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. Steep, unvegetated slopes during a heavy rain are often good sites to observe such small flows. At the other extreme, they can be a few hundred meters wide, tens of meters deep, flow at several tens of meters per second, and travel over 100 kilometers from a volcano. Since 1980, lahars have formed by all of these processes at Mount St. Helens. Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche , lateral blast , and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens.