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Geology

Rivers with a mind of their own

Frequent changes in course leave scars on the Pantanal landscape

Abandoned river channels drain off floodwaters in the Pantanal

MARIO ASSINE / UNESPAbandoned river channels drain off floodwaters in the PantanalMARIO ASSINE / UNESP

In the Pantanal wetland, one of the world’s largest floodplains, the rivers seem to have a mind of their own. They originate on the plateaus and flow down towards the lowlands in confined, well-established beds that nearly always meander along in a snaking pattern. But before long they rise up. As they flow along the plains, the rivers escape their rock-enclosed channels and take other courses, building new banks out of sediments from the plateau. This continuous remodeling has left imprints on the landscape in the form of channels, now abandoned, that look like giant fans.

Geologist Mario Luis Assine, who has been studying the rivers of the Pantanal since the 1990s, now knows that this remodeling phenomenon is a characteristic feature of the region. “Dating performed in some of the abandoned channels, known as paleochannels, has revealed ages that vary from tens to thousands of years,” Assine says. Working with his team at São Paulo State University (Unesp) in Rio Claro, Assine is attempting to reconstruct the changes that have taken place in the regional landscape over the past 100,000 years.

Although the nomadic movement of the Pantanal rivers is a natural feature throughout the region, the pace of this phenomenon has been accelerating. According to Assine, it is a consequence of changes in land use on the plateau. “Human occupation and farming are not the cause of the rivers’ changes in course,” he says. “But those activities speed up the process by increasing the rate at which sediments are transported from the plateau to the plains.”

Covered partly by Cerrado vegetation and partly by Amazon forest, the plateaus that are modeling the sedimentary basin are 200 to 300 kilometers from the heart of the Pantanal, where the terrain remains under water for up to four months each year. The rivers collect rainwater on the plateau, and then carry along the sediments and nutrients that feed into this area of about 150,000 square kilometers. And the transformations undergone by the Amazon Region and the Cerrado over the past five decades—loss of 13% and 40%, respectively, of their vegetation to agriculture and fishing—have increased the volume of sediments that reach the Pantanal.

As they cross the plains, the coarser, heavier sediments accumulate and block the riverbed. The water then breaches the banks and spreads out. Known as fluvial avulsion, this sudden change in course achieved by breaking through the banks is common along the final stretches of rivers in the Pantanal, where the terrain may lie two to four meters below the river channel.

Taquari megafan: area in light blue (center) shows the river’s most recent change in course

NASATaquari megafan: area in light blue (center) shows the river’s most recent change in courseNASA

Avulsions occur frequently and cause major changes in a river’s course in just a few decades. Assine and his colleagues first documented this phenomenon of the Pantanal on the Taquari River, which originates in the Caiapó Mountains near the city of Coxim, in the state of Mato Grosso do Sul. The Taquari runs westward to the Paraguay River, near the border with Bolivia. Nearly 800 kilometers long, this river features the second largest watershed in the Pantanal. The ongoing abandonment of channels along the final third of the river has etched a fluvial megafan that extends for some 50,000 square kilometers (37% of the Brazilian Pantanal wetland).

This and other megafans in the region are the record of both past and ongoing events. Satellite images analyzed by Assine have shown that a major avulsion of the Taquari occurred in 1990, near the Zé da Costa farm. At that point, the river broke through one of its banks—that is, it formed an avulsion, or arrombado, as it is called locally—and divided into two branches. Six years later, half of its water was flowing through the new channel, and by 2001 its earlier course had been abandoned. Within a decade, the mouth of the river had migrated nearly 30 kilometers in the direction of Corumbá, abandoning the section that had discharged near Porto da Manga. Today the old channel is covered by vegetation and receives water only during flood periods, which in the southern Pantanal reach their peak in June and July.

The phenomenon that Assine observed in the Taquari—whose headwaters are in an area of Cerrado savannah that has been considerably altered by agriculture—has also been documented by Assine and his team further to the north, in the São Lourenço, a river that originates on the Guimarães Plateau in the state of Mato Grosso, flows through the city of Rondonópolis and discharges into the Cuiabá River. After studying satellite images and conducting a flyover of the area, Assine, along with geographers Fabiano Pupim and Fabrício Corradini (who at the time were doctoral students under his advisorship) and American geologist Michael McGlue, collected sediments at different points along the river.

Sediment dating analysis showed that avulsions have been altering the course of the São Lourenço for tens of thousands of years, creating a 16,000-square-mile megafan. One avulsion occurred in the early 20th century, well before Central-West Brazil became an agricultural frontier. This change in course moved the mouth of the river a few dozen kilometers to the west. As a result, the river, which once flowed into the Piquiri, began to discharge its waters into the Cuiabá. “This is evidence that the rivers of the Pantanal change course through natural causes, independent of human activity,” Assine explains.

He and his colleagues estimated that this change occurred hundreds of years ago at most, but they were unable to pinpoint the timing more precisely. Only after finding a historic document did they discover that the event occurred between 1900 and 1910. In a 1942 article published in the Revista Brasileira de Geografia, engineer Virgílio Correia Filho presents information on the course of the São Lourenço since the 18th century. Although it was a significant event, it was only one of the river’s transformations. The history of the São Lourenço, reconstructed through analysis of the landscape and sediment dating, revealed a past marked by radical changes.

The researchers associate the changes in morphology and dynamics of the São Lourenço—which have also been observed in other rivers of the Pantanal—with past climate change events. Prior to 10,000 years ago, the São Lourenço did not meander as it does today. It flowed along straighter lines that bifurcated and then rejoined, forming an interlacing pattern similar to that of the Ganges Basin in India. In many cases, the channels created by these bifurcations disappeared from the landscape. “They were high-flow rivers during some periods, and then they disappeared, typical of a semiarid climate,” Assine points out.

One major change occurred at the end of the last glacial period, between 15,000 and 12,000 years ago, when the temperature rose about seven degrees in what is now Central-Southern Brazil. The higher temperature and humidity turned the São Lourenço into a perennial river. Assine, McGlue and other colleagues, including Sidney Kuerten of Mato Grosso State University and Aguinaldo Silva of the Federal University of Mato Grosso, collected sediments from three large lakes—Mandioré, Gaíva and Baía Vermelha—which confirmed the climatic oscillations in the Pantanal in the past 20,000 years that gave its rivers their present-day contours. “The dynamic landscape of the Pantanal is the consequence of changes that have been occurring since the late Pleistocene,” Assine explains. “That understanding is critical to the use, occupation and conservation of an area so susceptible to change.”

Project
Depositional systems of the Quaternary (late Pleistocene/Holocene) in the Pantanal Basin, Mato Grosso, Central-West Brazil (No. 2007/55987-3); Grant mechanism Regular Line of Research Project Award; Principal investigator Mario Luis Assine (Unesp/Rio Claro); Investment R$262,065.96 (FAPESP).

Scientific articles
ASSINE, M. L. et al. Channel arrangements and depositional styles in the São Lourenço fluvial megafan, Brazilian Pantanal wetland. Sedimentary Geology. 2014.
KUERTEN, S. et al. Sponge spicules indicate Holocene environmental changes on the Nabileque River floodplain, Southern Pantanal. Journal of Paleolimnology. 2013.
McGLUE, M. et al. Lacustrine records of Holocene flood pulse dynamics in the Upper Paraguay River watershed (Pantanal wetlands, Brazil). Quaternary Research. 2012.

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