Lahars, debris flows, and sediment-rich floods are frequent and deadly hazards at all mountain-forming volcanoes. Their hazard potential is traditionally assessed through mass-conserving closed-system models, where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks. This belies evidence of extremely high-energy and deadly catastrophes caused by such flows at large distances from volcanoes. Here we use the first high-resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass-flow structure. We show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path. We also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front.