Here is what experts believe happened so far:

- Glacier collapse -

A glacier collapse in Nepal's mountains near Tibet appears to have triggered the disaster.

"A large chunk of glacier fell from about 5,200 metres (about 17,000 feet)... into a valley," said Simon Cox, principal scientist at New Zealand's GNS Science.

It "transformed into this raging mass of debris... where it picks up rocks and sediment and trees and anything in the way and fires it on down through the canyon," he told AFP.

The wall of water first impacted a border post, as documented in terrifying CCTV footage showing mud and debris engulfing the area as people fled before it.

It then travelled downstream into Nepal, flooding villages and bringing down bridges and buildings.

- Seismic activity -

The glacier collapse was large enough to register as a seismic event, initially mistaken for an earthquake.

But after further analysis, the US Geological Survey determined "that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred.

The collapse registered as a 5.2-magnitude "seismic event", which took place at 8:37 am local time (0252 GMT).

Estimating just how much of the glacier collapsed will take time, Cox said, but it could range from "half a million cubic metres to tens of million cubic metres."

- Detective work -

With the affected area largely inaccesible, experts are using satellite imagery and any available measurements on the ground to reconstruct the disaster.

"We can look at satellite imagery taken before and after the event, and try and see what has changed, essentially looking for what is missing from the mountains in the 'after' imagery," said Lauren Vargo, a glaciologist at New Zealand's Victoria University of Wellington.

Soon after the seismic activity was recorded, a nearby water monitoring station stopped transmitting, Nepali authorities said.

Stations downstream began going offline one by one, though water was registered reaching up to 12.3 metres high in Kalikhola, near the Tibet border at 2:14 pm, they added.

- GLOFs -

Early speculation suggested a glacial lake outburst flood, or GLOF, a growing risk as glaciers melt due to climate change.

A GLOF occurs when a fragile debris wall holding back a lake of glacier melt suddenly collapses, unleashing the water behind it.

Some experts suggested a GLOF might have been involved.

"Water does flow within glaciers and flows beneath glaciers," said Adrian McCallum, a glaciologist at the University of the Sunshine Coast.

"But to my mind, the only reason why a large volume of water should all of a sudden flow down a valley is because it has been released," he told AFP.

Other experts said they did not detect existing glacial lakes near the site of the glacier collapse, and theorised that water collected under the glacier might have cascaded down with the falling chunk.

The falling glacier ice could also have briefly dammed the river, collecting water behind it that was released when it dislodged.

Summer glacier melt upstream and monsoon rains would mean the rivers were already high, and the volume of water was also swelled by debris and mud picked up as it travelled kilometres from the initial collapse, added Cox.

- Warnings -

Nepali authorities first received word of a flood around 9:00 am local time, and began alerting areas along the Bhote Khoshi river to "stay on high alert and stay in safe areas".

But is is unclear how many people received the warning, or how much time they would have had to respond.

"The reality is, if you are in a community underneath a large glacier and potentially a glacier lake where a GLOF could occur, then to some extent, it's a ticking time bomb," said McCallum.

And even for those receiving a warning, options would be limited, said Hatim Sharif, a hydrologist at the University of Texas San Antonio.

"Running will not help," nor will trying to flee by vehicle, he told AFP, because the mud and water moving at high speed form a lethal combination "like liquid concrete."

The only solution is to climb up a hill, at least 20 feet (six meters), he estimated.

- Climate change -

While it is hard to immediately attribute any individual glacial event to climate change, the warming planet is a constant background stress on these ancient ice rivers, expert say.

Glaciers are retreating globally, creating disaster risks but also depriving downstream communities of vital water sources.

"The biggest way to mitigate the risk would be to reduce our greenhouse gas emissions to limit warming," said Vargo.

burs-sah/jm