From natural to human-dominated floodplain geoecology – A Holocene perspective for the Dijle catchment, Belgium

Floodplain systems underwent important changes in many West and Central European catchments through the late Holocene. To better understand the relation between these changes and anthropogenic land cover changes, we evaluated detailed data on floodplain changes against detailed data on human impact for the Dijle catchment (758 km2), Belgium. The results show that during the Neolithic Period, human impact was nearly absent and floodplains consisted of a strongly vegetated marshy environment where organic material accumulated, which is considered as the natural state of the floodplain. The increase in human impact from Bronze Age onwards caused an increase in soil erosion and hillslope–floodplain connectivity. Consequently, sediment input in the floodplain system increased and floodplain geoecology changed towards an open floodplain dominated by clastic overbank deposits, mainly as the indirect result of an intensification of agricultural activities. Based on these data, a generalized model of floodplain development is presented. At the scale of the entire Dijle catchment, the gradual changes in floodplain morphology coincided with the gradually increasing human impact in the catchment, which suggests a linearity between the external forcing (human impact) and geomorphic response (floodplain change). However, at the narrow floodplains in the headwaters, the gradual increase in human impact contrasts with the abrupt change in floodplain geoecology, only triggered when human impact reached a threshold. Observed differences in the process-response model at catchment scale are attributed to differences in hillslope–floodplain connectivity, the location within the catchment and to differences in the timing and intensity of human activities between subcatchments.