Boundaries

This ecoregion includes the entire Guaporé-Iténez drainage upstream from Costa Marques, including the Río San Itonamas drainage. It is bounded to the north by the Serra dos Pacaás Novos and Chapada dos Parecis. The eastern limit is the drainage divide with the Paraguay River basin. To the west the limit is the drainage divide with the Mamoré system. The Parapeti basin forms the southern limit at the drainage divide with the Chaco ecoregion [342].

Topography

The Guaporé originates in the Mato Grosso Plateau in Brazil. It travels in a northwesterly direction toward the Río Mamoré, fed by tributaries draining the southwestern foothills of the Serra dos Pacaás Novos and Chapada dos Parecis. Most of the northern part of the ecoregion lies between 100-250 m asl on a peneplain composed of Tertiary sediments. In the far southwest elevations increase up to 3300 m asl along the eastern Andean slopes within the Parapeti basin. The Huanchaca Plateau lies in the eastern part of the ecoregion and is composed of Precambrian sandstone and quartzite rocks. This area is characterized by steep escarpments, canyons, and spectacular waterfalls.

Freshwater habitats

The Guaporé-Iténez basin forms the southwestern edge of the Amazon Basin, and includes the Rio Guaporé up to its confluence with the Marmoré. Unlike other rivers in the Madeira system that originate in the Andes or Beni plains, the Guaporé is a clearwater right bank tributary that drains the eastern Bolivian lowlands and southern Brazilian Shield. It has a mean annual discharge of 2100 m³/s, a fraction of the Marmoré discharge of 8400 m³/s further west. Its pH varies between 6.3 and 6.8.

Extensive areas of seasonally flooded forests and savannas line the Rio Guaporé, thus offering a variety of specialized habitats and abundant food sources. Other freshwater features include oxbow and subsidence lakes and permanent swamps and marshes. Numerous shallow, flat-bottomed lakes are unique to the Llanos de Moxos in the western part of the ecoregion.

Terrestrial habitats

This is a transitional ecoregion that ranges from Madeira-Tabajós moist forests in the north to Chiquitano dry forests in the center and dry Chaco in the south. In the northwestern part of the ecoregion is the Llanos de Moxos, an extensive area of seasonal savannas and wetlands interspersed with forested islands and gallery forests.

Description of endemic fishes

The ecoregion contains 25 endemic species in seven families. Over half of these are catfish in the genus Corydoras, whose species are well-known by aquarists, and Phreatobius sanguijuela, which is known only from a single well and apparently, is restricted to underground waters.

Other noteworthy fishes

The ecoregion contains a handful of Gymnotiformes, which are noted for their ability to produce electric fields for communication and navigation. Some of these include the black ghost knifefish (Apteronotus albifrons), banded knifefish (Gymnotus carapo), and glass knifefish (Eigenmannia virescens).

Ecological phenomena

Characins such Brycon, Colossoma , Leporinus, Schizodon, and Triportheus and have been noted to undergo spawning migrations and low-water upriver movements within the Madeira system. Spawning migrations occur when species move from the floodplains to the main rivers to spawn, usually early in the flood season.  Low-water movements occur as water levels decline and species move upriver intermittently.

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 encompasses the southern portion of the Madeira basin, the largest tributary of the Amazon. Unlike other tributaries of the Madeira that drain the Andes, the Guaporé-Iténez is the only basin that originates in the Brazilian Shield, resulting in a river with distinctly different chemical and physical properties.

Level of taxonomic exploration

Good in larger rivers, fair to poor in headwaters.