Scale dependency of the landscape’s morphometry in the Alpine Rhine

Mountain landscapes exhibit significant variability in their topographic properties, glacial cover, climate conditions, and the geological characteristics of the underlying bedrock, all of which contribute to the diversity of potential sediment sources. While it has been recognized that such variability is largely scale-dependent – meaning the spread of morphometric parameters, for instance, decreases as the size of the upstream drainage basin increases – there is limited research on this topic. However, understanding this dependency is crucial for determining the mechanisms driving catchment-wide erosion and identifying the origin of detrital material in the channel network.

Here we focus on the 4300 km²-large Alpine Rhine basin located in the European Alps of Switzerland. The Alpine Rhine basin itself is made up of >2000 tributary basins, which we delineated using the stream junctions as outlet points. For each of these tributary basins as well as for the progressively larger drainage basins farther downstream, we calculated a set of parameters including, but not limited to: mean hillslope angles, mean normalized steepness and concavity values of channels, glacial cover, annual precipitation rates and temperature ranges, and lithology. To explore the physical records of this variability, we determined the pattern of catchment-averaged denudation rates derived from concentrations of in-situ cosmogenic ¹⁰Be in detrital quartz at 49 sites.

The results reveal that the mean hillslope angles vary from c. 16° to 36° for basins smaller than 100 km². For larger basins, the upstream hillslope angles converge to a mean value of 27±3° (2-sigma standard deviation). The same pattern is also visible for other morphometric variables characterizing the shape of channels (e.g., concavity and normalized steepness values) and for parameters characterizing the hydroclimate and lithology of the basin. Similarly, a scale-dependent pattern is also visible for the ¹⁰Be-based denudation rates where a large variability spanning between <0.3 mm/yr to >2 mm/yr for basins <100 km² converges to a basin-averaged mean of c. 0.8 mm/yr at the downstream end of the Alpine Rhine. Mapping shows that the high rates are due to stochastic sediment input by landsliding, debris flows or glacial melt, while the low rates occur in basins where overland flow erosion dominates. This basin size, which is <100 km² for the Alpine Rhine, can thus be considered as a threshold value where detrital signals are produced and still well mixed, while for larger basins such primary signals may be diluted. This threshold is crucial as it provides constraints for both optimizing a sampling strategy for cosmogenic nuclides analysis and interpretation of the corresponding results.