Oct. 3, 2003 Conventional wisdom says a river's flood plain builds bit by bit, flood after flood, whenever the stream overflows its banks and deposits new sediment on the flood plain. But for some vast waterways in South America's Amazon River basin, that wisdom doesn't hold water.
Seasonal rains wash billions of tons of rock and soil from the Andes Mountains each year. But new evidence suggests that it's only about once every eight years, when the equatorial climate phenomenon known as La Niña is in full swing, that water rises rapidly enough to move huge amounts of that sediment to the flood plains.
Rolf Aalto, a University of Washington geologist, uses a lightweight tube-like device to extract core samples of such sediment deposits across the remote, thickly forested and unspoiled flood plains of northern Bolivia. He measures the amount and activity of lead-210, a radioactive isotope produced by the decay of naturally occurring radon. The isotope is deposited on the flood plain by fresh river sediment and by rainfall, and it can be used to date successive layers of sediment, for as long as a century after it is deposited, as the lead-210 decays further.
But when Aalto examined samples from the Beni River, which flows northward from the Andes into a Mississippi-sized Amazon tributary called the Rio Madeira, he was puzzled.
"I wasn't seeing a signal that looked anything like what I expected from previous studies on other river flood plains. I was befuddled," he said. "I would see a lot of lead-210 on the surface, and then the activity would drop down to an unchanging level of activity for a long way down, until the next big decrease."
The multitude of thin layers and gradually declining lead-210 activity that he expected to see simply didn't show up. The reason eventually occurred to him – a few thick sediment layers were filling most of the core samples. Once he came to that conclusion, he devised a new dating method to better reflect a more complex flood plain environment, then analyzed new samples that showed similar large deposits at various intervals going back nearly 100 years. Each of these large layers was deposited at a time that correlated with a La Niña event in the equatorial Pacific Ocean. La Niña is the cold phase of the El Niño Southern Oscillation climate phenomenon that now is recognized as having wide affects on weather throughout the Western Hemisphere.
Aalto is lead author of a paper documenting the findings that appears in the Oct. 2 edition of the journal Nature. Other authors are Laurence Maurice-Bourgoin and Jean-Loup Guyot of Institut de Recherche pour le Développement (IRD) in France and Brazil, who have conducted extensive field studies in Bolivia and Brazil; Thomas Dunne of the University of California, Santa Barbara; and David Montgomery and Charles Nittrouer, both UW Earth and space sciences professors.
Their research, funded by the National Science Foundation and IRD, has focused on two rivers, the Beni and the Mamoré, that flow from the Bolivian Andes into the Rio Madeira. The researchers used the same techniques to measure mercury concentration in the sediment and determine when it was deposited, a first step in tracking substances of environmental concern.
When the heavy rains of La Niña hit the Andes, great floods wash vast amounts of sediment from the mountains. When the water gushes from the mountains, the streams rise very rapidly and can create a crevasse, a breach in the naturally occurring earthen levees at the river's edge. Once a levee is breached, the river water rushes through the gap carrying huge amounts of sediment, which is splayed across and deposited on the flood plain by the flowing water.
"The whole system is being raised," Aalto said. "The river channel is being raised and the flood plain is being raised too, as these splays shoot out from the channel and deposit lobes of sediment. And in most cases this is happening in precise timing with large La Niña events. Sediment is moved and deposited throughout the river system in episodes orchestrated by the El Niño cycle."
Thousands of years ago, the Beni flowed into the Mamoré near the Andes. But its course has changed many times though the millennia and the main channel now is hundreds of miles away from the Mamoré. Relic channels, previous paths along which the Beni used to distribute water and sediment to its flood plain, are clearly visible on Landsat images from the National Aeronautics and Space Administration, which also helped finance the work.
Each shift in the Beni's course probably came after it filled in part of a huge sedimentary basin at the foot of the Andes, created by the tremendous weight of the mountains pushing down on the tectonic plate on which they sit. The portion of the plate next to the mountains is depressed to create what is called a foreland basin, essentially a huge repository for sediment carried by rivers. In Bolivia, the foreland basin can be more than three miles deep. By depositing sediment as a river shifts course across its flood plain, the stream fills in the foreland basin.
All of the water and some sediment from the Beni and the Mamoré, along with many other rivers, ends up in the lower main stem of the Amazon, the world's largest river. But the Amazon traps only about 10 percent of the sediment from its tributaries, while more than 50 percent is trapped within upstream tributaries crossing foreland basins near the mountains.
"Essentially all sediment in the Amazon comes from the Andes, and more than half is trapped in foreland basins along the mountains," Aalto said. "The Andes and its foreland basins cover about one-fifth of the area of the Amazon but account for most of the sediment transfer."
Major floods in which a river shifts course don't happen often on human time scales, but these Bolivian rivers still can migrate large distances during a single flood, he said.
"If the Beni River were in the United States, it would definitely be thought of as a problem channel," he said. "If people lived along it they would be appalled to have it migrating rapidly and moving many miles in just a generation. And the prospect of having the entire river channel jump tens of miles would be devastating."
Aalto believes such active channel migration, periodic large sediment deposits on the flood plain and large floods might all be part of the general mechanics of large rivers. However, that theory cannot be fully explored in the United States because few rivers of appreciable size still function naturally, without at least some significant human-caused disturbance. He hopes that studying largely unspoiled rivers such as the Beni and Mamoré will help to build a better understanding of how rivers behave in completely natural conditions, and that understanding can be applied to disturbed river systems in the United States and elsewhere.
"For a geologist interested in the redistribution of mass across continents, it's an exciting place to work," he said. "Rivers are the arteries of continents, and the Beni is about as close to the heart of the Amazon as you can get."
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