The role of multiple disturbances in the morphodynamics of an alluvial fan in the north Patagonian Andean forests, Argentina

Abstract

Alluvial fans have the potential to record climatic and neotectonic events. These depositional systems, characterized by distributary river patterns, are more frequently studied in arid environments due to better exposure and easier accessibility. In this study, we applied dendrogeomorphic methods on an alluvial fan within the wet temperate north Patagonian Andean forests to reconstruct the environmental history of the Derrumbe Creek Alluvial Fan. The specific objectives were: (a) to date the activation/inactivation of different ecogeomorphic units in the alluvial fan, and (b) to interpret the relative importance of various disturbances that have shaped its recent history. The Derrumbe Creek Alluvial Fan is located in the southern bay of Puelo Lake, in the northwest of Chubut Province, Argentina. We carried out two fieldwork campaigns for tree data collection and geomorphic analysis. Using standard dendrochronological laboratory methods, we obtained establishment dates for the different geomorphic units of the alluvial fan and developed a disturbance chronology. Three main disturbance events were identified: the first in the early 1920s, clearly associated with a fire; the second between 1957 and 1959, linked to sediment deposition (colmatation) and lateral migration of the active channel; and the third between 1961 and 1963, related to an outburst flood and avulsion of the active channel. We propose that the latter two disturbance events were facilitated or conditioned by the earlier fire, emphasizing the significance of fire history in the geomorphic evolution of the alluvial fan. Our findings highlight the role of multiple disturbances in the complex dynamics of alluvial fans in the southern Andes.