Controls on Valley-Floor Width in the western Andes

River-valley floors are low relief, confined parts of the landscape that store sediment en route from mountains to lowlands. Transient sediment storage influences sediment budgets, biogeochemical cycles, and the preservation of environmental signals. The controls on valley-floor width remain poorly understood, limiting the ability to reproduce natural valley shapes in landscape evolution models. Here, we examine the relationship between valley-floor width and thirteen proxies representing four previously identified controls on valley width - discharge, tectonic uplift, erodibility, and lateral sediment supply from hillslopes. Valley-floor width and proxies are quantified across 84 catchments along the western Andes between 5° and 40°S. A random forest regression model shows that proxies for water and/or sediment discharge exert the strongest control on valley width, followed by proxies for lateral sediment supply, tectonicuplift, and erodibility. When subdividing the dataset based on catchment boundaries and altitude, our correlation analyses reveal that approximated discharge is particularly influential at lower elevations, whereas uplift-related proxies exert a stronger control at higher elevations. Comparison with a theoretical model that predicts steady state valley width as a function of water discharge, uplift, and lateral sediment supply shows good agreement (R2 = 0.71), supporting the importance of these three parameters on valley width. Model-data mismatches occur in transient stream segments, where dynamic processes are not captured by the model. Our findings offer valuable insights for improving models of valley floor-formation and sediment storage estimations.