Fluvial architecture changed under the influence of increasing human impact throughout the Holocene in many north-west European catchments. Typically, peat formation - in a marshy environment during the Early and Middle Holocene - is replaced by clastic overbank deposition. In this study we show the importance of a detailed chronology of floodplain changes, with an example from the Belgian Dijle catchment. In total, 45 radiocarbon ages from 15 alluvial sites were used. Cumulative probability functions were made of the radiocarbon ages from the base (16 samples) and top (29 samples) of the peat layer. Two alluvial sites were studied in detail. The results indicate that the end of peat growth and the transition towards clastic overbank deposition is diachronous at the catchment scale, ranging between 6500 and 500 cal a BP. This diachronous pattern can be attributed to differences in timing and intensity of agricultural activities at the catchment scale, or to differences in hillslope-floodplain connectivity. Moreover, in the broad floodplain of the main valley, the top of the peat layer is even diachronous at cross-section scale. These results indicate that inferences on changes in sedimentation rate and floodplain changes should be based on a multi-transect and multi-core dating approach. © 2014 John Wiley & Sons, Ltd.