Valley morphologies of rivers crossing zones of active uplift range from narrow canyons to broad alluvial surfaces. They provide illuminating examples of the fundamental, but poorly understood, competition between relief creation and landscape flattening. Motivated by a field example of abandoned kilometre-wide, fluvially eroded platforms on active detachment folds in the Tian Shan foreland, we present physical experiments investigating the controls on the area of a growing fold that is reworked by antecedent rivers. These experiments reproduce the range of observed field morphologies, varying from wholesale bevelling of the uplifting fold to the formation of narrow, steep-walled canyons. A log-linear fit to a simple dimensionless parameter shows that the competition between lateral channel mobility and rock-uplift rate explains >95% of the variance in the bevelled fraction of the folds. Our data suggest that lateral bedrock erosion rates of 0.5-40 m yr â '1 are required to explain the formation of extensive platforms in the Tian Shan foreland and imply that varying water and sediment fluxes can cause striking changes in the degree of landscape flattening by influencing the lateral erosion rate.