Erupting volcanoes are among the most destructive forces in Mother Nature's arsenal. But where many people live on or near the flanks of such mountains, the real disaster often doesn't start until the eruption has subsided and the world has stopped paying attention. It is then that rain-swollen rivers emanating from volcanic peaks can send massive lahars – large waves of mud made up of water, ash and volcanic rock – careening down the mountainsides, often burying everything in their paths, even entire towns and villages. Such lahars can occur for years after an eruption, depending on how much debris the volcano deposits and how much rain falls, until the sediment has either been cleaned off the mountain or has stabilized so that it doesn't erode easily.
Mount Pinatubo, northwest of Manila on the Philippine island of Luzon, erupted with devastating force in June 1991 and now is proving to be an ideal laboratory for studying the "hydrologic aftermath" of a volcanic eruption, said Karen Gran, a University of Washington doctoral student in Earth and space sciences.
Gran has been studying data compiled from 1997 through 2003 from five rivers on Pinatubo's flanks. The streams are in various stages of recovery, with one almost back to its pre-eruption state because it didn't become as clogged by sediment. But others traverse areas that still have vast amounts of sediment that can be washed away easily. Pinatubo's location, in the tropics not far north of the equator, makes it subject to torrential rains from monsoons and typhoons.
"In one of the streams we're studying, nothing can live. If a big storm hits, the whole riverbed moves," Gran said. That means that more than 13 years after the eruption, some of the rivers studied have not recovered to the point of having stable channels, which are necessary for a return of aquatic species and a general ecological recovery.
Gran is a lead author of a paper detailing the research on how streams on volcanoes recover, published Jan. 5 in the Geological Society of America Bulletin. David Montgomery, a UW Earth and space sciences professor, is co-author.
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 some places, the water flows at a much higher elevation than it used to, placing the streambeds above where people live.
"There's been more loss of life and property at Pinatubo from lahars than from the eruption itself," Gran said. "And recovery comes slowly, in stages. It could be years before we start to see ecological recovery on some rivers."
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