One slope failed. Everything downstream of it was in the path.
A reconstruction of the Nepal flood — from the collapse on Langtang Lirung to the lakes still being watched upstream.
01The chain of events
A section of glacier and the bedrock beneath it breaks away high on the northern side of Langtang Lirung.
The mass becomes an ice-and-rock avalanche and plunges into the Lhende River basin.
Debris crashes into the valley floor, and water, mud and boulders accelerate downstream as a single flow.
The flow enters the Bhote Koshi, then the Trishuli.
Villages, roads, bridges and hydropower infrastructure along the corridor are destroyed.
~1,200 m
Vertical descent of the collapsed ice-and-rock mass into the Lhende basin — covered within seconds. That is close to a quarter of the height it started from.
Copernicus
02Where it went
?Collapse zone~5,127 m
?Timure—
?Syapru Besi—
?Hakubesi—
?Betrawati—
?Trishuli—
?Devighat—
?Kolpurtarbelow 500 m
~100 km
Distance travelled downstream USGS
~50 m/s
Debris-flow speed estimate, about 180 km/h Expert estimate cited by Reuters
03How fast the river rose at Galchhi
0 MIN
Normal
15 MIN
Rising
30 MIN
+9 m
Only the half-hour figure is reported, and it is an upper bound — a rise of as much as that. The intermediate step is drawn to show the shape of the surge, not a measured level. ICIMOD, cited by Reuters.
04What the imagery counted at Syapru Besi
?
240+ buildings destroyed, 32 damaged
Counted in the single mapped area around Syapru Besi. Elsewhere along the corridor, homes, bridges, roads, power stations and border infrastructure were carried away. These counts are imagery-based and subject to further validation — Copernicus rapid emergency assessment.
05The earthquake that wasn't
NOThe event was not an earthquake, though the seismic signal it produced was first reported as a magnitude 4.4 quake.
YESUSGS later attributed the signal to the collapse and debris avalanche itself, releasing energy equivalent to about a magnitude 5.2 earthquake. The ground shook because the mountain moved, not the other way round.
06Was it climate change?
What is known
Warming melts glaciers and permafrost, and can destabilise high-altitude slopes.
What isn't
Whether that is what loosened this particular slope. No direct attribution has been made.
Climate change may raise the likelihood of this kind of mountain instability, but scientists say there is not yet definitive evidence linking this specific collapse to it. Kristen Cook and Dan Shugar, speaking to AP, caution explicitly against drawing that line now.
07Why the damage was so extreme
A relatively open mountain corridor
Two decades of satellite imagery show how much has been added to the Lhende–Trishuli valley since.
Reuters analysis argues that development has packed critical infrastructure into a narrow mountain corridor with little room for a sudden surge. The physical event was extreme; the losses were extreme partly because of what had been built in its path.
08Why it isn't over
?
Two new lakes are being watched
99,000 sq m
The lake near the border by Saturday — roughly 14 football pitches, and 21,000 sq m smaller than on Thursday. It is draining.
Depth unknown
A second, larger lake further upstream. Its depth has not been established, so the volume behind it can't yet be judged.
09Why India is watching
Trishuli?Narayani / Gandak?Bihar
The river that carried the debris flow feeds the system that crosses into India. Around 540 foreign nationals are among the missing, Reuters reports, including Indian pilgrims.
Sources: Reuters · US Geological Survey · Copernicus (rapid emergency assessment and Sentinel-2 imagery) · ICIMOD · AP, quoting Kristen Cook and Dan Shugar. Figures are preliminary and, where noted, imagery-based estimates rather than measurements.