Linking highly variable Detrital 10Be Erosion Rates to heterogeneous Topography in Small and Alpine Catchments

Quantifying erosion and linking its topographic imprint to underlying controls is critical for understanding landscape evolution and sediment dynamics across time and space. Over the past decades, catchment-wide denudation rates from cosmogenic ¹⁰Be concentrations in river sediment emerged as a standard method for quantifying erosion. As such, ¹⁰Be-based erosion data have been instrumental in proposing different controlling mechanisms and processes for distinct mountain ranges. These include tectonic uplift, often modulated by periglacial processes, precipitation gradients, bedrock erodibility, landslide frequency, and most prominently, hillslope morphology, i.e., the slope angle. However, disentangling the individual effects of these processes has proven challenging, and often, the correlation between erosion rates and topography is observed only up to threshold values, e.g., slope angles <30° in the European Alps. One potential reason for this is that most data on the erosion rate are obtained for catchments on a regional scale (>10¹ to 10³ km²), where sediment mixing and storage, as well as the (dis)connectivity and stochastic nature of sediment sources, obscure the influence of individual processes on local erosion dynamics.

Here we present 16 new detrital ¹⁰Be erosion rates from two nested river catchments in Switzerland: the Luetschine in the Alps and the Schwarzwasser in the Pre-Alps. Despite being close (<50 km between outlets), these catchments display significant differences in topography within and between them. While the Schwarzwasser catchments are characterized by gentle slopes (mean values <21°) and mean altitudes of <1500 m a.s.l., the Luetschine catchments drain the northern rim of the highly elevated Aar Massif, resulting in steeper (up to 37° for mean slopes) and higher (mean altitudes of >2000 m a.s.l.) landscapes. To investigate how these topographic variations influence erosion rates, we sampled sub-catchments with progressively smaller drainage areas ranging from over 340 km² to less than 3 km² in the smallest upstream tributaries.

Overall, our resulting detrital ¹⁰Be erosion rates for the entire catchments are, 3-4 times higher for the Luetschine catchment than for the Schwarzwasser. They align with the general regional trend and the first-order control by the topographic uplift rate inferred by previous studies. In more detail, even in the topographically homogeneous Schwarzwasser basin, where erosion rates generally correlate with the average slope angle, the erosion rates internally vary by up to a factor of two, with the delivery of landslide material to the stream being the primary control. In the Luetschine basin, which comprises highly heterogeneous landscapes, the erosion rates differ by a factor of up to five among sub-catchments. Here, the highest and lowest values are obtained from the smallest catchments, and they do not correlate with slope angles in these steeper catchments (> 30 ° mean slope). Instead, locally different mechanisms, such as peri-glacial and glacial erosion, effectively modulate erosion rates. These findings reconcile contrasting control mechanisms for mountain range scale erosion on a local scale within the close geographical vicinity of our study area. These results underscore the need to sample smaller catchments and to consider topographic heterogeneity to link erosion and topography in steep environments.