Boundaries

This ecoregion includes the entire drainage basins of the Tocantins and Araguaia rivers upstream from Tucuruí.  It is limited to the west by the Serra do Roncador and Serra dos Gradaús, which is the drainage divide between the Araguaia and Xingu drainages. The southern limit is the Planalto Central drainage divide and Chapada dos Guimarães formation. To the east the limit is the Chapada das Mangabeiras and the Serra Geral de Goias drainage divides.

Topography

The undulating topography of the Tocantins and Araguaia basins reveal the eroded sedimentary and crystalline formations of the Brazilian Shield. Elevations range from less than 25 m in the lowlands of the Amazon Basin to more than 1600 m in the Planalto Central.

Freshwater habitats

The Rio Tocantins is the easternmost and third largest basin in the Amazon. Like the Tapajós and Xingu, it is a nutrient-poor clearwater river that drains the Brazilian Highlands. However, unlike the Tapajós and Xingu that empty into the Amazon’s main stem, the Tocantins empties into the Rio Pará south of Ilha Marajó. Its main tributary, the Araguaia, carries a mean annual discharge of 1680 m³/s. It contains many seasonal lagoons, marshes, and islands, including Ilha do Bananal, the largest fluvial island in the world. This low-lying savanna wetland lies between two channels of Rio Araguaia, and is flooded around six months out of the year, although the floodplain is inundated less than two meters. Other wetland areas in the ecoregion include the mouth of the Tocantins and the Tucuruí Reservoir. There are also numerous waterfalls, rapids, and cataracts along the rivers that reflect the geology of the ancient Brazilian Shield.

In the lowest reaches of the Tocantins, water levels are influenced by oceanic tides, although saltwater intrusion does not occur. As with other southern tributaries, the flood season lasts between January and May, with peak flood levels occurring around March-April. The average annual river fluctuation of the Tocantins-Araguaia basin ranges between 5-6 m.

Terrestrial habitats

The transition from Amazon lowland moist forests to dry forest and savanna scrub vegetation (cerrado) in the Planalto Central reflects the heterogeneity of this landscape. Cerrado dominates the higher elevations, whereas buriti (Mauritia flexuosa) palm swamps and gallery forests line rivers and streams.

Description of endemic fishes

More than 175 species are endemic to the ecoregion, representing a level of endemicity greater than 40%.  Families with the highest number of endemics include Characidae (36), Rivulidae (30), Loricariidae (26), and Trichomycteridae (10). Killifish in the genera Rivulus and Simpsonichthys are well represented with nine and eight endemics, respectively. Ituglanis is a monophyletic genus of catfishes known to inhabit leaf litter and caves. Six species are restricted to this ecoregion (I. bambui, I. epikarsticus, I. macunaima, I. mambai, I. passensis, I. ramiroi). The region contains the most diverse cave fish fauna in South America.

Ecological phenomena

Historically the Tocantins and Araguaia rivers have accommodated migratory fish species as they move upstream to spawn. The Tucuruí Dam and others have prevented the natural migration and spawning habitats for many species.

Justification for delineation

The Tocantins is the easternmost and one of the largest basins in the Amazon. Although it is related ecologically to the Amazon and is part of the Guyanan-Amazonian ichthyographic region, the Tocantins-Araguaia basin is sometimes considered to be distinct because it drains into the Rio Pará rather than the main stem Amazon.

Level of taxonomic exploration

Good