Rockfall-induced debris flow event in the Pyrenees: The case of Pineta valley of September 2022

• Post-event reconstruction of an extraordinary debris flow event was conducted in the Pyrenees, Spain. • Previous rockfall activity in the upper part of the catchment increased debris flow susceptibility. • Tree-ring records do not show evidence of similarly large events in recent decades. • Debris flow susceptibility in the analyzed supply-limited catchment has changed throughout the 20th century. • Analyzing sediment inputs and their connectivity with the channel is essential for predicting future similar events. Debris flow activity is expected to shift under global warming scenarios. However, linking changes in the frequency and magnitude of these events to climate change is challenging, especially in supply-limited catchments where sediment availability is a critical factor. Here, we reconstruct past and recent extreme debris flow events in a deglaciated catchment in the Central Pyrenees, where a massive debris flow event occurred in September 2022. By conducting detailed field-based, remote sensing and climate analyses, as well as examining tree-ring records and performing numerical models, we provide insights into the magnitude and triggering mechanism of this recent event and contextualize it within the 20th century. Our findings revealed a significant flow magnitude for the 2022 event (peak discharge ∼ 650 m 3 /s; catchment area ∼ 10 km 2 ). This event was linked to an intense, but relatively common rainfall in the Pyrenees (27.4 mm in 20 min). Thus, the debris flow magnitude seems to be enhanced by a rockfall that occurred two months earlier in the upper catchment (June 2022). Interestingly, tree ring records do not show events of similar magnitude in the catchment during the last six decades, but rather during the first half of the last century. The observed contrast in debris flow activity between the first (high activity) and second (low activity) halves of the 20th century, along with the potential role of permafrost thaw in causing a sudden sediment influx in the 2022 event, highlights the complex impact of climate change on debris flows in supply-limited catchments. These findings underscore the need for further research into how climate variability and change influence sediment availability in mountain catchments.