The legacy of historic damming in Central European floodplains

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Historic dams of largely unknown age are a widespread feature within German 1st to 3rd order streams. It has been proposed that accelerated slope erosion due to deforestation and agricultural land use is the reason for the aggradation of some Central European floodplains, however this process only explains the delivery of sediment to these rivers. We propose that valley bottom damming was also a critical mechanism for the effective trapping of this increased sediment load. In this study we will determine the onset and magnitude of this first impact of humans on riverscapes as a result of valley bottom damming, and examine the management implications. The extensive sedimentation of loam floodplains was precipitated by dam induced changes to the flow regime, and in turn caused the channels to grade from multithread to meandering. This floodplain loam now blankets palaeo-wetland soils, which would have formed an efficient carbon sink. The timing of this study is paramount, given that these dams are due to be removed according to the Water Framework Directive throughout the EU. Potentially, this may lead to large and sustained sediment pulses as legacy sediments and carbon rich wetland soils are remobilised via increased bank erosion. The quantification of sediments that may potentially be remobilised is therefore critical knowledge that can be used to support the management of rivers that will be affected by dam-removal.