We present new field data from rivers draining across active normal faults that incise across the same lithology at the fault, have been subjected to similar climatic regimes and tectonic settings, and were perturbed by a well-documented increase in fault slip rate ca. 1 Ma. In spite of these similarities, the rivers exhibit markedly different long profiles and patterns of catchment incision. We use channel slope and hydraulic geometry data for each river to calculate bed shear stresses (τb), and show that there is no simple relationship between peak τb and the relative uplift rates across the faults, U, which differ by a factor of four. The long-term average sediment supply to each channel (Qs), estimated from time-averaged catchment erosion rates, can explain the τb versus U data if bedload modulates bedrock incision rate, E, in a strongly nonlinear way. Together these field data allow us, for the first time, to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems.