Boundaries

This ecoregion, largely in Bolivia, includes the upper western portion of the Madeira River system from the Beni-Madre de Dios drainage in the north to the Mamoré drainage in the south.  The eastern limit is the drainage divide with the Guaporé system, the northern limit is the drainage divide with the Abunã and Purus drainages, the western limit is determined by the 750 m elevation contour along the eastern Andes, and the southern limit is the drainage divide with the Paraguay drainage.

Topography

This ecoregion forms part of the Madeira basin, which is bounded to the west by the steep slopes of the eastern Andes (Cordillera Oriental) and to the east by the Brazilian Shield. In the northwest the Fitzcarrald Arch forms a boundary between the rivers of the Beni-Madre de Dios system with those of the Ucayali system. The steep Andean slopes descend from the 800 m contour toward the lowlands, which range roughly between 100-250 m asl.

Freshwater habitats

This ecoregion includes steep-gradient rivers like the upper Madre de Dios and Inambari and low-gradient rivers like the Las Piedras, Beni, lower Madre de Dios, and Mamoré. These are characterized by their meandering courses and widespread floodplains with numerous oxbow lakes. Aquatic vegetation characteristic of the floodplain lakes include free-floating species (Eichhornia, Pistia, Salvinia), grasses (Hymenachne and Panicum), and submerged species (Utricularia foliosa).

Originating in the Bolivian Andes, the whitewater (muddy) Beni and Mamoré rivers provide around 60% of the Madeira’s flow, with mean annual discharges of 9000 and 8400 m³/s, respectively. This discharge is strongly seasonal, with fluctuations in water levels of the Mamoré averaging between 5-7 m. The remainder is provided by the Madre de Dios, a whitewater tributary of the Beni that originates in the Peruvian Andes. Average depths here range between 7-10 m during flood season and 2-5 m during low water season. Surface water temperatures in the Rio Madre de Dios basin average between 24.2º to 26.8º C, nearly 4º colder than surface temperatures recorded from the central Amazon. Oxygen levels are also relatively high, at 7 mg/l, and pH averages between 5.1 and 7.9 (Goulding et al. 2003).

Between the Beni, Mamoré, and Guaporé rivers is the vast Llanos de Moxos (Mojos) comprised of rivers, lakes (including Laguna Rogagua and Laguna Rogaguado), gallery forests, permanent swamps and marshes, and seasonally flooded savannas and wetlands. This area is flooded four to eight months a year due to rainfall and overflow from the Mamoré and its tributaries. Thousands of oxbow lakes line the floodplain of the Mamoré.

Terrestrial habitats

Three main terrestrial habitats stretch across this ecoregion. Along the eastern Andean slopes lies the Yungas, a transitional forest zone that ranges from montane to lowland forests. The lowland moist forests north and east of this area form part of the expansive Southwest Amazon moist forests ecoregion, whose major forest types include terra firme forests, seasonally flooded forests, and buriti or aguaje (Mauritia flexuosa) palm swamp forests. In the east lies the Llanos de Moxos, a vast area of flooded savannas and wetlands dominated by sedges and grasses.

Description of endemic fishes

There are nearly 80 endemic species, over a third of which are characids. There is also one endemic monotypic genera: Monotocheirodon (M. pearsoni). The cataract-studded Madeira may account for the many endemic species restricted to this basin.

Other noteworthy fishes

Ecological phenomena

Characoids 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. The tambaqui (Colossoma macropomum), for example, is found upriver as far as Río Beni and Río Mamoré. The piraíba (Brachyplatystoma filamentosum) is a goliath catfish that is also migratory.

Justification for delineation

This ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). The Amazon Basin was subdivided at finer scales using regionalized data on fish distributions. This ecoregion encompasses the western portion of the Madeira basin where tributaries flow from the Andes.

Level of taxonomic exploration

Good in large rivers, but remains fair to poor in higher reaches and headwaters.