New AMS radiocarbon age determinations indicate that overbank sedimentation along the lower reach of the Gelderse IJssel started ~950 AD. This environmental change most likely resulted from an avulsion of the Rhine into the IJssel valley. Prior to this avulsion, only local streams carrying minor amounts of sediment existed in the IJssel valley during most of the Holocene. Reinterpretation of previous conventional radiocarbon data from the upper IJssel floodplain leads to the conclusion that upper IJssel channel formation may have started ~600 AD. Our results contradict earlier suppositions, based on interpretation of archaeological data and historical accounts, that the Gelderse IJssel is much older and originated as a canal, dug under supervision of the Roman general Drusus.