Base image before the flood: Esri World Imagery · mud channel and buildings: measured on 11 October
Before dawn on 11 October, a blocked mountain stream in the Tsum valley gave way. A torrent of mud and rocks ran about 65 km down the Syar Khola and the Budhi Gandaki to Arughat. We read the open satellite data of that morning and every mapped building along the river.
Authorities provisionally blame a landslide that dammed a stream between Lokpa and Chumling, in the Tsum valley. The exact cause is still under study. The five people missing are herders from Chumchet.
Nepal's National Disaster Risk Reduction and Management Authority (NDRRMA), as of 13 October; trekkers: Manaslu Conservation Area Project.
Times in Nepal time.
A picture shows a flood. A response team needs to know where to check first. So every measure on this page is set against the same measure on a normal day.
A building near the channel is a place to check, not a confirmed loss. The authorities' counts and the teams on the ground come first.
1.8 km² of it was green on 6 October: forest and fields stripped or buried. 15% of the river corridor was under cloud, so this is a lower bound.
Mapped buildings within 20 m of the mud channel. On 6 October, 29 stood that close to the river. About 120 of them are between Salleri and Machhakhola.
Share of the river from Nyak to Jagat that changed sharply on radar at 06:03, during the flood. Between two normal passes, 2.5%. Same pixels, same masks.
Before and after, the mud channel and the radar pass, on the real relief, with every mapped building along the river. Follow the flood stop by stop, or fly it yourself.
Explore the valley in 3DOpen · about 6 MB
| Place | 6 Oct | 11 Oct | Reported |
|---|---|---|---|
| Sallerimay include the hydropower camp | 3 | 64 | Budhigandaki hydropower camp damaged |
| Machhakhola | 2 | 19 | |
| Jagat | 3 | 18 | about 115 trekkers were here |
| Aruarbang | 0 | 13 | |
| Sirdibas | 0 | 12 | |
| Dobhan | 1 | 11 | hydropower camp and Bailey bridge lost |
| Arughat Bazar | 5 | 7 | gauge and bridge swept away |
Mapped buildings within 20 m of the river on 6 October and of the mud channel on 11 October.
Early counts report 45 houses destroyed and 27 damaged from Ghattekhola to Machhakhola: the stretch where our map finds most of the buildings at the mud's edge.
Downstream, NDRRMA counts 64 damaged homes in Arughat and 58 in Tripurasundari (Dhading). Few buildings there stand at the channel on our map, so our measure misses most of that damage: it sees the channel, not the water and mud that reached houses beyond it. Our map points where to look first. It does not count damage.
The source area was under cloud on 11 October; it is shown as it was on 6 October. Sentinel-2 sees 10 m: a channel, not a house. The image of 10:56 caught a flood still running, so the channel is a snapshot, not its peak. Buildings are those mapped by OpenStreetMap volunteers; new or unmapped ones are missing. The radar reads a steep gorge, where slopes hide parts of the river from the satellite. The next NASA–ISRO NISAR pass over the valley is on 15 October.
Every analysis starts from a multi-source gather: hazard monitors, satellite imagery, population and food-security data, and our own measurements. Ask for one on your next event.
Sources: Nepal's National Disaster Risk Reduction and Management Authority (homes, households, missing, rescued; as of 13 October) · Department of Hydrology and Meteorology (Arughat gauge, flood onset) · Manaslu Conservation Area Project (trekkers) · contains modified Copernicus Sentinel data 2026 (Sentinel-2 L2A, 6 and 11 October; Sentinel-1) · OPERA RTC-S1, NASA JPL, distributed by the Alaska Satellite Facility DAAC (11 October 06:03, 29 and 17 September) · Copernicus DEM GLO-30 © DLR e.V. 2010–2014 and © Airbus Defence and Space GmbH 2014–2018, provided under COPERNICUS by the European Union and ESA · buildings, rivers and places © OpenStreetMap contributors, ODbL · opening image: Esri World Imagery (Esri, Maxar, Earthstar Geographics and the GIS User Community), taken before the flood, at true relief. Mud channel, buildings near it and radar change: Watchfloor measurements. Relief in the explorer and the film exaggerated 1.5 times; white on the relief: snow or cloud.