Debris flows move so quickly because they are fluidized masses of saturated soil, boulders and rock fragments, wood, and water, which causes them to erode or scour their flow path and accumulate more material than initially failed on the slope. Comparing pre- and post-flow imagery shows erosion by the flow clearly, with initial texture in the channel scoured away where the flow was most concentrated. A final example of flow behavior from McDowell County, North Carolina, shows how flow material can completely overwhelm a shallow channel due to volume or velocity and spread out over a flat slope.