By developing an extremely high-resolution database of the Rhine-Meuse delta, researchers at the Water, Climate and Future Deltas hub have reconstructed its sediment history and development over thousands of years.
The Rhine-Meuse delta is the product of a 9000-year development
Kim Cohen, assistant professor of paleogeographic evolution of the Rhine-Meuse delta, explains, "The Rhine-Meuse delta, as existing today, is the product of a 9000-year development". Since the last Ice Age, sea-level rise filled the North Sea and the Rhine-Meuse delta was then formed by many meandering branches of the intertwined Rhine and Meuse rivers. The rate of sea-level rise, division of discharge, flow dynamics of the rivers, land-use change in the upstream basin and delta itself and land subsidence together influenced the routes of new river branches and the amounts and composition of sediment received and deposited in the delta.
Reforestation in the river basin
From ca. 15,000 years ago, large parts of the Rhine river basin became forested which reduced soil erosion compared to prior ice-age conditions. Once rates of sea-level rise decreased, the Holocene sediment flux determined the size and position of the delta. By 5000 years ago, the Rhine supplied the Dutch delta with about 1.1 Mt/year of fine sediments (silt and clay), while the Meuse added a further 0.3 Mt/year. Most of this sediment was deposited on the delta plain, with the largest amounts trapped in the central Rhine-Meuse delta where flooding was more widespread.
Replacement of the delta branch network
From the Bronze Age onwards (~3500 years ago) large-scale deforestation in the Rhine basin for agriculture increased the delivery of sediment to the Rhine-Meuse delta. By Roman times (2000 years ago), the rivers carried almost twice as much fine sediment as before, approximately 2 Mt/year for the Rhine and 0.55 Mt/year for the Meuse. The muddier conditions caused a complete replacement of the delta branch network to occur, splitting the discharge over a greater number of branches and river mouths than before. As new branches emerged and continued to develop, the channels of older river branches gradually silted up. Kim Cohen concludes, "It was during this pulse of sedimentation that the network of river branches of the Rhine-Meuse delta took the form we know and see today".
Land reclamation and embankments
From the Middle Ages onwards, humans intensified their interventions in the Rhine-Meuse delta through construction of embankments, draining and reclaiming wetlands. This drastically changed sediment delivery and trapping in the Rhine-Meuse delta. Former floodplains no longer received sediment from flooding and deposition rates increased in the narrow, embanked floodplain zones that remained. Of the estimated 2.5 Mt/year of fine sediment delivered to the delta, about 25% was trapped in the embanked floodplains and the majority passed through to the estuaries in the lower coastal plain.
Back to prehistorical sedimentation levels
Since the industrial revolution, human activities in the upstream catchment, such as reforestation and construction of reservoirs and flood retention basins, have gradually decreased the sediment delivery to the delta towards levels seen in prehistory. The amount of fine sediments dropped from 3.5 Mt/year in the 1950s to less than 2 Mt/year by the beginning of the 21st century, while the trapping efficiency of the embanked river branches remained low. Given this low river input, sediment input from the North Sea is now the main sediment source in the modern estuarine reaches.