Unravelling mixed sediment signals in the floodplains of the Rhine catchment using end member modelling of grain size distributions

Abstract

During sediment transport downstream, river systems mix sediments from different parts of their catchments. During deposition, sediments are often unmixed again in different depositional environments (facies). During fluvial transport, between erosion and deposition of sediment, the sediment is sorted too. Sediment released by erosion during agricultural practises may be different than the sediments that erode under conditions of forest cover. If this is true, the Late Holocene floodplain sediments have different characteristics in terms of grain size and texture than older floodplain deposits (Middle Holocene). For this study, we used an end-member modelling algorithm EMMA, which is frequently used on for instance deep marine sediment cores and loess-paleosol sequences to unravel different source of sediment input. We collected 15 cores from three large stretches along the trunk Rhine River: the Upper Rhine Graben, the Lower Rhine Valley, and the Rhine Delta. On a local sale, grain sizes vary strongly as a result of river processes. Using the recognized end-members, comparison between Upper and Lower Rhine Valley sites shows the overall composition to be very similar, despite that they are spread over ~300 km length of river. Across the transition towards the delta, downstream fining is observed for the first time in suspended sediments. In time, human land use resulted in more soil erosion, causing coarser sediment to be deposited in the floodplains. Human impact is shown to be of impressive scale and magnitude, and has to be regarded a forcing factor that acts drainage basin wide already millennia ago.