The role of fluvial systems in coastal sediment dynamics, particularly regarding sediment storage in floodplains, the flux of sediment to the coast, and its influence on coastal evolution, is currently being widely considered. This study contributes to this debate through a geoarchaeological approach to reconstructing floodplain sedimentation rates in the Olway-Usk Valley system in South Wales, which is a tributary to the Severn Estuary, a coastal feature of global importance because it has the world's second highest tidal range. Archaeological and radiocarbon-dated horizons within alluvium indicate that floodplain deposition was initiated as Holocene sea levels rose toward present levels around 6500 yr B.P. River drainage appears to have been impeded at this time, leading to ground surface-water logging, sporadic peat formation in valley bottoms, and subsequent overbank deposition of alluvium. The mean Holocene sedimentation rate is 0.44 ± 0.02 mm yr −1 , but this includes a rise to around 1 mm yr −1 after the medieval period, rising again to around 10 mm yr −1 since the early nineteenth century. This increase reflects human activity, especially agricultural land use within the catchment such as widespread ploughing, which was not introduced until the nineteenth century. The volume of accumulated fluvial fine sediment stored within the Olway-Usk valley system is estimated to be 5.25 × 10 7 m 3 (7.09 × 10 7 t). These data are used here as a proxy for total fluvial sediment input into the Severn Estuary since marine inundation first occurred around 8500 yr B.P. The combined sediment contribution from rivers and intra-estuarine bedrock erosion is estimated to be 1.13 × 10 10 t. This corresponds precisely to a previously estimated amount of Holocene sediment lying beneath the coastal wetlands of the Severn Estuary. These independent estimates give confidence to our understanding of the sediment dynamics in the Severn Estuary through the Holocene, and the investigation provides a new case study to contribute to the wider debate of alluvial sediment storage and fluvial fine sediment flux to the coast.