Controls on valley-floor width in the western Andes

River valley floors are low-relief, confined landscapes that temporarily store sediment moving from mountains to lowlands. This transient storage affects sediment budgets, global element cycles, and the preservation of environmental signals. However, the factors controlling valley-floor width remain poorly understood, preventing landscape evolution models from reproducing and forecasting those diverse landforms. In this study, we analyze valley-floor width and four potential controlling factors—river discharge, tectonic uplift, lithology, and lateral sediment supply from hillslopes—across ~126,000 locations in 84 catchments along the western Andes (5°–40°S). Using a random forest regression model, we identify river discharge as the dominant control on valley width, followed by lateral sediment supply, tectonic uplift, and erodibility. When organizing the data by catchment and elevation, correlation analyses reveal that discharge has greater influence at lower elevations, while uplift plays a stronger role at higher altitudes. A comparison with a theoretical model—predicting steady-state valley width based on discharge, uplift, and sediment supply—shows over 95% agreement, reinforcing the significance of these parameters. Discrepancies between model predictions and observed data mostly occur in transient stream segments, where dynamic processes are not captured by the model. Overall, our findings improve understanding of valley floor formation and sediment storage and aid in interpreting past climate and tectonic activity through valley morphology.