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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 …