Our legacy: how human impact takes on climatic changes to control floodplain dynamics in the Upper Rhine area

While the dynamics of central European fluvial systems were originally controlled mainly by climatic changes, human impact has fundamentally altered the catchments’ sediment dynamics. During the Middle to Late Holocene, a transition occurred to anthropogenically controlled fluvial systems, which got stored in floodplain sediments as these record both past river dynamics and human activities (e.g., settling, mining, logging).

Since insights from the meso-scale rivers are largely lacking, two rhenian tributaries were studied with a higher (Kinzig river, Black Forest, Germany) and lower (Fecht river, Vosges, France) impact of mining and timber drift and raft. We use geophysical surveys, sedimentological investigations, luminescence dating, and geochemical analyses to study floodplain stratigraphies and heavy metal contaminations.

Our sedimentological data reveal distinct phases of floodplain accumulation (Kinzig: 0.1 mm/a for Late Pleistocene/Early Holocene till 9.00 ka, 0.3 mm/a for Mid/Late Holocene 9.00-0.82 ka, 1.1 mm/a for modern 0.81-0 ka). Additionally, floodplain sediment contaminations and historic land use seem closely related as suggested by the timing and rise in heavy metal concentrations (Kinzig: enrichment factors of Ba, Pb, Cu peaking at ca. 2.5, 4.0, 3.0). Preliminary results from the Fecht also point to increasing accumulation rates over a similar time span.

Hence, cross-referencing floodplain stratigraphy with land use history supports our understanding of a gradual shift to an anthropogenically dominated fluvial system. Said transition occurred in the Kinzig after human settlement in the Upper Rhine plain, finding higher anthropogenic impact on floodplains over the last ca. 2500 years as found in the unprecedented accumulation rates.