Quantifying Sediment Dynamics in an Alpine River Catchment using a 10Be Tracing Method

In this study, we quantify the spatial variation in sediment generation for the c. 12 km2 large Grbe catchment situated at the northern margin of the Swiss Alps. We particularly trace the sediment transfer from the hillslope to the channel network in the headwaters, and finally to the depositional fan at the downstream end of the catchment. Mapping shows that sediment production in the Grbe catchment occurs through three primary mechanisms: (1) overland flow erosion generating sand and silt, contributing to the generation of suspension loads; (2) shallow and deep-seated landslides linked to the main channel, both supplying a mixture of gravel, boulders, and silt/sand during floods, thus generating sediment for both the bedload and suspension load of the Grbe River; and (3) incision of the river into glacial till in the upper headwaters and into landslides farther downstream. The bedrock of the Grbe catchment comprises Molasse, Flysch, and Quaternary deposits, posing challenges in tracing the origin of the material and estimating the relative importance of the various processes for sediment generation. However, previous research has shown that the cosmogenic 10Be concentration can differ for various sediment sources (Cruz Nunes et al. 2015; e.g.). Therefore, we measured 10Be concentrations in the sand fraction (0.25 2 mm) in the main channel and in the tributaries, aiming to capture suspension load signals generated through overland flow erosion and landslides. As a novel approach, we also determined the bulk 10Be concentration of gravels (2 10 cm) collected from the same sampling locations in the Grbe channel, in the three tributaries as well as from the landslide tongues reaching into the Grbe. The results point to three different conclusions: First, there exists a clear difference between the signals measured in the sand fraction and the gravel samples. In particular, the 10Be concentrations in the sand fraction are two to four times higher than those measured in the gravel at the same sites. This grain size dependence aligns with previous findings by Puchol et al. (2014). Second, the sand samples in the main channel show a downstream decrease in 10Be concentration, thereby reflecting the supply of material from the tributaries and particularly from the landslides with low 10Be concentrations. Third, bulk gravel samples reveal a larger variability in 10Be concentrations than the sand samples at the same locations. This suggests that the supply and downstream transport of the coarse-grained bedload material occurs more episodic than the generation and transfer of the suspension load. REFERENCES Cruz Nunes, F., Delunel, R., Schlunegger, F., Akar, N., Kubik, P. (2015): Bedrock bedding, landsliding and erosional budgets in the Central European Alps. Terra Nova, 27(5), 370-378. Puchol, Nicolas; Lav, Jrme; Lupker, Maarten; Blard, Pierre-Henri; Gallo, Florian; France-Lanord, Christian (2014): Grain-size dependent concentration of cosmogenic 10Be and erosion dynamics in a landslide-dominated Himalayan watershed. In: Geomorphology 224, S. 5568.