A mudline left behind on trees shows depths reached by the mud. This view is along the Muddy River, southeast of Mount St. Helens. January-February 1985, p.20. Both types of flows contain a high concentration of rock debris to give them the internal strength necessary to transport hugh boulders as well as buildings and bridges and to exert extremely high impact forces against objects in their paths.
Debris flows are coarser and less cohesive than mudflows .
As lahars become dilute in downstream direction they become hyperconcentrated streamflows . Lacking internal strength, the mixture of rock debris and water takes on different flow properties. The coarser debris in this type of flow is no longer held in suspension by matrix strength and therefore settles to the bottom of the flow. Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second.