Geomorphological evidence of a historical Landslide-dammed Lake Outburst Flood in the Cajón del Bayo basin, Southern Central Andes (36°S)

Landslide dammed lakes are created when mass movements block river valleys, posing potential hazards in the form of Landslide-dammed Lake Outburst Flood (LLOF). In the Southern Central Andes (SCA) of Argentina (36°S), a rock avalanche dammed the Cajón del Bayo basin in 1986, creating Baya Lake. This study reconstructs the historical evolution of the basin, focusing on key geomorphological changes including the rock avalanche, lake formation and subsequent outburst flood. The analysis included: a) a geomorphological mapping, identification of outburst flood deposits and remote sensing of landscape evolution; b) sedimentological description of landslide facies and lake-level variations; c) landslide-dam characterisation and stability assessment; and d) a hydrometeorological analysis of temperature and precipitation to contextualise the LLOF event. Results show that a 4.7 hm³ rock avalanche impounded the Baya Lake. Geomorphic stability indices yielded contrasted results: while one classified the dam as stable, another indicated instability. Ultimately, the dam collapsed, triggering a LLOF that released 0.14 hm³ of water from the lake and 0.3 hm³ of sediments eroded from the dam, exposing its internal structure. Although geomorphic impacts were evident immediately downstream the lake, the event was not detected at the nearest gauging station, and a regional flood frequency analysis indicate low probability of occurrence of the empirically estimated peak discharge. This highlights how LLOFs occurred in remote areas may go unnoticed and how the increased discharge from small tributaries, though significant locally, can be attenuated in larger rivers. These findings underscore the importance of identifying and studying LLOFs at both local and regional scales in the SCA, where such events are underreported and may differ from global cases.