Transboundary outburst floods from giant supraglacial lakes over the Himalayan China–Nepal border

Supraglacial lakes are widespread on Himalayan debris-covered glaciers, yet their drainage, including glacial lake outburst floods (GLOFs), remains sparsely observed. Based on field surveys, remote sensing analysis, and interviews with local residents, we documented two exceptional supraglacial GLOFs that occurred along the China–Nepal border near Mt. Xixabangma in 2025, resulting in severe transboundary impacts. On 7 July 2025, a giant supraglacial lake (0.64 ± 0.02 km2 in area, 11.8 × 106 m3 in volume) on the Nielangpu Glacier, 24 km upstream of Nyalam county, drained through an englacial conduit following the hydrofracture of its ice dam, releasing ∼3.84 × 106 m3 of water over ∼30 h. Inefficient englacial drainage produced a low-magnitude GLOF that caused no measurable downstream river channel erosion, nor damage to settlements. Tragically, On 8 July 2025, a second supraglacial GLOF occurred on the Purepu Glacier, located 16 km northwest of Nielangpu. This lake of comparable size (0.68 ± 0.03 km2, 9.09 × 106 m3) released ∼7.22 × 106 m3 of water within ∼35 h. The high-energy outflow initially traveled over the glacier surface and a downstream gorge, triggering an extreme hydrological response. Severe loss of life, infrastructure damage, and channel erosion were caused by this event, and inland transportation between China and Nepal was disrupted. Both the failed lakes were at least an order of magnitude larger than typical Himalayan supraglacial lakes. Their formation and eventual outburst reflect the combined influence of short-term weather forcing and longer-term climatic drivers, probably acting through glacier-wide differential ablation, tributary-induced dynamic blocking, perturbed englacial drainage networks, rapid meltwater impoundment, and powerful hydrofracturing. A full understanding of these unprecedented GLOF mechanisms highlights the urgent need to strengthen supraglacial lake monitoring and pertinent risk management within existing frameworks.