A steadily flowing glacier and no detectable precursor before the 11 October 2026 Budhi Gandaki rock–ice avalanche and flood, Nepal

Ajay Kumar Sah 

On 11 October 2026 at about 04:20 local time, a rock–ice avalanche at the head of the Ungilung Khola, on the southern slopes of Chamar (7,165 m) in Gorkha District, Nepal, became a debris flow and flood that travelled down the Syar Khola and the Budhi Gandaki. I reconstruct the event from free satellite data (Sentinel-1, Sentinel-2, Landsat 8/9, ITS_LIVE and NISAR), official records and my own reading of historical Google Earth imagery, in the four days after the event, before any clear 10 m image. Each change test is compared with a control, where possible a fake event made from images taken before the event. The change mapped at the source on 11 October happened between 6 October 10:41 and 11 October 10:42 local time. A river gauge 23 km downstream recorded no flood pulse between 5 and 11 October. On 13 October, Landsat 9 shows a 0.31 km² block of snow loss at 5,020–5,470 m on an unnamed valley glacier, absent in the fake test, and a candidate avalanche track at 6,350–6,600 m above face B; face A is unchanged. The glacier had flowed at about 60–130 m yr⁻¹ for a decade, and no speed-up or other precursor was detected. Newly public L-band interferograms from NISAR kept face B within about ±2 cm along the line of sight in every pair from May to 7 October, and showed nothing unusual on the hanging glacier above it in the pair that ended 9.5 hours before the collapse. Radar at 06:03 recorded a fresh rough surface, most likely deposit or debris-laden water, 0.5–1.5 km up the Syar Khola from its confluence with the Ungilung, where the blockage and a 1.5 km backwater were reported. The flood reached Arughat, 63 river km downstream, in at least three rises, the largest last. The source scar, volume and any chain of small lakes remain unmeasured until clearer images arrive.