Unravelling the mechanisms behind the spatial and temporal trends of suspended sediment in the Rhine basin

River deltas are reliant on upstream fluvial sediment delivery for their survival. The ultimate sediment delivery to deltas and the changing bed dynamics of river channels are strongly dependent on climate and anthropogenic changes within the entire river basin that control the increase (due to e.g. increased erosion, climate change) or decrease (due to e.g. sand mining, dam construction) of sediment supply. In the case of the Rhine-Meuse basin, suspended sediment delivery to the delta apex at Lobith has decreased since the 1950s. Therefore, we investigated changes in suspended sediment concentrations (SSC) and suspended sediment loads (SSL) over time along the main Rhine branch and its major tributaries (the Aare, Main, Mosel and Neckar) to determine the cause of the decline. We hypothesis and mathematically demonstrate that the spatial pattern in the temporal change can explain and determine specific mechanisms that are causing the decline.

Using collated SSC data of varying frequency from 1997-2014, we explored the suspended sediment transport within and along branches using the rating curve method & discharge-suspended sediment relations for a total of 26 measurements stations in the basin. These were compared with bed level data from Ylla-Arbós et al. (2021), to examine the interaction of SSC with bed dynamics.

A clear spatial trend emerged: the decrease in SSC strongly increases in an upstream direction. In the Alpine Rhine SSC has increased. There is negligible change in the upper basin/impounded section of the Rhine. However, SSC decreases emerge after the confluences with the Main and Mosel branches and this decrease becomes stronger moving towards the delta.

We find that contrary to many other river basins which are showing declining fluvial sediment delivery to deltas due to upstream dams or sediment management activities, in the Rhine-Meuse basin  the cause is actually the changing retention of channels and differing erosion rates from the river bed. Since the 19^th century there have been activities to straighten and narrow the Rhine river to embank and fix the river course for navigation. This created high amounts of incision in the river bed in the early 20^th century, but as proven by Ylla-Arbós et al. (2021) and others, this incision is now decreasing. These changes in suspended sediment supply from the river bed can be correlated to the changing supply at the delta apex. Since the 1980s efforts have been made to stabilize bed erosion and this ‘fixing’ of the river beds has led ultimately to a declining suspended sediment supply to the delta apex. This suggests that response to human interventions is not only relevant at a centurial timescale but is likely to be a defining feature of sediment supply for the coming century.  

 

References

Ylla Arbós, C., Blom, A., Viparelli, E., Reneerkens, M., Frings, R. M., & Schielen, R. M. J. (2021). River response to anthropogenic modification: Channel steepening and gravel front fading in an incising river. Geophysical Research Letters, 48(4), e2020GL091338.