AMNA NAWAZ: Welcome to the "News Hour."

The scale of the disaster in the Himalayas is becoming clearer tonight after catastrophic flooding tore through parts of Nepal and neighboring Tibet.

Scientists now believe the disaster was likely triggered by the collapse of a glacier high in the mountains, unleashing a torrent of water, mud, and debris that swept away bridges, roads, and entire communities.

GEOFF BENNETT: President Trump today called the devastation in Nepal brutal.

The death toll stands at least 390, with more than 1,400 others still missing.

That's as rescue crews race to reach survivors in some of the world's most difficult terrain.

In the valleys that carve through the highest peaks in the world, villages are now buried in mud.

This is Nepal's Nuwakot district.

Yesterday's floods left behind a layer of sludge so high, it reached rooftops and trapped trucks as if in quicksand.

Survivors recounted the terror as it all unfolded without warning.

JANAK BAHADUR, Flood Survivor (through translator): It came in suddenly and exploded like a bomb.

The river looked small from a distance, but it suddenly started overflowing and swept through this place.

GEOFF BENNETT: Geological experts believe a glacier collapse some 30 miles north of Kathmandu was likely what caused the disaster.

These satellite images show the glacier along those snowy peaks there at the bottom before the collapse.

And this is what it looks like now, the surrounding rivers swollen by the floodwaters and debris rushing downstream.

Cell phone video showed the torrent starting to barrel toward this hydroelectric dam.

Within seconds, it bursts through the other side and engulfs the entire site, this border crossing between Nepal and Tibet, the autonomous region in China, now completely destroyed by the ensuing landslides.

Nepalese officials say the floods have swept away dozens of bridges and damaged some 25 miles of roads.

That, combined with the already treacherous terrain of the Himalayas, have made rescue efforts especially difficult.

Nepal's army says it's airlifted more than 100 people to safety using helicopters.

On the ground, crews trudge through knee-deep mud with the aid of search dogs to locate the missing, hoisting them onto their backs, many survivors transported to hospitals in Kathmandu.

But as the search grinds on, many families have no choice but to wait for news they fear may never come.

SUBINA TAMANG, Wife of Missing Man (through translator): All I want is that my husband comes back to me safely in one piece.

I do not want anything else.

CHAMINDI SHRESHTHA, Mother of Missing Man (through translator): My son is stuck in the floods.

He is a truck driver.

And he said he was driving.

There is no news of him.

We are not able to find out anything.

GEOFF BENNETT: The scale of this catastrophe is being felt across the region.

Officials in China say they're searching for hundreds of people still missing in Tibet, where floodwaters stretched for miles across the vast plateau.

Authorities said a new lake was formed in the deluge.

A spokesperson for China's Foreign Ministry vowed to send aid to Nepal, which has significantly fewer emergency resources than its neighbor.

LIN JIAN, Chinese Foreign Ministry Spokesperson (through translator): The mudslide has caused massive casualties and left many missing in both China and Nepal.

The two sides are having close communication and coordinating efforts for search-and-rescue.

GEOFF BENNETT: Among the missing, some 500 foreign nationals.

The U.S.

State Department this morning confirmed three Americans had been rescued, but as many as 90 may have been affected by the floods.

SADHGURU JAGGI VASUDEV, Indian Spiritual Leader (through translator): I wish to bring to your notice that there has been a terrible... GEOFF BENNETT: In an emotional message today, the Indian spiritual leader known as Sadhguru said a group had been on a holy pilgrimage in the area and got caught in the floods.

He said 22 of them were U.S.

citizens.

And for more on what caused the devastating floods, we're joined now by Dan Shugar, who heads the waterSHED Lab at the University of Calgary.

Thanks for being with us.

DAN SHUGAR, waterSHED Lab Director, University of Calgary: Thank you.

GEOFF BENNETT: So scientists believe the deadly flood in Nepal may have started when a massive chunk of ice snapped off a glacier high in the Himalayas, unleashing this devastating cascade into the valley below.

Walk us through how that might have happened.

DAN SHUGAR: That was certainly our initial thought as to what happened based on the information that we had yesterday from some preliminary satellite information.

It's changed a little bit since then, but the general story is still the same.

At about just under 5,000 meters elevation is a glacier that in the satellite imagery that we saw yesterday essentially two days ago was there, yesterday was gone, and the flood resulted downstream.

Since then, we have found that glacier ice - - chunk of glacier ice that essentially disappeared downstream was part of a much larger mass of bedrock, so a gigantic chunk of the mountainside that collapsed, essentially exploding as it descended a vertical kilometer to the valley bottom, melting all that ice, and then flowing out and generating the floods that we saw in those horrific videos.

GEOFF BENNETT: What makes that part of the Himalayas so vulnerable?

DAN SHUGAR: That's a good question.

I suppose it would be a couple of things, one, incredibly steep topography and then huge relief, so massive differences between the low points and the high points, lots of glaciers, lots of glacial lakes, which weren't an issue in this particular disaster, but certainly in other ones.

A lot of seismic activity.

The Gorkha earthquake wasn't that long ago, and we saw very similar sort of disasters unfold, in fact, just on the other side of this mountain.

And the human part of that equation, I think, is important.

Unlike in Western North America, in much of the Himalaya, you have settlements in these incredibly high, narrow valleys that essentially are putting people and infrastructure and livelihoods at risk when these landslides or these floods come down.

GEOFF BENNETT: The other human element at play, it strikes me, is climate change.

What role does climate change play here?

What can scientists say with confidence about the connection between rising temperatures, retreating glaciers, and disasters like this one?

DAN SHUGAR: We can speak in generalities, because it's a little bit hard to speak about the specific event just yet because we haven't done any sort of attribution study.

As temperatures rise, glaciers will melt, will thin vertically, destabilizing the valley walls that they formerly were sort of help -- hoping -- or helping to hold up.

Additionally, as air temperatures rise, the permafrost is changing, and so permafrost is just ground that remains frozen year-round year after year.

But in some areas, both at high altitudes, but also in the north, permafrost is destabilizing because of warming temperatures.

And that can cause large landslides like this as well.

And then a third factor, I think would be changes in precipitation patterns, not only in terms of intensity, but in terms of when during the year they occurred.

Water can get underneath the glaciers, allowing them to slide a little bit more rapidly, any number of ways that that extra water can sort of enhance or increase the likelihood of one of these kinds of events.

GEOFF BENNETT: Dan, what lessons can be learned from this disaster?

DAN SHUGAR: I think that these landscapes, these steep mountain landscapes, are continually evolving due to climate change.

And the global community, not just the government of Nepal or China or India, need -- the global community, we need to be stepping up to help mitigate these risks and to enhance the resiliency of people living in these environments that are subject to these disasters that, when there's a climate change link, we in the rich West are largely responsible for.

GEOFF BENNETT: Dan Shugar with the waterSHED Lab at the University of Calgary, thanks for joining us this evening.

DAN SHUGAR: Thank you.