The effects of changing channel retention: Unravelling the mechanisms behind the spatial and temporal trends of suspended sediment in the Rhine basin

Deltas are reliant on upstream fluvial sediment sources for their survival. In the Rhine basin, the suspended sediment transport governs the supply of fines to the delta, a supply that has been dwindling for several decades. We investigate the changes in suspended sediment (SS) fluxes along the main Rhine branch and its tributaries since 1997 and link these to past and ongoing human activities in the basin. We demonstrate that the spatial pattern in the temporal change can allow us to discount and determine specific mechanisms of SS delivery, transport and trapping that are causing the recent decline. A clear spatial trend in the temporal change emerges: there is an increasing loss of SS from the upper basin towards the delta apex. For the last two decades, this is contributed to the introduction of retention basins, which increase the trapping of overbank fines during high flow. However, the declining sediment flux to the delta extends further in the past (at least since 1950); thus, we also examine potential historical mechanisms, which are different from the short-term explanation. The longer term decline can be explained by the compound effects of dam construction and the decreasing fine sediment uptake by the river from its channel bed. The Rhine River has demonstrated incision in response to normalization works (finalized in the early 1920s). The incision rates declined over the second half of the 20th century corresponding to a declining SS supply from the bed to the delta apex. The importance of the channel bed as a source contributing to the total SS in human-affected rivers and the contribution of fines through varying channel bed incision has not yet been identified or considered in global river basin studies. However, it may become increasingly relevant as more rivers are urbanized and controlled for flood protection purposes in the face of climate change.