TRENDS AND CONTROLS IN HOLOCENE FLOODPLAIN SEDIMENTATION IN THE RHINE CATCHMENT

Climate and human impact have profoundly altered the functioning of rivers. Varying sedimentation rates in the floodplain of the River Rhine during the Holocene are assumed to be the direct result of variations in sediment flux to the River Rhine that seem to be related to climate-induced and human-induced vegetation changes in the upstream drainage basin.

We will present evidence of changing Holocene floodplain sedimentation in the River Rhine catchment based on a database with 219 alluvial ¹⁴C-samples and quantification of floodplain sediment volumes. The ¹⁴C-data were analysed in terms of: i) sedimentation rates and ii) fluvial activity and stability. Based on ages indicating activity and stability, cumulative frequency distributions were calculated. Floodplain sediment volumes were quantified for successive Holocene timeslices based on valley wide cross sections in major sedimentary sinks along the River Rhine (i.e. the Upper and Lower Rhine Graben and the Rhine delta).

The results of the probability frequency analysis suggest a tendency towards stable fluvial and colluvial systems during the early and middle Holocene and increased activities during the Late Pleistocene and the last 4000 years in the River Rhine catchment. In general, phases of increased activity coincide with phases of decreased stability (and vice versa) suggesting a homogenous response of the studied system to external forcing. The analysis of the ¹⁴C-ages in terms of sedimentation rates show low base line rates before 2000 BP and strong increases afterwards. It will be discussed whether the increase rates are either a result of increasing human impact or an artifact of the method. Results of the floodplain sediment volumes quantification show an increase in sedimentation between 4 ka and 2 ka BP, and a strong increase after 2 ka BP. This trend is visible in all three large sediment sinks in the non-alpine River Rhine, and it exceeds amplitudes expected from Holocene climatic forcing. Therefore, we interpret this as a drainage basin-wide response to human impact in the Rhine catchment.

In general, the results show a coherent shift from a natural controlled to a human dominated fluvial system. Due to the long history of human impact in the Rhine catchment, the shift already started at 4 ka BP and increased after 2 ka BP.