Controls on valley-floor width in the western Andes

River valleys range from narrow and deeply incised canyons to broad and open valleys with valley floors that are many times wider than the river and accommodate large amounts of sediment. Floodplains in wide valley floors are areas of high biodiversity and have been important places in the development of human settlements. The intermediate storage of sediment along valley floors plays an important role in modifying chemical weathering fluxes and global element cycles, but also impacts the propagation of sedimentary signals. Despite the importance of valley floors as habitats and sediment traps, we still have a limited understanding on what controls the width, and hence the accommodation space, of valley floors. This knowledge gap currently prevents reliable predictions of how valley floors will evolve under changing environmental conditions.

Previous studies suggest that water discharge, lithology, uplift rate, and/or lateral sediment supply from hillslopes might exert some control on valley-floor width. But the results are ambiguous and the relative importance of each parameter remains unknown. Here, we quantify valley-floor width and each of the four potential control parameters at ~126,000 locations in 84 catchments along the Western Andes between 5° and 40°S. We rely on remote-sensing data and digitized maps. On the mountain belt scale, discharge exerts the strongest controls on valley-floor width, while lithology plays only a minor role. We investigated regional difference in the relative control on valley width by subdividing the data based on catchment boundaries and elevations. Correlation analyses suggest that discharge is a strong control especially at low elevation where discharge is highest, while uplift becomes a stronger control at higher altitudes. As such, our data are in line with observations from the Himalaya, where uplift has been shown as the strongest width control in higher elevations (Clubb et al., 2023). Our findings suggest that future changes in the discharge regime will preferentially impact valley-floor evolution at sites of low elevation and high discharge and help to interpret past tectonic and climatic boundary conditions from valley geometries.

 

Clubb, F. J., Mudd, S. M., Schildgen, T. F., van der Beek, P. A., Devrani, R., & Sinclair, H. D. (2023). Himalayan valley-floor widths controlled by tectonically driven exhumation. Nature Geoscience, 16(8), 739-746.