Ice-patch collapse and early-warning implications from a Himalayan flash flood: emerging cryo-hydrological hazards under deglaciation

Emerging cryo-hydrological hazards linked to deglaciation are increasingly affecting high-mountain environments, yet their precursors remain poorly constrained. We investigate the 5 August 2025 flash flood at Dharali, Uttarkashi (Central Himalaya), and present evidence that it was triggered by the rapid collapse of an exposed ice patch within the nivation zone of the Srikanta Glacier. Integration of high-resolution digital elevation models, multi-temporal satellite imagery, and publicly available video records enabled reconstruction of the event chronology and ridge-to-valley hazard propagation. Pre-event imagery during the ablation period revealed exposed ice patches on steep north- to northeast-facing slopes, indicating thinning seasonal snow and firn cover consistent with ongoing deglaciation. Post-event observations confirmed complete ice-patch disappearance and the formation of fresh downslope erosional scars. Conservative estimates of ice volume, gravitational potential energy, and meltwater yield, combined with steep longitudinal gradients and channel confinement, explain the short-duration, high-intensity debris-laden surge that produced severe downstream impacts. These findings identify ice-patch collapse in nivation zones as an under-recognized cryospheric hazard and demonstrate the value of integrated satellite and terrain analysis for precursor detection and early warning in rapidly deglaciating mountain catchments.