Down stream of a glacier, in the run off water shed.

As I've heard tell, this was a glacial collapse into and blocking a river, which then built up a head of pressure behind it, bursting to release a hammer blow of river water and debris that travelled a great distance.

Similar things can happen due to landslides into rivers; it depends on the topology of the land, here long narrow valleys carved out by heavy water flows.

The warning being, don't live in a natural "drainpipe".

In mountainous regions that's often not much of an option, there's a lot of vertical sides and valley bottoms.

The specific geological cause is not yet clear.

We do know that a glacier substantially collapsed.

What contributed to the amount of water in the debris flow isn't clear. Speculation, from trained geologists, is that the friction of the glacier flow itself contributed to melt. I'm speculating that recent rains or higher temperatures might have contributed to melting on the glacier (and have seen some commentary speculating about this).

Speculating on my part: There might also have been accumulated water below the glacier, liberated by the ice flow, or perhaps groundwater which was evacuated along with other material. Events seem to have precipitated too quickly (roughly 7 minutes from galcial collapse to Gyirong Port inundation) for the collapse to have formed, then broken, a dam on tributary waterways upstream of the port.

GLOF is a recognised threat, but this event seems to have been at best only partially explained by that, and may be an entirely different mechanism. The follow-on studies should be fascinating, in the way that horrific disasters often are.

The rivers in that area are pretty small. You would have to block them for days before there is a risk of a massive flash flood.

From what I've understood, the collapsing glacier either contained enough liquid water or melted enough from the impact that it continued flowing downstream. It covered ~20 horizontal kilometers in about 7 minutes before hitting the border crossing.

It sounds like there may have been some opportunity for warning then? If the thing was more of a "build up" ?