Lahar is a term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and/or river valleys.

When moving, a lahar looks like a collection of wet concrete that carries rock debris ranging in size from gravel to boulders. When a lahar rushes downstream from a volcano, the size, speed, and the amount of water and rock debris it carries is always changing. The starting surge of debris erodes rocks and plant life from the side of the volcano or mountain. Lahars can also carry part of a mountain, water, and snow and ice, which can overflow the rivers. By picking up debris as they go down hill, lahars can grow to over 10 times their own size. As a lahar moves away from a volcano or mountain, it starts to size down.

Lahars cause serious economic and environmental change. They can level anything in their path. Buildings can be crushed and completely ruined. Lahars can shear off hills or anything else in their path. The aftermath of a lahar is burial of the city or wildlife. Stones, boulders, tree trunks, and building rubble are picked up on the way. Mud flows, a type of Lahar, are often more powerful that rivers, killing thousands of people and causing enormous amounts of damage over a large area. After a the mud sets it becomes as hard as concrete.

American Red Cross

http://volcanoes.usgs.gov/Products/Pglossary/lahar.html
http://volcanoes.usgs.gov/Hazards/What/Lahars/lahars.html
http://www.uccs.edu/~geogenvs/ges199/rainier/lahars.htm