Cross-Sensor Characterization of the 26 August 2026 Nepal High-Mountain Hazard Cascade Using Landsat and Sentinel-1

On 26 August 2026, a mass-movement and flood cascade affected the Nepal–China border region and propagated downstream through the Lende Khola–Trishuli river system. Rapid characterization of such high-mountain events is challenging because optical observations may be limited by cloud cover, while synthetic aperture radar (SAR) amplitude changes can reflect multiple surface and geometric effects. This study combined pre- and post-event Landsat 8/9 imagery with multi-temporal Sentinel-1 VV observations to determine whether the 2026 disturbance produced a spatially coherent radar response. A 5.52 km2 affected-area footprint was independently delineated from Landsat imagery and used to compare Sentinel-1 changes inside and outside the mapped disturbance along a 14.4 km valley corridor. A spatially matched 2025 sequence, which included a documented GLOF, was used to characterize spatial variability during a separate observation period. Within the mapped footprint, median absolute VV change during the 2026 event was 2.19 dB, compared with 1.66 dB outside, while median event anomaly was 0.56 and 0.06 dB, respectively. The inside–outside anomaly contrast was positive in 27 of 29 profile segments. After adjustment for the corresponding 2025 spatial contrast, the reference-adjusted spatial contrast remained positive in 25 of 29 segments, with a median of 0.344 dB. Direct recomputation of SAR pixel statistics over a common 14 km profile yielded positive reference-adjusted spatial contrasts in 85.7%, 92.9%, and 85.7% of units at 500 m, 1 km, and 2 km scales, respectively. The results indicate that the optically mapped 2026 disturbance coincided with a predominantly positive Sentinel-1 spatial contrast across the examined scales, although its magnitude and spatial prevalence depended partly on the aggregation procedure. A supplementary cross-orbit analysis showed positive inside–outside anomaly contrasts in both ascending (0.499 dB) and descending (0.251 dB) Sentinel-1 observations over common valid-pixel coverage. However, the descending acquisitions jointly covered only 59.9% of the Landsat-mapped footprint, limiting the spatial representativeness of this comparison. The proposed cross-sensor framework provides a simple approach for rapid characterization of high-mountain cascading hazards without interpreting SAR amplitude change as a direct event inventory.