Lahars are highly hazardous volcanic wet flows, owing to their potentially large volumes, rapid velocities, long runouts, frequent occurrence during and following eruptions, and unpredictability. However, lahars are not a specific flow type. They are, instead, time-discrete flow events like flash floods, which involve up to three distinct types of flow—debris flow (~ 50 to ~ 80 volume percent solid particles), hyperconcentrated flow (~ 10 to ~ 50 volume percent solid particles), and water flow (0 to ~ 10 volume percent solid particles). These concentration boundaries vary slightly for different particle-size distributions of sediment mixtures. All three flow types can occur simultaneously in different parts of the same lahar, and therefore lahars can evolve in unique ways in time and space as they flow downstream. Debris flow, hyperconcentrated flow, and water flow have fundamentally different flow behaviors, bulk properties, particle transport mechanics, and modes of deposition. These differences can lead to distinctly different kinds of deposits, as well as different lithofacies zones within any single lahar deposit.