Process-landform relationships at variable spatio-temporal scales in the Southern Chaco fluvial megafans (Argentina)

The Chaco Plain (South America) is an aggradational lowland (40-600 m asl) with divergent drainage patterns, at present under a tropical–subtropical climate. Late Quaternary coalescent fluvial megafans constitute the most extensive depositional system of the Central Andean foreland basin. A complex assemblage of macro- and mesoscale landforms of the Southern Chaco plain (22°–31°S) was analyzed at variable spatio-temporal scales, based on remote-sensing data coupled with field and sedimentological datasets. The Pilcomayo, Bermejo, Salado-Juramento and Dulce River megafans are between the largest distributive fluvial systems (DFS) in modern landscapes of Earth. The surface covered by these megafans reaches ca. 5,5 million km², deeply advancing basinward and achieving megafan radii of 600/750 km, most of these to the axial fluvial trunk (Paraná).  They are characterized by highly dynamic fluvial processes related to flooding and partial avulsions. A process-based geomorphological research focusing on Late Quaternary-Present fluvial and wetland dynamics from fluvial valley scale to DFS scale is presented. The spatio-temporal pattern of fluvial aggradation and progradation on these megafans is related to climatic and neotectonic forcings. The influence of the South American Summer Monsoon System during the Late Quaternary produced high seasonal fluctuations in the discharge in their Andean rivers, associated with the high sediment input and short-term sedimentation of the formative rivers. Fluvial distributive channels, alluvial ridges, aggradational lobes and crevasse and terminal splays are the typical landscape elements of the megafans. Foreland basin configuration determined the landscape patterns along these megafans. The mosaic of the Late Pleistocene and Holocene landforms influences the present hydro-geomorphological dynamic of each megafan. Extensive permanent to temporary wetlands were developed in the middle/distal reaches of these DFS, which are characterized by surfaces of extremely low gradients (0.03°–0.18°). These wetlands functioned as sediment sinks. Late Pleistocene (MIS 5/3) wetland sedimentary facies and hydromorphic palaeosols were analyzed from cores obtained from research boreholes in distal areas, considering the lack of exposed stratigraphy across the extensive and flat megafan surfaces. Partial avulsions are frequent in the Holocene river belts in middle/distal areas of the megafans, where large, floodplain-filling splays dominate (overbank processes).  The accelerated expansion in deforestation and agricultural land use on the Chaco megafans enhanced their flood vulnerability under climate change. Hence, understanding the landscape dynamics of the Chaco Plain is fundamental to risk mitigation strategies.