{"id":523249,"date":"2024-06-28T17:00:55","date_gmt":"2024-06-28T20:00:55","guid":{"rendered":"https:\/\/revistapesquisa.fapesp.br\/?p=523249"},"modified":"2024-06-28T17:00:55","modified_gmt":"2024-06-28T20:00:55","slug":"the-long-history-of-the-sao-francisco-river","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/the-long-history-of-the-sao-francisco-river\/","title":{"rendered":"The long history of the S\u00e3o Francisco river"},"content":{"rendered":"

The S\u00e3o Francisco river was once a long, meandering, shallow river, nothing like the Xing\u00f3 canyons formed hundreds of millions of years ago on the border between the states of Sergipe and Alagoas, where its waters today flow between rock walls up to 50 meters (m) high.<\/p>\n

Over the last 90,000 years, in response to changes in rainfall patterns and vegetation on the riverbanks, the shape of the S\u00e3o Francisco river, known locally as \u201cVelho Chico,\u201d has changed significantly. The 2,863-kilometer (km) river cuts through 521 municipalities from its source in the Serra da Canastra Mountains of Minas Gerais.<\/p>\n

When rain was more abundant between 90,000 and 66,000 years ago, the river carried large amounts of sediment and carved through the land. When sediment volumes were particularly high, sand was deposited on the riverbed and banks, creating an intertwined pattern of multiple channels flowing at the same time. When there was less sediment and more moderate rainfall between 66,000 and 39,000 years ago and between 19,000 and 9,000 years ago, it formed broad sinuous curves called meanders. Meandering rivers are common in tropical and subtropical environments, exemplified by the Purus and Juru\u00e1 rivers in the Amazon, which are the most winding in the world, alongside the Mississippi in the USA.<\/p>\n

The river\u2019s current path, some 20 m below its old position, took shape around 5,000 years ago, according to sediment analyses conducted by researchers from the University of S\u00e3o Paulo (USP), S\u00e3o Paulo State University (UNESP), the Federal University of S\u00e3o Paulo (UNIFESP), and the Federal University of Alagoas (UFAL).<\/p>\n

Geologist Patr\u00edcia Mescolotti points out that the method used to determine when sunlight last fell on the quartz crystals found in the sand on the riverbanks before they were covered by more recent sediment is limited to 90,000 years. \u201cThe river is likely older, but we cannot say for sure,\u201d says the researcher, who has been working at the Federal University of Mato Grosso do Sul (UFMS) in Campo Grande since May 2022. Describing the river’s geomorphological dimensions in a January 2022 article in the Brazilian Journal of Geomorphology<\/em>, geologists Landerlei Santos of UNESP and Edgardo Latrubesse of the Federal University of Goi\u00e1s (UFG) observed that some of the floodplains may have formed at the end of the Pleistocene epoch, between 10,000 and 82,000 years ago.<\/p>\n

During her PhD at UNESP in Rio Claro, under the guidance of geologist Mario Assine and UNIFESP geographer Fabiano Pupim, Mescolotti examined sediments collected from 51 points along the riverbank in Bahia and from dunes in the municipality of Xique-Xique, in Bahia (see picture<\/em>). She also studied aerial photos and satellite images that showed the old riverbed \u2014 shaped like small horseshoes, and abandoned as the water found easier paths to follow \u2014 and helped to reconstitute the terraces (former floodplains), which extended beyond the old riverbanks.<\/p>\n<\/div>

\n \n \n \n <\/picture>Alexandre Affonso \/ Revista Pesquisa FAPESP<\/span><\/div>
\n

\u201cDespite the river being a symbol of the Northeast, the flow of the S\u00e3o Francisco appears to be controlled by the South American monsoon system [characterized by intense rainfall in the summer and dry winters], which takes water from the Amazon to the Southeast of Brazil,\u201d explains Pupim. Unlike other large rivers, most of which are supplied by tributaries along their entire course, the S\u00e3o Francisco receives most of its water near its source, from tributaries such as the Rio das Velhas (the largest), the Paracatu, and the Urucuia. \u201cAs has been the case for thousands of years, around two-thirds of the river\u2019s water continue to come from the region of the river\u2019s source in Minas Gerais.\u201d By the time it reaches the municipality of Janu\u00e1ria, less than a thousand km from its source, the river already has almost 70% of its volume, as described in an article published in Quaternary Science Reviews <\/em>in April 2021.<\/p>\n

At the same time, USP geologist Cristiano Mazur Chiessi and colleagues studied the behavior of the S\u00e3o Francisco river basin by examining the proportion of two distinct forms of hydrogen and carbon in tree and grass remains accumulated in marine sediments, which they collected from depths of 1,897 m, less than 1 km from the river mouth on the border of Alagoas and Sergipe, in 2016. Their conclusions were similar to those of Pupim’s group: \u201cRainfall in the S\u00e3o Francisco river drainage basin, particularly at the headwaters and in the middle section, comes mainly from the Amazon,\u201d says Chiessi. \u201cWe didn\u2019t notice any significant long-term changes in the water source.\u201d<\/p>\n

The hydrogen and carbon distribution indicated periods of more or less intense rainfall in the S\u00e3o Francisco basin. \u201cWhen the dry season was shorter, trees occupied a greater area in the Cerrado in the area of the river basin near the source. When dry seasons were longer, conversely, grasses predominated,\u201d says USP geologist Jaqueline Quirino Ferreira, lead author of an article detailing the group\u2019s results, published in Quaternary Science Reviews <\/em>in March 2022.<\/p>\n

An particular aspect of Velho Chico is that it crosses three natural environments \u2014 the Atlantic Forest, Cerrado (savanna), and Caatinga (semiarid scrublands). \u201cLong rivers that flow from north to south or south to north generally cross more than one type of natural environment, such as the Paran\u00e1 River, which cuts through the Atlantic Forest and southern Brazil and Argentina,\u201d explains Jos\u00e9 C\u00e2ndido Stevaux, a geologist from the Federal University of Mato Grosso do Sul (UFMS) in Tr\u00eas Lagoas who worked with Mescolotti.<\/p>\n

S\u00e3o Francisco has one of the largest artificial lakes in the world, the 4,214 km2<\/sup> Sobradinho dam reservoir, part of the power plant of the same name in northern Bahia. This and four other large hydroelectric power plants (Tr\u00eas Marias, Lu\u00eds Gonzaga, Xing\u00f3, and Paulo Afonso) alter the width, depth, speed, and flow of the river, and consequently change the lives of residents in nearby towns.<\/p>\n

\u201cThe effects of the dams are clearer near the mouth, especially in three municipalities in Alagoas (Piranhas, P\u00e3o de A\u00e7\u00facar, and Traipu) and one in Sergipe (Propri\u00e1),\u201d observes geographer Genisson Panta, a PhD student at the Federal University of Pernambuco (UFPE) and high school teacher at a state public school in Macei\u00f3. Motivated by Stevaux, who he met at a conference in Fortaleza, Cear\u00e1, he has been studying these changes since 2019 and presented them in an article published in the Journal of South American Earth Sciences <\/em>in January.<\/p>\n

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Walter Antonio do Livramento\u2009\/\u2009Wikimedia Commons<\/span>In Piranhas, Alagoas, the river has become shallower in response to the increased flow caused by the Xing\u00f3 damWalter Antonio do Livramento\u2009\/\u2009Wikimedia Commons<\/span><\/p><\/div>\n

Based on field research and the analysis of data from hydrometric stations that measure the river\u2019s flow, he concluded that in Piranhas, the depth decreased in response to an increased flow caused by the Xing\u00f3 hydroelectric plant, built on the border between Alagoas and Sergipe in 1994. \u201cBefore, there was a layer of sediment at the bottom of the river, the depth of which was adjusted depending on the flow rate,\u201d he says. When the dam was built, the sediment layer was removed, the bed rocks were exposed, and the river could only make lateral adjustments, increasing its width.<\/p>\n

In the region between Propri\u00e1 in Sergipe and Porto Real do Col\u00e9gio in Alagoas, the river\u2019s banks widened by 250 m from 1969 to 2022. \u201cThis is a very high erosion rate, around 5 m per year,\u201d Panta emphasizes. According to him, erosion is not continuous \u2014 it is episodic. \u201cA single flood can wash away tons of sediment.\u201d<\/p>\n

In Propri\u00e1, the river’s flow was maintained, but its width increased from 600 m to 720 m after the Xing\u00f3 power plant began operating 150 km downstream in 1994. In Carinhanha, southwest of Bahia, the depth of the river increased from 2 m to 3 m after the construction of the Tr\u00eas Marias dam 700 km to the south, which was also completed in 1994. \u201cThe figures are similar to those of other tropical river systems, such as Tocantins-Araguaia and Paran\u00e1,\u201d he noted.<\/p>\n

In Pia\u00e7abu\u00e7u, a municipality in Alagoas located 10 km from the river mouth, an unusual number of teenagers and adults suffered hypertension, the cause of which was discovered in 2017: during a drought, seawater flowed back into the river and people consumed untreated brackish water. \u201cThe sea advances more easily into the river mouth now because since the dams were built, the river flow is controlled by the demand for electricity,\u201d says Panta.<\/p>\n

Given its current name by Italian explorer Am\u00e9rico Vesp\u00facio (1454\u20131512) in 1501 and still today the stage for events such as a boat procession in Penedo, Alagoas, every January, the S\u00e3o Francisco continues to be transformed by both human and natural forces. One ongoing major project is a transfer of its waters, started in 2007 and partially completed in 2022, through the construction of 700 km of concrete canals to supply farms and residents in the interior of northeastern Brazil.<\/p>\n

\u201cTo be successful, a strong plan is needed on how much water to take and how to distribute it, perhaps not taking it at certain times of the year so as not to harm the river,\u201d says Stevaux. \u201cHundreds of rivers have been diverted around the world. Dams cause much worse impacts.\u201d<\/p>\n

Shifting dunes <\/strong><\/p>\n

\"\"

Patricia Colombo Mescolotti<\/span>Now divided by rivers, the dunes of Xique-Xique once formed a singular areaPatricia Colombo Mescolotti<\/span><\/p><\/div><\/p>\n

Based on analyses of sand collected from depths of up to 2 meters, the dunes in Xique-Xique, Bahia, which today occupy an area of 8,000 km2<\/sup>, began to be formed by sand from the S\u00e3o Francisco river at least 150,000 years ago, when the climate was dry, as described in an article published in the journal Geomorphology <\/em>in January.<\/p>\n

\u201cIn several places, the dunes were once more extensive and formed a single area, today divided by the rivers that flow into the S\u00e3o Francisco,\u201d says Patricia Mescolotti, from UFMS. Different reptile and mammal species live on either side, differentiated from a single species.<\/p>\n

The dunes, up to 30 m high, shift in response not only to the river sands that feed them, but also to the removal of vegetation from the riverbanks and the intensity of droughts, according to a paper published in Geomorphology <\/em>by Santos and Latrubesse in November 2021. They observed that the dunes migrated an average of 15 meters per year between 2002 and 2010 and 9.4 meters per year from 2010 to 2019.<\/p>\n

Roughly 15 years ago, sand from the Geleia dunes covered the roads and houses in the village of Icatu, Bahia, forcing inhabitants to move to previously settled dunes.<\/div>\n

Projects
\n1.<\/strong> Historical perspectives on critical thresholds of the climate system: The Amazon rainforest and the Atlantic Meridional Overturning Circulation (PPTEAM) (n\u00ba 18\/15123-4<\/a>); Grant Mechanism<\/strong> Research Grant \u2012 Program for Research on Global Climate Change \u2012 Young Investigator Award; Principal Investigator<\/strong> Cristiano Mazur Chiessi (USP); Investment<\/strong> R$3,123,253.45.
\n2. <\/strong>Assessing the effects of past and future climate changes on Amazonian biodiversity (CLAMBIO) (n\u00ba 19\/24349-9<\/a>); Grant Mechanism<\/strong> Research Grant \u2012 Biota Program; Principal Investigator<\/strong> Cristiano Mazur Chiessi (USP); Investment<\/strong> R$230,317.74.<\/p>\n

Scientific articles<\/strong>
\n<\/strong>FERREIRA, J. Q. et al<\/em>. Changes in obliquity drive tree cover shifts in eastern tropical South America<\/a>. Quaternary Science Reviews<\/strong>. vol. 279, 107402. mar. 1, 2022.
\nMESCOLOTTI, P. C. et al<\/em>. Fluvial aggradation and incision in the Brazilian tropical semi-arid: Climate-controlled landscape evolution of the S\u00e3o Francisco river<\/a>. Quaternary Science Reviews<\/strong>. vol. 263, 106977. july 1, 2021.
\nMESCOLOTTI, P. C. et al<\/em>. The largest Quaternary inland eolian system in Brazil: Eolian landforms and activation\/stabilization phases of the Xique-Xique dune field<\/a>. Geomorphology<\/strong>. vol. 420, 108516. jan. 1, 2023.
\nPANTA, G. et al<\/em>. Morphohydraulic of a dam-impacted large river: The S\u00e3o Francisco River, Brazil<\/a>. Journal of South American Earth Sciences<\/strong>. vol. 121, 104167. jan. 2023.
\nSANTOS, L.A.; LATRUBESSE, E.M. Unidades geomorfol\u00f3gicas da plan\u00edcie aluvial do M\u00e9dio Rio S\u00e3o Francisco, Nordeste do Brasil<\/a>. Revista Brasileira de Geomorfologia<\/strong>. vol. 23, no. 1, pp. 1097\u2013115. jan. 13, 2022.
\nSANTOS, L.A.; LATRUBESSE, E.M. Aeolian mobility in the Middle S\u00e3o Francisco Dune Field, Northeast Brazil, as a response to Caatinga\u2019s droughts and land-use changes<\/a>. Geomorphology<\/strong>. vol. 393, 107940. nov. 15, 2021.<\/p>\n","protected":false},"excerpt":{"rendered":"In the last 90,000 years, the S\u00e3o Francisco river has straightened and deepened under the influence of erosion and dams","protected":false},"author":17,"featured_media":523254,"comment_status":"closed","ping_status":"closed","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":[],"coauthors":[5968],"class_list":["post-523249","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-science"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/523249","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\/17"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=523249"}],"version-history":[{"count":1,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/523249\/revisions"}],"predecessor-version":[{"id":523266,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/523249\/revisions\/523266"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media\/523254"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=523249"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=523249"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=523249"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=523249"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}