Boundaries

This ecoregion includes the entire drainage basin of the Rio São Francisco and all of its tributaries.

Topography

The headwaters of the São Francisco rise in the Serra da Canastra among the ancient sedimentary and crystalline plateaus of the Coastal Brazilian Shield. The basin lies to the west of the Chapada da Diamantina and Serra do Espinhaço, and east-southeast of the Serra dois Irmãos, Serra da Tabatinga, and Espigão Maestre. Elevations range from sea level to 1785 m asl. The soils of the upper and middle basin are predominantly latosoils and podozolic soils.

Freshwater habitats

Stretching 2870 km from its headwaters in the Serra da Canastra to the Atlantic Ocean, the Rio São Francisco is the fourth longest river in South America. There are 36 tributaries, of which 19 are perennial. Left bank perennial tributaries include the Abaeté, Paracatu, Urucuia, Carinhanha, Corrente, and Grande, whereas right bank perennial tributaries include the Pará, Paraopeba, das Velhas, and Verde Grande (Sato & Godinho 2004). The basin also includes more than 33 reservoirs, two of the largest being Sobradinho and Itaparica on the main stem (Junk 2007).

Waters in the upper basin are fast-flowing and well oxygenated with rapids and falls. Toward the middle reaches the flow is slower and the river is subject to flooding from rainfall in the headwaters. The lower middle reaches are characterized to a large extent by reservoirs and seasonal tributaries, and the lowest course is defined as a plains river with slow flow that is subject to flooding. This area also has many marginal lagoons and floodplains. The total discharge of the São Francisco is 3,150 m3/s (Sato & Godinho 2004).

Terrestrial habitats

The Rio São Francisco has its headwaters in the cerrado, which represents one of the most diverse savannas in the world. Caatinga dominates much of the lower basin. The threatened Atlantic dry forests, which occur to a large extent within the São Francisco basin, form a transition between cerrado and caatinga with characteristic species such as the Barriguda tree (Cavanillesia arborea), Cedrela fissilis, Schinopsis brasiliensis, Astronium urundeuva, Aspidosperma macrocarpa, and Tabebuia sp. (WWF 2001).

Description of endemic fishes

With 113 endemic species (60%), the São Francisco basin is one of the major areas of fish endemism in South America, and may indicate an aquatic refuge during the late Miocene (Hubert & Renno 2006). There are also seven endemic genera, represented by the species Lophiosilurus alexandri, blind tetra (Stygichthys typhlops), Franciscodoras marmoratus, Pseudotatia parva, Conorhynchos conirostris, Hysteronotus megalostomus, Bagropsis reinhardti, and three near-endemic genera with species shared with the Northeastern Mata Atlantica ecoregion [328]. The ecoregion contains the largest assemblage of endemic annual pearlfishes of the family Rivulidae, with 41 species, 30 of which in the genus Simpsonichthys.

Other noteworthy fishes

The pirá (Conorhynchos conirostris) is an endemic monotypic catfish that is distinct from other catfish by its lack of dorsal flattening, a proboscis-like mouth, and blue soft skin. It feeds mainly on mollusks, but also eats insect larvae and pupae, worms, and micro-crustaceans (Sato & Godinho 2004). Its evolutionary position within the Siluriformes is still unclear, and it does not belong to any of the currently recognized families.

Ecological phenomena

Around 8% of the fish species in the São Francisco basin undergo spawning migrations (Sato & Godinho 2004). Some of these species include the golden dorado (Salminus franciscanus), piapara (Leporinus elongates), pirá (Conorhynchos conirostris), and spotted sorubim or pintado (Pseudoplatystoma corruscans).

Justification for delineation

The São Francisco basin forms one of South America’s ichthyographic provinces defined by Ringuelet (1975) and Gery (1969).

Level of taxonomic exploration

Good