Boundaries

This ecoregion includes the mid-upper drainage basins of the tributaries of the Rio Madeira draining the northern and western slopes of the Brazilian Shield and upper portions of the Rio Maués-Açu and Rio Abacaxis, between the Madeira and Tapajós.  The lower limit of this ecoregion is the contact zone between the Amazon lowlands sedimentary basin with the northern border of the Brazilian Shield. It is limited to the south by the drainage divide of the Guaporé and Paraguay basins, along the Serra dos Pacaás-Novos and the Chapada dos Parecis.

Topography

The geologic substrate ranges from the soft sedimentary lowlands of the Amazon Basin to the hard crystalline basement of the Brazilian Shield. Whereas most of the ecoregion lies below 250 m, elevations exceed 1080 m in the Serra dos Pacaás-Novos.

Freshwater habitats

The Madeira is the largest tributary of the Amazon, accounting for roughly 15% of its freshwater input. This ecoregion encompasses the interfluve between the Madeira and Tapajós rivers. Whereas the Madeira carries a heavy sediment load from the Andes, most of the of its right bank tributaries are nutrient-poor clearwater rivers that drain the Brazilian Shield. In addition to large rivers such as the Ji-Paraná (Machado), Maués-Açu, Abacaxis, and Aripuanã are many small rivers and streams. Peak flows in the region occur around March or April, although nearly all rivers below 1000 m are flooded between December and May. This results in deeply flooded forests along the lower courses of these rivers. Such seasonally flooded forests are uncommon for clearwater rivers, such as the Tapajós and Xingu, which have relatively few flooded forests.

A number of cataracts occur between the Bolivia-Brazil border and the city of Porto Velho – a sign of the underlying Brazilian Shield. The Teotônio rapids and others are barriers to the distribution of many fish species.

Terrestrial habitats

This area encompasses a variety of vegetation types, including dense lowland rain forest throughout a majority of the ecoregion, as well as submontane rain forest and woodland savanna in the upland areas. White-sand igapó forest lines clearwater rivers like the Rio Aripuanã.

Description of endemic fishes

The ecoregion contains 18 endemics in four families. The royal tetra (Inpaichthys kerri) is the only monotypic endemic. Genera with multiple endemic species include Leporinus, Utiaritichthys, Sternarchorhynchus, Aequidens, and Crenicichla. Other endemics include Creagrutus petilus, Ancistrus verecundus, Parotocinclus aripuanensis, and Scoloplax baskini.

Other noteworthy fishes

A popular aquarium fish, the pineapple discus (Symphysodon discus willischwartzi) is found in the Rio Abacaxis.

Ecological phenomena

Characoids represent more than 50% of the species in this ecoregion. This reflects spectacular adaptive radiations that started during South America’s long period of isolation in the Tertiary, which is said to rival that of the African lake cichlids.

Justification for delineation

Good in large rivers, fair to poor in headwaters.