Sediment storage in the Upper Rhone Valley, Switzerland

Abstract

High-mountain areas feature the largest potential energy gradients and are thus characterized by the highest sediment fluxes. Vast amounts of Postglacial sediment are stored in this environment on all spatial scales. Pleistocene glaciations left behind a system with “empty” sediment sinks, but also a high availability of unconsolidated, potentially transportable sediment. Today, sediment covers large shares of high-mountain areas. Investigating these storages is of importance, since mountains are home to > 10% of the global population. Nonetheless, little is known about the distribution, volumes, morphology and residence time of sediment storages. Triggers, such as strong rainfall, snowmelt or earthquakes, can cause rapid changes in sediment storage posing a hazard to human society and infrastructure. <br /> Due to often pointed out (up-)scaling issues in sediment budget studies and geomorphology in general, knowledge on sediment storages on larger scales is restricted. Existing studies focus on sediment storage in smaller scale catchments (mostly up to 102 km2) and emphasize their (long-term) importance or investigate macro-scale sediment storages (e.g. valley fills) on the larger scale; hence, neglecting the relevance of headwater sediment storage. In this thesis, I approach the problem of transferring knowledge and bridging the gap between different scales, by applying multiple methods on multiple scales in the large catchment of the Upper Rhone, Switzerland. I contribute to this aim with the ensuing objectives: (1) understanding and analyzing the formation of Postglacial landforms on the small scale, (2) investigating the landform assemblage in medium-sized tributaries within the Upper Rhone Basin, and (3) creating scale linkages for sediment distribution between smaller scales and a large catchment scale through a hierarchical upscaling. <br /> On the small scale, three outsize fans, i.e. strikingly large fans with unlikely small feeder basins, are investigated to contribute to a better quantitative understanding of these landforms and their effect on larger spatial scales. Morphometric analysis of the fans and their source areas, geophysical surveys of the subsurface material and cosmogenic radionuclide exposure dating are combined in a multi-method approach. Outsize fan formation was rapid, but incremental and non-catastrophic. The largest part of the fan was deposited through high-energy, high-magnitude debris flow processes with average yields of up to 73 kt km-2 yr-1 within only ~4 kyr soon after deglaciation (~10-6 ka). Afterwards, lower-magnitude debris flows deposited a second sedimentary facies across the initial fan. Over time, erosion and sedimentation rates strongly lessened transitioning into an ongoing phase of very limited geomorphic activity. The outsize fans, which are deposited rapidly and occupy the full valley width, have a potent influence on sediment transfer, storage and therefore landscape evolution on a larger scale. <br /> On the regional scale, sediment storage and bedrock (total 360 km²) were mapped in five key sites of the Upper Rhone Basin to bridge the gap between the regional and the large catchment scale. This dataset serves as valuable contribution in the upscaling approach. <br /> On the large catchment scale, the Upper Rhone Basin, the spatial sediment distribution was studied in great detail through a combined field-based and statistical modeling approach. Best prediction results are achieved with a generalized additive model. Sediments cover 53.5 ± 21.7% of the land surface, but the distribution is by no means even. Small headwaters feature a very strong variability in sediment coverage. More than 90% of the sediment cover is located outside the main valley, where the macro-scale sediment storages are situated. The majority of sediment cover account for a significant part of the sediment stored in large-scale catchments, thus highlighting the importance of sediment storage in low-order catchments and headwaters. <br /> Based on my observations, results and in-depth discussion of collected data from different spatial scales, I conclude that geomorphic systems have a nested hierarchical structure, which I utilize to transfer knowledge on sediment storages to larger scales in the Upper Rhone catchment, resulting in a detailed analysis of all sediment storages in this geomorphic system. This is synthesized in a framework for resolving scaling issues in sediment storage analysis comprising conceptual and applied notions, which is intended to serve as a guiding concept for further large-scale sediment storage analysis.