It was already too late when the call came.
Rajan Khatri, 41, a police officer, was working at his station in a mountain village in northern Nepal when a friend rang on Wednesday morning to warn that a flash flood was thundering downriver in his direction. As soon as he hung up, giant waves slammed into the station.
“I cannot explain how big the waves were,” Mr. Khatri said from a hospital bed in Nepal’s capital, Kathmandu, on Thursday. “They engulfed several meters-high buildings in a fraction of seconds. It wasn’t just water. It was a combination of boulders, sand and thick sludge.”
The onslaught flung Mr. Khatri out of the building, and he lost consciousness and suffered chest injuries. Two of his colleagues survived, but four were missing on Thursday.
They were among the more than 1,400 people missing in the devastating deluge of rock, ice, trees, mud and water that blasted through river valleys on the border between Nepal and Tibet, washing away bridges, smashing homes, schools and hydropower plants, and swallowing people who tried to run away.
On Thursday, emergency workers climbed across steep Himalayan valleys and used helicopters to ferry shocked survivors to safety as they carried out a sprawling search for anyone who had managed to escape the flood.
Nepali officials said at least 389 people had been killed, while the authorities in China, where far less information was being released, reported three deaths in Tibet.
The death toll was expected to rise.
In Nepal, more than 900 people, including about 500 foreign visitors, were unaccounted for, the country’s disaster management agency said. The missing foreigners included at least 288 from India, as well as travelers from the United States, Australia, Britain, Canada and Malaysia.
In Tibet, at least 558 people were missing in Gyirong County, including 260 foreigners, Chinese state media reported. China’s foreign ministry said that 100 Chinese citizens were missing in Nepal.
The State Department said in a statement that about 90 U.S. citizens “may be impacted by the floods” and three had been rescued in Nepal. It said it was not aware of any Americans who had died.
Survivors described having almost no time to react before giant waves came roaring down the Bhotekoshi and Trishuli Rivers.
Smriti Ojha, 17, said she had been getting ready for school on Wednesday morning when she felt the ground shaking. She ran out of her house, believing an earthquake had struck. Then she saw the flood. She said her immediate family had survived, but four of her relatives were missing.
“It was terror,” Ms. Ojha said from a hospital bed in Kathmandu. “I cannot believe everything was swept away before my eyes.”
Officials and aid organizations were still assessing the scale of the destruction in the remote region, renowned for its high-altitude lakes and steep peaks.
New York Times journalists traveling with the Nepali military on Thursday surveyed the devastating aftermath. One highway, near the Bhotekoshi River, had mostly disappeared. Scars on hillsides showed that in some places, the river had risen more than 650 feet during the flood.
Rescue efforts were being hampered by communication lines that had been cut and the destruction of dozens of bridges and about 25 miles of roads, Nepal’s government said.
The Nepali Army’s search-and-rescue base was packed with hundreds of army personnel and survivors, some of whom wept after arriving by helicopter. The area had no electricity or phone networks after the flood, and soldiers and other rescuers were using military radios to communicate.
Nepal’s foreign minister, Shisir Khanal, called it “one of the largest natural disasters we have experienced in recent memory.”
President Trump, speaking in the Oval Office, said he had offered U.S. aid to Nepal. “We are sending aid, and we’ve spoken to the leadership, and we let them know anything they want will happen,” he said.
Scientists piecing together satellite imagery and seismological data said that a high-altitude landslide coinciding with a glacial collapse most likely caused the flash flood. The collapse happened on the north face of Langtang Lirung, a Nepalese mountain near the border with Tibet, at an elevation of around 17,000 feet, scientists said.
After a huge chunk of the mountainside broke off, it sent millions of tons of rock and ice crashing into the river valleys below, said Kristen Cook, a geomorphologist at the Université Grenoble Alpes in France.
“It was a much larger collapse than we were initially able to see in the satellite imagery,” she said.
The shaking caused by the falling rock and ice was so powerful that the U.S. Geological Survey registered it at a magnitude of 5.2, as if it were a moderately powerful earthquake. Warming temperatures caused by climate change may have destabilized the glacier and rock face, scientists said, adding that it was too early to determine a precise cause.
“Earthquakes, landslides, slope failures all could happen even without climate change,” said Austin Lord, an anthropologist and senior fellow at the Stimson Center in Washington who has studied natural disasters in Nepal. “It’s just that climate change is this additional factor that increases the likelihood, scale, frequency and intensity of these events.”
Nepal has a flood warning system, but it relies on a conventional network of river and lake gauges that was not equipped to alert people to the sudden collapse of part of a mountain, according to scientists and disaster response researchers.
The river gauges are designed instead to detect slower-rising water levels that typically precede a monsoon-driven flood or glacial lake outburst, which occurs when a pool of glacial meltwater bursts its banks.
“We didn’t have the right information at the right time, so we couldn’t save more people,” said Basanta Raj Adhikari, director of Tribhuvan University’s Centre for Disaster Studies in Nepal. “The Nepalese government is very good on monsoon floods and perennial floods, but we were not prepared for this fast-moving flash flood,” he added.
Reporting was contributed by Mujib Mashal, Laura Chung, Robin George Andrews, Erica L. Green, Michael Crowley and Judson Jones.
