Probing sedimentary layers in the Karoo Basin of South Africa, the scientists found evidence that, with the loss of deep-rooting plants, meandering river systems changed rapidly to braided systems. Braided streams run much straighter and faster and branch out for short distances before merging back to the primary stream. They also cause much faster sediment buildup because vegetation is not holding streamside soil in place and it is easily swept away by the faster-moving water.

Using data from the Karoo and elsewhere, the scientists attribute the drastic change in river character to a catastrophic global die off of vegetation that likely resulted from the same cause as the mass extinction among marine and land animals.

Peter Ward, a UW geological sciences professor, along with David Montgomery, a UW associate geological sciences professor, and Roger Smith, the South African Museum's curator of geology, publish their findings in the Sept. 8 issue of the journal Science.

Sedimentary layers from the Permo-Triassic boundary were examined at seven different sites scattered across 250 miles of the Karoo Basin, and the researchers found striking similarities in the evidence for a rapid shift from meandering to braided streams.

Major tectonic activity could change streams from meandering to braided, Ward said. However, recent studies have shown there was no major tectonic activity at the time of the Permo-Triassic extinction, which occurred when the Earth's land was still locked in a supercontinent called Pangea.

Braided streams were common until the Silurian Period some 400 million years ago, but then gave way to meandering streams as plant life evolved. Today it is rare to see a braided stream unless it is in a place, like Mount St. Helens in Washington state, where the landscape has been denuded by a catastrophic event such as a volcanic eruption.

"The thing we take so for granted now -- meandering rivers -- is a very recent feature on Earth," Ward said. "This didn't appear until the Silurian, when land plants started to take over the Earth."

The sudden reappearance of braided streams, probably on a global scale, 250 million years ago is strong evidence of a major catastrophe that wiped out plant life as well as much of animal life, Ward said. He noted that plant life emerged again relatively quickly at the beginning of the Triassic Period.

A number of potential causes for the mass extinction have been postulated, including the impact of an asteroid or comet, environmental shifts, volcanism or the overturning of the oceans to release trapped gases into the atmosphere. In their research, paid for by a National Science Foundation grant, Ward, Montgomery and Smith do not speculate which, if any, of the theories is correct. But they say that the way plant life disappeared indicates events happened very quickly on a geological scale.

"Whenever you describe something as happening in thousands, rather than millions, of years, that's very fast geologically speaking," Ward said. "The new evidence helps us understand how rapid this was, because the transition from meandering to braided streams was quick. And I think the most important thing is that it tells us how catastrophic this was. It was the most catastrophic event in Earth history, or at least in the history of life."

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