Controls of Andean valley-floor width

Abstract

River-valley cross sections range from deeply incised gorges with narrow or no floodplains to wide valley floors of kilometer wide, densely populated plains. The diversity of valley geometries is the product of the interplay between tectonic uplift and erosion by rivers. Rivers deepen valleys through vertical incision into underlying bedrock or sediment deposits and widen valley floors by lateral erosion of enclosing valley walls. While the rate of incision is thought to mainly compensate tectonic uplift, comparably little is known about processes and controls of valley widening and valley-floor width. Due to this knowledge gap, we are currently unable to reproduce the wide range of valley shapes that we observe in nature and fail to predict valley floor and floodplain evolution under changing environmental conditions.Field measurements of valley floors are sparse, but generally indicate that valleys are narrower at sites of enhanced uplift and grow wider with greater river discharge and in softer lithologies. However, order of magnitude scatter in those datasets suggest further, so far unknown controls on valley-floor width. Here, we systematically quantify valley-floor widths of 82 river valleys draining the Western Andes between 5°S and 38°S. At each site, we estimate potential control parameters on valley-floor width including river discharge, rock erodibility, uplift rates, total sediment discharge, and lateral sediment supply from valley walls. The respective influence of each of these parameters on valley-floor widths is investigated using a random-forest approach. A better understanding of controls on valley-floor evolution will both enhance future prediction of floodplain response to climate change and enable past climate and tectonic reconstructions from valley topography.