Boundaries

This ecoregion includes the drainage basins of the Rio Xingu flowing through the northern slope of the Brazilian Shield. The northern limit is Belo Monte at the zone of contact between the sedimentary basin and the Brazilian Shield. The southern limit is the drainage divide with the Paraguay drainage. The eastern limit is defined by the drainage divide with the Araguaia basin along the Serra do Roncador, Serra dos Gradaús, and Serra dos Carajás. The Serra do Cachimbo and Serra Formosa form the western limit between the drainage divide with the Tapajós basin.

Topography

The elevation rises from the mouth of the Xingu in the Amazon lowlands to more than 800 m at the northern slopes of the Mato Grosso Plateau (Planalto Mato Grosso). Rivers flowing over these granite and sandstone tablelands produce some of the most spectacular waterfalls in the Amazon Basin.

Freshwater habitats

The Xingu is the fourth largest basin in the Amazon (roughly 7% in area), and is one of the largest clearwater rivers that drain the crystalline plateaus and sedimentary plains of the Brazilian Shield. It accounts for roughly 5% of the freshwater flowing into the Amazon. The transparency of its green-tinted water (from phytoplankton blooms) ranges between 0.6 to 4 m and the pH ranges from 4.5 to 7.8. Water levels begin to rise in September or early October, reaching maximum levels in March or April. The average annual river fluctuation is between 2-5 m. Because there is little sedimentation along the main stem, the river is wide at its mouth, resembling a large lake. Oceanic tides occur 100 km up the Xingu, as its confluence with the Amazon River is 420 km from the Atlantic Ocean.

The headwaters below 10ºS are nestled between the Serra Formosa and Serra do Roncador in a depression characterized by extensive wetlands that are periodically flooded. This area contains lakes, marshes, and seasonally flooded forests and savannas. Compared to downriver where nutrient levels are low, the headwater basin is more nutrient-rich, supporting a greater diversity of herbaceous aquatic vegetation and mollusks. Cataracts and waterfalls occur downstream and along tributaries of the Xingu. Notable areas include the Serra do Cachimbo waterfalls and the Volta Grand rapids (downstream from Altamira), which acts as a barrier for aquatic fauna between the central Amazon Basin and the upper Xingu basin.

Terrestrial habitats

The ecoregion ranges from Amazonian moist forests in the north to seasonal forests and Planalto cerrado as one moves further south.

Description of endemic fishes

Currently 36 endemic species and two endemic genera (Ossubtus and Phallobrycon) are recognized. More than half of the endemics are cichlids and characids.

Other noteworthy fishes

Ecological phenomena

Volta Grande rapids act as a barrier to migratory fish from the central Amazon, resulting in different species assemblages above and below the cataracts.

Justification for delineation

This ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). It contains a diverse and unique assemblage of species from the eastern portion of the Brazilian Shield draining into the Amazon.

Level of taxonomic exploration

Good in large rivers, but fair to poor in headwaters.