Videos show crushing walls of water overtake buildings and bridges on Nepal-Tibet border

Crowds attempt to flee a surging wall of mud but have nowhere to go as it hits them with horrific force. Rows of buses and trucks parked nearby are swept aside like toys. Moments later, there is only wreckage and destruction.

The shocking surveillance footage from the Rasuwagadhi border crossing, which links China’s semiautonomous Tibet region to Nepal, captures the scale of destruction as flash floods swept through Himalayan mountain towns and villages Wednesday, leaving ruins in their wake.

In Nepal’s hardest-hit Rasuwa region, videos verified by NBC News showed bridges being cracked apart and swept away by the force of raging waters filled with mud and debris. Peaceful lush green valleys were transformed in an instant with the violence of churning water.

Aboard a bus in Bidur, a passenger captures chaos as the surge appears in the distance: People scream and shout, some running away on foot, as vehicles on the road attempt to make their escape. Elsewhere, in videos shared by Nepali police, roads and buildings are swept away completely.

Satellite imagery showed the scale of the flooding, with the basin of the Trishuli River dramatically swelled by mud.

Hundreds of tourists are among the missing in the popular mountain regions north of Nepal’s capital, Kathmandu. At least 157 people have been killed, the country’s police service said. The true scale of the casualties may take days to become clear, however, with power and communication lines swept away in some areas.

The office of Nepal’s prime minister said that 484 tourists were missing, of whom 391 were foreigners and 93 Nepalis.

Sunil Sharma, a tourism board spokesperson, said 133 were from India on pilgrimage to Mount Kailash in Tibet, a sacred site for Hindus. Also missing were 47 from the United States, 34 from Australia, 33 from Britain and 24 from Canada, he said. How many local people were also ‌missing was not clear.

A State Department spokesperson said it “is aware of Americans impacted by the floods in Nepal and is closely tracking the situation.”

Canada was also closely following the situation, said Canadian Prime Minister Mark Carney on X, calling the floods “absolutely devastating.”

High casualty counts are also feared in Tibet, where hundreds more are missing, with both Nepal and China declaring emergencies and scrambling rescue teams.

“There is devastation everywhere we look,” Tula Bahadur BK, a health worker in Rasuwa, told Reuters. “The settlements next to the river have been completely swept away. Five of my own relatives are missing.”

Sasmit Pokharel, a Nepali government spokesperson, said they have asked both India and China for help with the rescue efforts.

The cause of the inundation was not immediately clear, but climate experts at the Kathmandu-based International Centre for Integrated Mountain Development said an avalanche from a glacier likely caused Wednesday’s flash flood.

The U.S. Geological Survey reported a magnitude-4.4 earthquake early Wednesday, about 60 miles north of Kathmandu.

“The Lhende Khola has flooded twice in 14 months, this time triggered by an ice rock avalanche from a glacier on the Nepal side that blocked the river and released a sudden surge downstream,” said Saswata Sanyal, a disaster risk reduction specialist at the climate research group.

He noted the importance of early warnings in “a warming Himalaya.”

Alton Byers, a faculty research scientist at the Institute of Arctic and Alpine Research at University Colorado Boulder, has studied glacial outburst floods and other alpine hazards in Nepal. Byers told NBC News this flood was the worst he’d ever seen.

“It’s just horrific. I’ve never seen a flood of this magnitude,” Byers said.

Byers also said it’s not exactly clear what happened yet, though initial reports suggest that an earthquake might have triggered an avalanche in Nepal, which somehow led to the release of floodwaters.

Permafrost is thawing in the alpine because of climate change, according to Byers, which destabilizes mountain slopes and makes dangerous landslides, icefalls and avalanches a growing risk to downstream communities.

The Himalayan region, which spans several countries, including India, is especially vulnerable to heavy rains, floods and landslides because it is warming faster than the rest of the world due to human-caused climate change. Research shows that extreme weather events, including heat waves, rains and melting glaciers, have become more frequent.

Researchers in Kathmandu found earlier this year that glaciers across the Hindu Kush Himalaya region are melting at an accelerating rate, with ice loss rates doubling since 2000, primarily due to rising global temperatures and climate change.