Impact of river management on grain size patterns: example of the Sense and Gürbe Rivers in the Swiss Alps

Can river management practices impact the grain size patterns along mountain streams? If yes, such practices may have important consequences for the gravel industry, fluvial ecosystems and for the interpretation of grain size patterns in geosciences. Since the motion of particles in a riverbed is dependent on the applied shear stress, which in turn depends on river slope and depth, there are reasons to expect some impact on the riverbed grain sizes after a river channelization or construction of check dams. With the aim to answer the presented question we analyse and compare a large dataset of grain sizes and slopes of exposed gravel bars obtained in two mountain streams, i.e. Sense and Gürbe Rivers. While the Sense River is maintained in its natural character, the Gürbe River has been engineered with more than 60 check dams along c. 5 km on its sediment supply area. Both rivers are situated at the northern border of the Swiss Alps, they share the same source area and experience identical hydroclimatic conditions. In addition, for both streams, landslides and high-concentration flows from steep tributaries supply the clastic material to the trunk. Field work in the Sense River has been carried out c. 8 km downstream of the main sediment supply area, and the corresponding catchment area is c. 120 km² large. The selected stretch of interest on the Gürbe River lies above an alluvial fan situated right downstream of the supply area. The corresponding size of the drainage area in the Gürber River is c. 12 km². We collected more than 15’000 grain sizes by applying the Wolman’s pebble count method conducted on orthoimages of exposed gravel bars. The orthoimages were generated from digital photos that were taken by an unmanned aerial vehicle (UAV) and processed with standard photogrammetric techniques. These photos were also used to create digital elevation models, which allowed us to calculate the surface slope at various scales. Despite the differences in catchment area and distance from the supply area, the results from both rivers reveal similar bar-scale slopes (2.0 ± 0.1 cm/m) and grain size D50 (4.2 ± 0.1 cm) and D84 (12.7 ± 0.7 cm) percentiles. Additionally, by calculating the slope around each grain within a 2 m diameter circle, called here « local slope », we found a linear dependence between the local slopes (0.5 to 20 cm/m) and the grain size percentiles in both rivers. Since the check dams are built to reduce the mass fluxes, we consider that the grain size data of the Gürbe River reflects a greater attenuation of the landslide signals than the Sense River data. Furthermore, we suggest that the local slope dependency of the grain size percentiles rather reflects the effects of hydrodynamic processes than those of hillslope processes as controls on the motion of the riverbed particles. It thus appears that the check dams can have an impact on the sediment routing as they attenuate signals related to mass movements.