{"id":166173,"date":"2015-01-25T13:35:32","date_gmt":"2015-01-25T15:35:32","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/?p=166173"},"modified":"2015-07-03T13:44:27","modified_gmt":"2015-07-03T16:44:27","slug":"rivers-with-a-mind-of-their-own","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/rivers-with-a-mind-of-their-own\/","title":{"rendered":"Rivers with a mind of their own"},"content":{"rendered":"
\"Abandoned

MARIO ASSINE \/ UNESP<\/span>Abandoned river channels drain off floodwaters in the PantanalMARIO ASSINE \/ UNESP<\/span><\/p><\/div>\n

In the Pantanal wetland, one of the world\u2019s 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.<\/p>\n

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. \u201cDating performed in some of the abandoned channels, known as paleochannels, has revealed ages that vary from tens to thousands of years,\u201d Assine says. Working with his team at S\u00e3o 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.<\/p>\n

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. \u201cHuman occupation and farming are not the cause of the rivers\u2019 changes in course,\u201d he says. \u201cBut those activities speed up the process by increasing the rate at which sediments are transported from the plateau to the plains.\u201d<\/p>\n

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\u2014loss of 13% and 40%, respectively, of their vegetation to agriculture and fishing\u2014have increased the volume of sediments that reach the Pantanal.<\/p>\n

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.<\/p>\n

\"Taquari

NASA<\/span>Taquari megafan: area in light blue (center<\/em>) shows the river\u2019s most recent change in courseNASA<\/span><\/p><\/div>\n

Avulsions occur frequently and cause major changes in a river\u2019s 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 Caiap\u00f3 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).<\/p>\n

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 Z\u00e9 da Costa farm. At that point, the river broke through one of its banks\u2014that is, it formed an avulsion, or arrombado<\/em>, as it is called locally\u2014and 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 Corumb\u00e1, 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.<\/p>\n

The phenomenon that Assine observed in the Taquari\u2014whose headwaters are in an area of Cerrado savannah that has been considerably altered by agriculture\u2014has also been documented by Assine and his team further to the north, in the S\u00e3o Louren\u00e7o, a river that originates on the Guimar\u00e3es Plateau in the state of Mato Grosso, flows through the city of Rondon\u00f3polis and discharges into the Cuiab\u00e1 River. After studying satellite images and conducting a flyover of the area, Assine, along with geographers Fabiano Pupim and Fabr\u00edcio Corradini (who at the time were doctoral students under his advisorship) and American geologist Michael McGlue, collected sediments at different points along the river.<\/p>\n

\"\"<\/a>Sediment dating analysis showed that avulsions have been altering the course of the S\u00e3o Louren\u00e7o for tens of thousands of years, creating a 16,000-square-mile megafan. One avulsion occurred in the early 20th<\/sup> 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 Cuiab\u00e1. \u201cThis is evidence that the rivers of the Pantanal change course through natural causes, independent of human activity,\u201d Assine explains.<\/p>\n

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<\/em>, engineer Virg\u00edlio Correia Filho presents information on the course of the S\u00e3o Louren\u00e7o since the 18th<\/sup> century. Although it was a significant event, it was only one of the river\u2019s transformations. The history of the S\u00e3o Louren\u00e7o, reconstructed through analysis of the landscape and sediment dating, revealed a past marked by radical changes.<\/p>\n

The researchers associate the changes in morphology and dynamics of the S\u00e3o Louren\u00e7o\u2014which have also been observed in other rivers of the Pantanal\u2014with past climate change events. Prior to 10,000 years ago, the S\u00e3o Louren\u00e7o 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. \u201cThey were high-flow rivers during some periods, and then they disappeared, typical of a semiarid climate,\u201d Assine points out.<\/p>\n

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 S\u00e3o Louren\u00e7o 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\u2014Mandior\u00e9, Ga\u00edva and Ba\u00eda Vermelha\u2014which confirmed the climatic oscillations in the Pantanal in the past 20,000 years that gave its rivers their present-day contours. \u201cThe dynamic landscape of the Pantanal is the consequence of changes that have been occurring since the late Pleistocene,\u201d Assine explains. \u201cThat understanding is critical to the use, occupation and conservation of an area so susceptible to change.\u201d<\/p>\n

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

Scientific articles<\/em>
\nASSINE, M. L. et al<\/em>. Channel arrangements and depositional styles in the S\u00e3o Louren\u00e7o fluvial megafan, Brazilian Pantanal wetland<\/a>. Sedimentary Geology<\/strong>. 2014.
\nKUERTEN, S. et al<\/em>. Sponge spicules indicate Holocene environmental changes on the Nabileque River floodplain, Southern Pantanal<\/a>. Journal of Paleolimnology<\/strong>. 2013.
\nMcGLUE, M. et al<\/em>. Lacustrine records of Holocene flood pulse dynamics in the Upper Paraguay River watershed (Pantanal wetlands, Brazil)<\/a>. Quaternary Research<\/strong>.<\/em> 2012.<\/p>\n","protected":false},"excerpt":{"rendered":"Frequent changes in course leave scars on the Pantanal landscape","protected":false},"author":16,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[159],"tags":[200,240],"coauthors":[105],"class_list":["post-166173","post","type-post","status-publish","format-standard","hentry","category-science","tag-environment","tag-geology"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/166173","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/users\/16"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=166173"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/166173\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=166173"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=166173"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=166173"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=166173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}