The floors of river-valleys range from kilometer wide, densely populated plains to deeply incised gorges with narrow or no floodplains. Valley cross-sections are the product of the interplay between tectonic uplift and erosion by rivers. Rivers deepen valleys by cutting into the underlying bedrock or sediment deposits and widen valley floors by lateral erosion of enclosing valley walls. While the incision rate is thought to mainly compensate tectonic uplift, little is known about the controls of valley widening and valley-floor width.
Field measurements of valley floors are sparse, but generally indicate that valley floors are wider at sites of greater river discharge and in softer lithologies and narrower with enhanced uplift. 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 along 84 rivers draining the Western Andes. At each of the ~126,000 measurement sites, we quantify four potential control parameters on valley-floor width (river discharge, uplift rate, lateral sediment supply from valley walls, and rock erodibility) and investigate their respective influence. In general, river discharge seems to be the most important control parameter on valley-floor width at locations close to the ocean at low elevations, while uplift and/or wall height becomes more important at high elevations farther away from the ocean. A better understanding of controls on valley-floor evolution will both enhance future prediction of valley floor response to climate change and enable past climate and tectonic reconstructions from valley topography.