the São Francisco river has straightened and deepened under the influence of erosion and dams
nothing like the Xingó 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
in response to changes in rainfall patterns and vegetation on the riverbanks
known locally as “Velho Chico,” 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
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 Juruá rivers in the Amazon
according to sediment analyses conducted by researchers from the University of São Paulo (USP)
the Federal University of São Paulo (UNIFESP)
and the Federal University of Alagoas (UFAL)
Geologist Patrícia 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
but we cannot say for sure,” 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
geologists Landerlei Santos of UNESP and Edgardo Latrubesse of the Federal University of Goiás (UFG) observed that some of the floodplains may have formed at the end of the Pleistocene epoch
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
She also studied aerial photos and satellite images that showed the old riverbed — shaped like small horseshoes
and abandoned as the water found easier paths to follow — and helped to reconstitute the terraces (former floodplains)
    Alexandre Affonso / Revista Pesquisa FAPESP“Despite the river being a symbol of the Northeast
the flow of the São 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,” explains Pupim
most of which are supplied by tributaries along their entire course
the São Francisco receives most of its water near its source
from tributaries such as the Rio das Velhas (the largest)
“As has been the case for thousands of years
around two-thirds of the river’s water continue to come from the region of the river’s source in Minas Gerais.” By the time it reaches the municipality of Januária
the river already has almost 70% of its volume
as described in an article published in Quaternary Science Reviews in April 2021
USP geologist Cristiano Mazur Chiessi and colleagues studied the behavior of the São 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
Their conclusions were similar to those of Pupim’s group: “Rainfall in the São Francisco river drainage basin
particularly at the headwaters and in the middle section
comes mainly from the Amazon,” says Chiessi
“We didn’t notice any significant long-term changes in the water source.”
The hydrogen and carbon distribution indicated periods of more or less intense rainfall in the São Francisco basin
trees occupied a greater area in the Cerrado in the area of the river basin near the source
grasses predominated,” says USP geologist Jaqueline Quirino Ferreira
lead author of an article detailing the group’s results
published in Quaternary Science Reviews in March 2022
An particular aspect of Velho Chico is that it crosses three natural environments — the Atlantic Forest
“Long rivers that flow from north to south or south to north generally cross more than one type of natural environment
which cuts through the Atlantic Forest and southern Brazil and Argentina,” explains José Cândido Stevaux
a geologist from the Federal University of Mato Grosso do Sul (UFMS) in Três Lagoas who worked with Mescolotti
São Francisco has one of the largest artificial lakes in the world
part of the power plant of the same name in northern Bahia
This and four other large hydroelectric power plants (Três Marias
and consequently change the lives of residents in nearby towns
“The effects of the dams are clearer near the mouth
especially in three municipalities in Alagoas (Piranhas
and Traipu) and one in Sergipe (Propriá),” observes geographer Genisson Panta
a PhD student at the Federal University of Pernambuco (UFPE) and high school teacher at a state public school in Maceió
he has been studying these changes since 2019 and presented them in an article published in the Journal of South American Earth Sciences in January
Walter Antonio do Livramento / Wikimedia CommonsIn Piranhas
the river has become shallower in response to the increased flow caused by the Xingó damWalter Antonio do Livramento / Wikimedia Commons
Based on field research and the analysis of data from hydrometric stations that measure the river’s flow
the depth decreased in response to an increased flow caused by the Xingó hydroelectric plant
built on the border between Alagoas and Sergipe in 1994
there was a layer of sediment at the bottom of the river
the depth of which was adjusted depending on the flow rate,” he says
and the river could only make lateral adjustments
In the region between Propriá in Sergipe and Porto Real do Colégio in Alagoas
the river’s banks widened by 250 m from 1969 to 2022
erosion is not continuous — it is episodic
“A single flood can wash away tons of sediment.”
but its width increased from 600 m to 720 m after the Xingó power plant began operating 150 km downstream in 1994
the depth of the river increased from 2 m to 3 m after the construction of the Três Marias dam 700 km to the south
“The figures are similar to those of other tropical river systems
such as Tocantins-Araguaia and Paraná,” he noted
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
“The 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,” says Panta
Given its current name by Italian explorer Américo Vespúcio (1454–1512) in 1501 and still today the stage for events such as a boat procession in Penedo
the São 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
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,” says Stevaux
“Hundreds of rivers have been diverted around the world
Patricia Colombo MescolottiNow divided by rivers
the dunes of Xique-Xique once formed a singular areaPatricia Colombo Mescolotti
Based on analyses of sand collected from depths of up to 2 meters
began to be formed by sand from the São Francisco river at least 150,000 years ago
as described in an article published in the journal Geomorphology in January
the dunes were once more extensive and formed a single area
today divided by the rivers that flow into the São Francisco,” says Patricia Mescolotti
Different reptile and mammal species live on either side
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 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
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