Evidence for Self‐Similar Bedload Transport on Andean Alluvial Fans, Iglesia Basin, South Central Argentina

Abstract

Self-similar downstream grain-size fining trends in fluvial deposits are being increasingly used to simplify equilibrium sediment transport dynamics in numerical models. Their ability to collapse time-averaged behavior of a depositional system into a simple mass balance framework makes them ideal for exploring the sensitivity of sediment routing systems to their climatic and tectonic boundary conditions. This is important if we want to better understand the sensitivity of landscapes to environmental change over timescales >102 years. However, the extent to which self-similarity is detectable in the deposits of natural rivers is not fully constrained. In transport-limited rivers, stored sediment can be remobilized or “recycled” and this behavior has been highlighted as a mechanism by which externally forced grain-size fining trends are distorted. Here we evaluate evidence of self-similarity in surface gravel-size distributions on three geomorphically diverse alluvial fans in the Iglesia basin, south Central Argentine Andes. We find that size distributions are self-similar, deviating from that condition only when significant variability occurs in the coarse tails of the distributions. Our analysis indicates a strong correlation between the degree of sediment recycling and the proportion of coarse clasts present on the bed surface. However, by fitting a relative mobility transfer function, we demonstrate that size-selectivity alone can explain the bulk size distributions observed. This strengthens the application of self-similar grain size fining models to solving problems of mass balance in a range of geomorphic settings, with an aim for reconstructing environmental boundary conditions from stratigraphy.