Why do rivers leap from their banks? Scientists strive to predict deadly flooding events

By Fred Pearce

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Rumors that the Kosi River was about to burst were spreading fast in Kusaha, a Nepalese village on the border with India. The river's levees, towering over the village, were being eroded quickly by the cresting waters. At 2 p.m. on 18 August 2008, the east bank ruptured. People ran for their lives as the breach grew. Soon, the entire river, one of the largest tributaries of the Ganges River, had overrun Kusaha and was spilling into India, drowning farm after farm in search of a faster path to the sea.

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For all their destructiveness, avulsions bring benefits to both nature and society. They unleash regular floods that nourish many of the world's great wetlands. For example, the vast Pantanal, in the heart of South America, is kept rich and muddy by the avulsing Taquari River.

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Although avulsions will never be wholly predictable, the day is nearing where engineers can know where and when the dangers are at their greatest. Adrian Hartley, a rivers researcher at the University of Aberdeen, is pleased with how far the field has come. “We are getting much closer to understanding avulsions.” But he warns that just as researchers begin to grasp these natural river cycles, human influences are altering them. “Few river systems are without significant human influence anymore, so simple generic models, however good they are, won't do all the job needed to predict avulsions.”

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Today, as the river rises as much as 10 meters above the surrounding flood plain, China is testing out new tactics in the vigil to avoid an avulsion. It has reduced sediment loads through soil conservation projects on the Loess Plateau. And it tested using fast flows of sediment-depleted water from dammed reservoirs to flush out sediment from the riverbed downstream and deliver it to the ocean, says Jeff Nittrouer, a hydrologist whose team at Rice University has been studying China's efforts to tame the Yellow River.

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