A mudflow is a moving mass of earth made motile—fluid—by rain or melting snow. These are the rain-driven landslides of mud that often take out houses and roads in California and Oregon. Not so much a formation as an event, mudflows, though temporary—fast-moving, even—are often given names, such as the Kautz Creek Mudflow or the Osceola Mudflow. One type of mudflow is a lahar: pyroclastic material or water-saturated volcanic debris sliding down the flank of a volcano. Midway between a mudflow and a lahar is what happened on Mount Shasta’s Konwakiton Glacier in 1924: meltwater built up until the land in front of it gave way, releasing all the water at once and triggering a sudden, dangerous laharlike mudflow. A distinct though related type of flow is a debris flow, a mass of earth moving downslope. Only here, the debris and the slope are both significantly drier.

Stephen Graham Jones