Abstract

Climate change is altering sediment dynamics of mountain fluvial systems, with implications for sediment transport, connectivity, and geomorphic responses. The semi-arid Andes of central Chile provides a natural laboratory to study these processes under the influence of the current megadrought and extreme hydroclimatic events. This study investigates the provenance of sediment transported along the Maipo Alto Basin (33 degrees S) by analyzing 87Sr/86Sr and eNd compositions from sediment samples collected during two seasons of high discharge (spring and summer of 2019-2021). Additionally, an extraordinary precipitation event in January 2021 that triggered widespread debris flows, provided a unique opportunity to evaluate shifts in sediment sources and transfer pathways. Results indicate that under current hydroclimatological conditions, sediment is sourced from a restricted area dominated by rocks of Quaternary volcanic and Mesozoic formations located in the upper reaches of the basin, mainly glacierized catchments. This provenance signal indicates that glacial meltwater plays a relevant role in sediment mobilization, enhancing connectivity between subglacial sources and the fluvial network. However, the extreme summer precipitation event altered this dynamic by mobilizing sediment from lower-altitude hillslopes, dominated by rocks of Cenozoic and Mesozoic formations. The rapid transfer of debris flows to the channel network significantly changed sediment composition within the basin, highlighting the role of high-magnitude, low-frequency events in modulating sediment connectivity. The findings underscore the role of the megadrought in sediment generation and transfer mechanisms, with increased dependence on glacial sources. Future climate scenarios predict intensified storms and changes in seasonal runoff patterns, suggesting that sediment transport will become more event-driven, challenging infrastructure, water resources, as well as geomorphological and ecological stability along the Maipo River.