A mudflow or mud flow is a form of mass wasting involving "very rapid to extremely rapid surging flow"[lower-alpha 1] of debris that has become partially or fully liquified by the addition of significant amounts of water to the source material.[1]

Mailboxes caught in a mudflow following the May 1980 Mount St. Helens volcanic eruption.

Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows; thus, they are able to travel farther and across lower slope angles. Both types are generally mixtures of various kinds of materials of different sizes, which are typically sorted by size upon deposition.[2]

Mudflows are often called mudslides, a term applied indiscriminately by the mass media to a variety of mass wasting events.[3] Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called flow slides.[4]

Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).[5]

A statutory definition of "flood-related mudslide" appears in the United States' National Flood Insurance Act of 1968, as amended, codified at 42 USC Sections 4001 and following.

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