São Francisco River ecosystem

Category: Inland Aquatic Biomes.

Geographic Location: South America.

Summary: The longest river contained entirely within Brazil, the São Francisco is considered the river of national unification; it serves as a lifeline for agriculture and economic development in an area often beset by drought.

The São Francisco is the longest river contained entirely within Brazil, and the fourth-longest river in South America. It originates in the Serra de Canastra mountain range in the states of Minas Gerais and Bahia at about 2,400 feet (730 meters) in elevation, and winds north and east through the countryside before emptying into the South Atlantic Ocean. The total length of the São Francisco is nearly 1,900 miles (3,000 kilometers), and its watershed occupies more than 240,000 square miles (630,000 square kilometers).

The states of Alagoas, Pernambuco, and Sergipe also provide drainage into the São Francisco basin, bringing water from 168 rivers and streams, of which 90 are on the right bank and 78 on the left bank. Principal tributaries of the river include the Abaeté, Carinhanha, Das Velhas, Jequitaí, Paracatu, Corrente, Salitre, and Verde Grande Rivers.

Much of the São Francisco river basin is arid or semiarid; with its average water flow of 100,000 cubic feet (2,830 cubic meters) per second, the river provides a lifeline for the region known as Brazil’s “drought polygon.” This dry zone receives about 15–30 inches (37–75 centimeters) of precipitation a year, whereas much of eastern Brazil receives 40–70 inches (100–180 centimeters) annually. The Sao Francisco’s flow is perennial, although its water level can change dramatically over the course of the year. The upper river basin—the Bahia interior forests—have average annual temperatures of 64–72 degrees F (18–22 degrees C). The lower river basin, primarily a semidesert ecoregion, has an average maximum temperature of 92 degrees F (33 degrees C) and average minimum temperature of 66 degrees F (19 degrees C).

Flora and Fauna

The upper river basin of the São Francisco is primarily savanna and forest; hardwoods grown in this area include the vinhatico, jacaranda, and the Brazilian cedar. The upper-middle basin is an agricultural region in which cotton, beans, rice, and corn (maize) are grown. The region also produces pineapples, potatoes, maté tea, melons, coffee, castor and cottonseed oils, and sugarcane (grown here mainly for rum). Other plant types growing in this region include the aloe plant, the cochineal cactus, and the vanilla plant.

Also in the middle river basin, stunted, thorny forest is typical; dominant trees in this area include the barriguda, the catingueiras, and the juremas (a palmlike tree). Many types of cacti and bromeliads also grow in this zone, along with rubber and cashew trees. Along the riverbanks in the lowlands here, cassava (manioc), corn, beans, and melons are cultivated in shallow waterbeds. On the coastal lowlands, rice and sugarcane are grown. The dry lowlands of the basin (or the sertão) are used largely for livestock grazing, with herds mainly featuring cattle, goats, sheep, and donkeys.

Non-domesticated mammal species that may be found in the upper, forested river basin include Coimbra’s titi monkey, blond titi monkey (critically endangered), southern masked titi monkey, maned three-toed sloth (endangered), maned wolf, prehensile-tailed porcupine, ocelot, oncilla, lion tamarin (an endangered monkey species), ring-tailed coati, Brazilian gracile opossum, South American tapir, white-lipped peccary, and a variety of small mammals such as rodents and many bat species. There are several hundred bird species identified here, as well as many frogs (especially treefrogs), turtles, lizards, and snakes.

The lower, semiarid portion of the watershed supports smaller faunal communities. Mammals include several marmoset species, hairy-rumped agouti, seven-banded armadillo, several opossum species, tapir, peccary, mice and other rodents, and bats. There are also many birds, amphibians, and reptiles here. The river itself sustains more than 140 identified fish species, including anchovy, knifefish, catfish, tetra, and piranha. Although fish are plentiful, and there is an extensive fish-farming economy, damming of the river and use of the water for irrigation and hydroelectricity has greatly diminished fish population numbers.

Human Impact

The São Francisco is navigable for about 1,100 miles (1,800 kilometers) of its total length; a railroad line has been constructed to carry goods and passengers around the Paulo Alfonso Falls; other breaks include the rapids of Pirapóra in Mina Gerais, and on the border of the state of Bahia. In the 19th century, the São Francisco served as an important means of migration from the southeast to the northeast in Brazil; it was at this time the river achieved its reputation as the “river of national unification.”

Because many tributaries of the São Francisco flow intermittently, based on the annual cycle of rain and drought, water is stored in reservoirs to assure a more consistent supply; however, this causes accelerated loss of water due to evaporation. The largest of these reservoirs is the Sobradinho Reservoir, located in the state of Bahia; it has a storage capacity of over 8 cubic miles (34 cubic kilometers) and a surface capacity of more than 1,500 square miles (4,000 square kilometers), making it the twelfth-largest artificial lake in the world.

The primary modern uses for the waters of the São Francisco river are generation of hydroelectricity and irrigation for agriculture. Demand for both is expected to increase in the future. The local development authorities project an increase of at least 2,000 square miles (500,000 hectares) in irrigated agricultural areas within the river basin. Somewhat counterintuitively, in relatively dry years, demand is expected to be greatest during the wet (rather than dry) season because more crops are planted during the wet season, and because of greater variability of rainfall during the wet season.

The São Francisco also provides most of the electricity used in northeastern Brazil; as of 2019, hydroelectric plants in the São Francisco River and its tributaries had an installed capacity of 10.3 gigawatts. The Paulo Alfonso Dam, built in 1955, now provides electric power for the whole of northeastern Brazil. Another dam with a large hydroelectric plant serving the region is the Tres Marias in Minas Gerais, built in 1961.

Development and economic activities in the river basin have caused environmental deterioration of the river. In Bahia, the chief threats are deforestation, discharge of raw sewage, and construction of dams. In Pernambuco, desertification is well advanced; and in Alagoas, deforestation has almost completely cleared the land. (As of 2006, an estimated 95 percent of the Atlantic rainforest of this region was already destroyed.)

The largest government-funded project on the river was the diversion of 1.4 percent of the São Francisco’s water to the more than 13 million people who live in the dry sertão lowlands. The project included 435 miles (700 kilometers) of canals, tunnels, and several dams. Construction began after protest in 2007 and was completed in two pahses by 2017 and 2018. Impacts from climate change will have to be taken into consideration, as evaporation rates will likely increase in the arid regions, even as water usage demand also increases.

Bibliography

Eastham, J., M. Kirby, and M. Mainuddin. “Water-Use Accounts in Challenge Program on Water and Food (CPWF) Basins: Simple Water-Use Accounting of the São Francisco Basin.” CPWF Working Papers 10 (2010).

Kelman, Jerson. “The Brazilian San Francisco River Basin Narrative.” Gavin Publishers, 18 Sept. 2019, www.gavinpublishers.com/article/view/the-brazilian-san-francisco-river-basin-narrative. Accessed 1 Sept. 2022.

Maneta, M. P., M. Torres, W. W. Wallender, S. Vosti, M. Kirby, L. H. Bassoi, and L. N. Rodrigues. “Water Demands and Flows in the São Francisco River Basin (Brazil) With Increased Irrigation.” Agricultural Water Management 96, no. 8 (2009).

Osava, Mario. “Environment: River of National Integration is Dividing Brazil.” Inter Press Service News Agency. http://www.ipsnews.net/2005/01/environment-river-of-national-integration-is-dividing-brazil.

Tortajada, Cecilia. “Sao Francisco Water Transfer.” United Nations Human Development Office. http://hdr.undp.org/en/reports/global/hdr2006/papers/tortajada%20cecilia.pdf.