Within upstream reaches of incised valleys, fluvial sedimentation occurs where it is controlled by interaction between climate and tectonics. This study focuses on a Plio-Pleistocene fluvial paleovalley, which drained the northeastern margin of Siena basin (northern Apennines, Italy). Valley filling resulted from the interaction between river drainage and active normal faults striking perpendicular and parallel to the main valley. Through numerical modeling, this study aims to refine temporal and spatial mesoscale deposit variations, which highlight the upset of fluvial architectures derived from the interplay between the river system and uplift. Geomorphological and hydrodynamic parameter calibration was performed integrating field studies with paleohydraulic and paleomagnetic data. The numerical model simulates the evolution of valley formation with the development of (i) a pre-tectonic steady state system, followed by (ii) a syntectonic aggradation and avulsion phase, and (iii) a post-tectonic relaxation phase. The syntectonic phase shows fine sediment back-filling upstream of the uplifted area and coarse sediment down-filling downstream of the upwarping. The recorded aggradations are asynchronous with upstream deposition preceding downstream deposition.