This study aims to test models of landform evolution by analyzing the relationship between topography and the long-term denudation rate of mountainous watersheds. We measured terrestrial cosmogenic ^<10>Be in quartz grains in fluvial sediment sampled from the outlet of 31 watersheds underlain by granite or granodiorite in Japan, to determine the millennial-scale average rate of denudation over the catchment areas. Topographic analysis of the watersheds was conducted using a digital elevation model with a grid size of 10 m. Denudation rates show a strong nonlinear increase with increasing mean slope angle of the watersheds, indicating a transition in the dominant erosion process at a critical slope gradient that yields an infinite sediment flux by persistent landsliding on hillslopes. This result suggests that the sediment yield from a watershed cannot be explained by typical models of landform evolution employing a linear formulation in relationship between soil transport and the hillslope gradient, however, it seems concordant with a nonlinear model simulating a process transition from soil creep to active landsliding, thereby forming threshold hillslopes. The datasets presented in this study also reveal the existence of steep but slowly eroding watersheds beyond the threshold slope condition, indicating the importance of interactions between erosion and weathering in controlling the denudation of mountainous watersheds.