The Rhine delta-a record of sediment trapping over time scales from millennia to decades

At the land-ocean interface, large river deltas are major sinks of sediments and associated matter. Over the past decennia, many studies have been conducted on the palaeogeographic development of the Rhine delta and overbank deposition on the Rhine floodplains. This paper aims to synthesise these research results with special focus on the amounts and changes of overbank fines trapped in the Rhine delta and their controls at different time scales in the past, present and future. Sediment trapping in the Rhine delta throughout the Holocene was quantified using a detailed database of the Holocene delta architecture. Additional historic data allowed the reconstruction of the development of the river's floodplain during the period of direct human modification of the river. Using heavy metals as tracers, overbank deposition rates over the past century were determined. Measurements of overbank deposition and channel bed sediment transport in recent years, together with modelling studies of sediment transport and deposition have provided detailed insight in the present-day sediment deposition on the floodplains, as well as their controls. Estimated annual suspended sediment deposition rates were about 1.4 x 10(9) kg year(-1) between 6,000 and 3,000 years BP and increased to about 2.1 x 10(9) kg year(-1) between 3,000 and 1,000 years BP. After the rivers were embanked by artificial levees between 1100 and 1300 AD, the amount of sediment trapped in the floodplains reduced to about 1.16 x 10(9) kg year(-1). However, when accounting for re-entrainment of previously deposited sediment, the actual sediment trapping of the embanked floodplains was about 1.86 x 10(9) kg year(-1). Downstream of the lower Waal branch an inland delta developed that trapped another 0.4 x 10(9) kg year(-1) of overbank fines. Since the width of channel was artificially reduced and the banks were fixed by a regular array of groynes around 1850, the average rates of deposition on the embanked floodplains have been 1.15 x 10(9) kg year(-1). Scenario studies show that the future sediment trapping in the lower Rhine floodplains might double. The variations in amounts of sediment trapped in the Rhine delta during the past 6,000 years are largely attributed to changes in land use in the upstream basin. At present, the sediment trapping efficiency of the floodplains is low and heavily influenced by river regulation and engineering works. Upstream changes in climate and land use, and direct measures for flood reduction in the lower floodplains, may again change the amounts of sediments trapped by the lower floodplains in the forthcoming decades.