Shortly after 8:30 on Wednesday morning, a wall of water, mud and rock came down the Lhende Khola river valley on the border between Nepal and Tibet. It washed away homes and buried entire villages. More than 350 people are confirmed dead and over 1,300 remain missing. At one point along the river, the water rose by as much as eight metres in half an hour.

It is the deadliest disaster the region has seen since the earthquake of 2015. The trigger has been reported as the collapse of a glacier upstream.

What kind of flood this is

Floods of this kind behave differently from the ones most people picture. A river in flood rises over hours and gives some warning. What happened on Wednesday is closer to a dam failure.

Glaciologists describe the mechanism roughly as follows. As a glacier retreats, meltwater pools behind the ridge of loose rock and ice the glacier has pushed ahead of it. That ridge is not engineered; it is rubble. If part of the glacier or the slope above collapses into the pool, or if the ridge gives way, the whole volume can release at once. The water picks up boulders, gravel and trees as it goes, so what arrives downstream is not water but an abrasive slurry heavy enough to move buildings. Researchers call these events glacial lake outburst floods.

The speed is the killing feature. An eight-metre rise in thirty minutes leaves no time to move uphill if you are asleep, indoors, or on the valley floor where the flat ground — and therefore the housing, the road and the fields — happens to be.

Why this valley

The Himalayan border corridors between Nepal and Tibet share a set of features that make them exposed. The valleys are steep and narrow, which concentrates flow instead of spreading it. Settlements, roads and small hydropower installations cluster along the river because there is nowhere else level to build. Many of the glacial lakes upstream sit inside China, while much of the population at risk is downstream in Nepal, so monitoring and warning have to cross a border to be useful.

Scientists studying the region have warned for years that the number and size of these lakes has grown as glaciers have retreated, and that the hazard is therefore increasing rather than holding steady. That is a statement about the general trend. Attributing any single event to that trend takes months of investigation, and researchers are careful to distinguish the two.

The numbers will keep changing

Reports on Thursday carried significantly different figures — one wire service put the death toll at 160 while others reported more than 350. That is normal at this stage and is worth explaining rather than ignoring.

Official death tolls generally count bodies recovered and identified. Other tallies include deaths reported by local authorities or families but not yet confirmed. “Missing” is looser still: it can mean someone whose home was destroyed, someone who has not answered a phone in a valley where the phones no longer work, or someone counted twice under two spellings of their name. In past disasters, a substantial share of the missing have turned out to be alive, having walked out or sheltered elsewhere. That is a reason to treat 1,300 as a measure of how little is known rather than as a forecast of the final toll.

Recovery in a valley like this is slow for practical reasons. The road is usually the first thing the flood destroys, so heavy equipment cannot reach the worst-hit villages. Debris is metres deep. Helicopters depend on weather that in late August is often against them.

What can be done about it

The engineering answers are known and are not cheap. Dangerous lakes can be lowered by siphoning or by cutting a controlled channel through the moraine, which has been done at a handful of sites in Nepal. Sensors upstream can trigger sirens downstream, buying minutes — which in a flood that arrives in half an hour is the difference between a village that got up the hillside and one that did not.

The obstacles are money, maintenance and jurisdiction. A siren network only works if someone services it for decades. A cross-border warning system only works if two governments agree to share data quickly, in the middle of the night, without a diplomatic process. Both of those are harder than the physics.

Sources: Democracy Now; Havana Times; Anadolu Agency. Casualty figures are provisional and are expected to change.