Diego Cusicanqui, Edwin Loarte, Wilson Gómez, Hilbert Villafane, Kristen Cook, Guillaume Piton, Benjamin Lehmann, Florence Magnin, Adina Racoviteanu, Swann Zérathe, Simon Filhol, Katy Medina, Rolando Cruz, Ronald Concha, Juan C. Torres-Lázaro, Pascal Lacroix
The Cordillera Blanca (Peru) is among the world’s best documented GLOF regions. Yet hazard prioritisation frameworks still focus mainly on large lakes (>0.1 km²), while the numerous small (<0.01 km²) and medium (0.01–0.1 km²) lakes remain comparatively underassessed. In Huascarán National Park, 67% of the 882 inventoried lakes are small and 26% are medium-sized. In this study, we reconstruct the 28 April 2025 Vallunaraju cascade event in the Casca valley, where a rockfall impacted two small lakes and triggered an outburst flood reaching the Huaraz–Independencia conurbation 14 km downstream. Documented impacts included two fatalities and damage to five bridges, twenty-nine houses and water infrastructure. Although the modest initial lake-water release (~42 × 10 3 m 3 ), progressive channel-bed entrainment increased the total moving mixture volume by ~6–7 times to 296 × 10 3 m 3 . Depending on plausible drainage scenarios, entrained sediment accounted for ~73–84% of the total mixture volume, whereas lake water contributed ~13–15% and background discharge ∼ 1–5%. We interpret Vallunaraju as a sediment-amplified cascade showing that small-lake outbursts can generate severe downstream consequences where triggers, abundant erodible sediment and exposure coincide, supporting integrated assessments that include small lakes, trigger susceptibility and propagation and exposure controls.