Nikhil Sharma , Alexander C Whittaker, Stephen E. Watkins, Luis Valero, Jean Vérité, Cai Puigdefàbregas, Thierry Adatte, Miguel Garcés, François Guillocheau, Sebastien Castelltort 

Ancient fluvial deposits typically display repetitive changes in their depositional architecture such as alternating intervals of laterally-stacked, high-amalgamation (HA) channels, and floodplain-dominated intervals with vertically-stacked, low-amalgamation (LA) channels. Such patterns are usually ascribed to slow and high rates of base-level rise respectively, but “upstream” factors such as water discharge and sediment flux have also been recognized for their potential role in controlling stratigraphic architecture but have not been tested in ancient fluvial systems. Here, we use palaeohydraulic reconstructions to document riverbed gradient evolution within three middle Eocene (~40 Ma) fluvial HA-LA sequences in the Escanilla formation in the south-Pyrenean foreland basin. We show, in an ancient fluvial system, that river slope was primarily driven by climate-controlled water discharge variations rather than base-level changes as commonly assumed. These results have fundamental implications for the interpretation of the fluvial stratigraphic record and for our ability to reconstruct ancient hydroclimates.