DEFINITION OF MUDFLOWS

Mudflows resemble debris flows, except that they have higher

concentrations of water (up to 30 percent), making them

more fluid, with a consistency ranging from soup to wet concrete.

Mudflows often start as a muddy stream in a dry

mountain canyon, which, as it moves, picks up more and

more mud and sand, until eventually the front of the stream

is a wall of moving mud and rock. When this comes out of

the canyon, the wall commonly breaks open, spilling the

water behind it in a gushing flood, which moves the mud

around on the valley floor. These types of deposits form

many of the gentle slopes at the bases of mountains in the

southwest United States.

Mudflows have also become a hazard in highly urbanized

areas such as Los Angeles, where most of the dry

riverbeds have been paved over, and development has moved

into the mountains surrounding the basin. The rare rainfall

events in these areas then have no place to infiltrate, and rush

rapidly into the city picking up all kinds of street mud and

debris, and forming walls of moving mud that cover streets

and low-lying homes in debris. Unfortunately, after the storm

rains and water recedes, the mud remains and hardens in

place. Mudflows are also common with the first heavy rains

after prolonged droughts or fires, as residents of many California

and other western states know. After the drought and

fires of 1989 in Santa Barbara, California, heavy rains

brought mudflows down out of the mountains filling the

riverbeds and inundating homes with many feet of mud. Similar

mudflows followed the heavy rains in Malibu in 1994,

which remobilized barren soil exposed by the fires of 1993.

Three to four feet of mud filled many homes and covered

parts of the Pacific Coast highway. Mudflows are part of the

natural geologic cycle in mountainous areas, and they serve

to maintain equilibrium between the rate of uplift of the

mountains, and their erosion. Mudflows are only catastrophic

when people have built homes, highways, and businesses in

places that mudflows must go.

Volcanoes too can produce mudflows: layers of ash and

volcanic debris, sometimes mixed with snow and ice, are easily

remobilized by rain or by an eruption and may travel many

tens of kilometers. Volcanic mudflows are known as lahars.

Mudflows have killed tens of thousands of people in single

events and have been some of the most destructive of

mass movements.

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