The 26 August 2026 Langtang Lirung ice–rock avalanche and its transboundary cascade: a multi-source observational reconstruction in the Bhote Koshi basin

Donghua Xu 

On 26 August 2026, an ice–rock avalanche detached from the northern flank of Langtang Lirung (7,227 m) at about 5,200 m a.s.l., transformed into a debris flow that travelled roughly 22 km in 6–7 minutes, destroyed the Gyirong border port (China) and dammed the river; a flood then propagated a reported ~170 km down the Bhote Koshi–Trishuli–Narayani system into Nepal and India. At least 16 people died in China (546 missing) and 903 bodies were recovered in Nepal (4,247 missing; NDRRMA, 31 August). Using only open data (seismic records, Sentinel-1 SAR backscatter change, emergency mapping, and official reports from both countries), we reconstruct the full cascade and assess the origin of the initial flood wave: it reached the first inhabited stream gauge 12.8 min after the trigger, far too fast to be the overflow of the main barrier lake (began only on 28 August), and was generated by the avalanche-emplaced debris flow and/or a rapid release from the debris dam — indistinguishable with open data. SAR change detection identifies about 7.5 km² of strongly modified surface in the source sector; assuming 1–5 m affected-layer thickness, this corresponds to a first-order volume-equivalent scale of ~10⁷ m³, not a direct estimate of the detached source volume. ERA5-Land shows a modest warm anomaly (+0.59 K, July–August 2026 vs. 1991–2020). The event recurred at the same transboundary site 14 months after the July 2025 Gyirong glacial lake outburst flood, highlighting slope-scale monitoring and cross-border data sharing as priorities.