Soil erosion can severely degrade landscapes, and concentrated flows such as rills and gullies can be the dominant contributor to the soil losses. This paper examines the growth, development, and spatiotemporal evolution of rills and rill networks using a soil-mantled experimental landscape subjected to simulated rain and downstream base level lowering. Rill incision and network development and extension occurred due to actively migrating headcuts formed at the flume outlet by base level lowering. The communication of this wave of degradation due to this exogenic forcing occurred very quickly in space, and resulted in nearly the same amount of bed incision throughout the network. Rill incision, channel development, and peaks in sediment efflux occurred episodically, yet these were in direct response to the downstream base level adjustments. Although flows were supply limited, most of the sediment efflux was genetically linked to headcut development and migration. The geometry of the eroded rills and the rates of headcut migration were well correlated to overland flow rate. These findings have important implications for the prediction of soil loss, rill network development, and landscape evolution where headcut erosion can occur.