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.