Causes, consequences and implications of the 2023 landslide-induced Lake Rasac glacial lake outburst flood (GLOF), Cordillera Huayhuash, Peru

Glacierised Peruvian mountain ranges are experiencing accelerated, climate-change-driven glacier ice loss. Peru’s second highest mountain range, the Cordillera Huayhuash, has lost about 40 % (∼ 34 km²) of its glacier cover since the 1970s. Newly exposed landscapes are prone to a number of hazard processes including the formation and evolution of glacial lakes, changing stability conditions of mountain slopes, and rapid mass movements. In this study, we integrate the analysis of meteorological data, remotely sensed images and field observations in order to document the most recent (February 2023) large mass-movement-induced glacial lake outburst flood (GLOF) from the moraine-dammed Lake Rasac. The GLOF was triggered by a mass movement from the failure of a part of an arête ridge (i.e. narrow ridge separating parallel glacial valleys) with an estimated volume of 1.1×10⁶ m³–1.5×10⁶ m³. This occurred in a rock zone where climate information – primarily from reanalysis data – indicates cold, deep permafrost and was preceded by several small-magnitude precursory rockfall events. The reduced stability of the frozen rocks in the detachment zone is most likely related to deep warming but not to especially critical conditions of warm permafrost with higher amounts of unfrozen water. Further, we describe the surprisingly short-distance process chain (attenuated by Lake Gochacotan, located 3.5 km downstream from the detachment zone) and analyse the transport of large boulders with the use of hydrodynamic modelling, revealing that flow velocities > 5 m s⁻¹ must have been reached in the case of translational motion and > 10 m s⁻¹ in the case of rotational motion of the largest transported boulders (diameter > 3.5 m). In addition, we analyse climate trends over the past 8 decades and meteorological conditions in the days prior to the GLOF, revealing a statistically significant temperature increase trend over the past 8 decades, as well as an atmospheric temperature rise and positive thermal anomaly in different soil layers before the event. We argue that climate change effects (warming air and permafrost temperatures) served to hasten the failure. This study helps us to understand (i) mechanisms, amplification and attenuation elements in GLOF process chains and (ii) frequency–magnitude relationships of extreme geomorphic processes that undergo alteration due to the rapidly changing high-mountain environments on a regional scale (both large-magnitude rockfalls and GLOFs). This study supports earlier work that indicated an increasing frequency of large mass-movement-induced GLOFs originating from the warming cryosphere in recent decades.