We observed the incisional response of an alluvial river to base-level fall. We conducted the experiment in a 3.9 × 2.4 × 0.4 m box that we filled with uniform 0.140±0.04 mm sand. We dropped base level by lowering the elevation of an "ocean" pool at the river outlet. As the initial condition, we cut a 10±2 cm wide channel to a steadily increasing depth, from 3±0.5 cm at the inlet, where we supplied water...