangpo river in Nepal. By Thursday morning, about 2 million cubic metres of water had accumulated, and water had begun flowing out of the lake, the Global Times reported.

China Central Television reported that another 3 million cubic metres could enter the lake over the following three days, raising concerns that the natural dam could fail.

Chinese authorities are monitoring the lake continuously and running flood simulations to assess the consequences of a possible breach. They are also preparing emergency assistance and rescue operations.

The Chotchen and Purupe rivers originate in Tibet and merge before crossing into Nepal, where the river is known as the Bhotekoshi. The Bhotekoshi is the principal upper tributary of the Trishuli river.