Human impact onto the sediment budget of the upper Rhône River, Central Swiss Alps

Human impact onto our landscapes is widely visible, but yet hard to quantify due to its short time scales and unpredictable feedbacks. In this debate, the world’s river networks are of special concern, because they regulate... [ view full abstract ]

Human impact onto our landscapes is widely visible, but yet hard to quantify due to its short time scales and unpredictable feedbacks. In this debate, the world’s river networks are of special concern, because they regulate the transport of sediment between sources and sinks. One of the main human impacts onto the river network is the construction of hydropower dams and connected storage reservoirs, which are believed to retain up to 25% of the global sediment delivery to the ocean.
The upper Rhône basin located in southwestern Switzerland is one of the major sediment factories of the Central Swiss Alps. Sediment is generated in around 50 tributary basins, which join the main Rhône River along its c. 150 km long course from the headwaters in the glaciated Aar massif to the primary sedimentary sink in Lake Geneva. Since the beginning of the last century, hydropower dams have been constructed in many of the largest tributary basins.
In this study we investigate the role of hydropower dams onto the sediment budget of the upper Rhône basin. The tributary basins are located in mainly three lithological units: (1) (meta)oceanic rocks in the South, (2) granitoid rocks in the East and West, and (3) carbonates in the North. The study area consists to 56% of unit 1, 27% of unit 2 and 18% of unit 3. Most of the hydropower dams and the largest facilities in particular, are located in the southern area within the lithological unit 1.
We sampled the sediment generated in the major tributary basins at the basin outlets, and fingerprint them using heavy and light mineral analysis, as well as bulk geochemistry on the fine-sand fraction. Then, we analyse the sediment at the outlet of the Rhône River close to the sedimentary sink in Lake Geneva with the same techniques. Using compositional modelling, we calculate the contributions of the source areas. We combine these data with calculations of sediment load and cosmogenic nuclide dating to understand the theoretical sediment load of each tributary basin. Result show that the southern units, which theoretically should supply over 50% of the material, are highly under-represented in the sediment. We therefore conclude that hydropower dams and the connected sediment storage have an impact onto the sediment budget of the Rhône River.