At 11:18 h (New Zealand time, GMT +12) on 18 March 2007 an impoundment of 0.01 × 106 m3 of tephra collapsed, releasing 1.3 × 106 m3 of water from Crater Lake at 2536 m elevation on Mount Ruapehu. The lahar traveled 200 km along the Whangaehu River. Aerial LiDAR surveys of the upper 62 km of flow path were made before and after the lahar. We present here the first large-scale quantification of the geomorphic impact of the dam-break flood along with the rates and controls on its sediment entrainment and deposition. The flood mobilized a net value of 2.5–3.1 × 106 m3 of boulders, gravel, and sand over the first 5 km of travel to form a lahar of at least 4.4 × 106 m3 passing instruments at 6.9 km. LiDAR volume-transfer calculations match dynamic measurements made. After a logarithmic increase in cumulative net sediment entrainment, the lahar appeared to reach its maximum sediment-carrying capacity at 22 km. Patterns of alternating sediment erosion and deposition occurred that dominantly reflect a combination of channel morphology and confinement on the local sediment-carrying capacity of the flow.