Mount Pinatubo's eruption, the second largest recorded in the 20th century, deposited nearly 1.5 cubic miles of volcanic ash and rock on its flanks, about 10 times more than Mount St. Helens in Washington state deposited in its eruptions in 1980. The town of Bacolor at the edge of Pinatubo was buried repeatedly by major lahars. Today a large church in Bacolor must be entered through the choir loft everything below is filled with sediment.

"The thing about a mud flood is that it doesn't recede. It just stays," Montgomery said.

Eventually, all the river channels will stabilize and lahars will occur infrequently, Gran said. That's because the fine-grained ash and pumice typically are the first to be carried away by water. As a river cuts deeper into the sediment, coarser material eventually will form a more solid streambed and the amount of sediment in the water will decline steadily.

Sediment runoff comes from three primary areas, Gran said. One is the vast amount of material hundreds of feet thick deposited in a river valley, which the river can wash away as it meanders across the valley. Another is high terraces formed from a combination of fine and coarse material, which stand above the river level but erode into the river during heavy rainfall and eventually can be stabilized by vegetative growth. The third source is high cliffs that are unstable and can drop large chunks of sediment into the water at any time.

The frequency of lahars is much lower now than in the first five years after the eruption, she said, and the indigenous Ayta people are moving back to their ancestral home on the mountain's uplands. But the large amount of sediment remaining on Pinatubo's flanks still poses danger. Some of the rivers still can flood wide areas, making it difficult to site and build bridges. And in
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Contact: Vince Stricherz
vinces@u.washington.edu
206-543-2580
University of Washington
5-Jan-2005