Brazilian Shield Amazonian Rivers & Streams

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Paraguay River, Brazil.
© WWF-Canon / Juan PRATGINESTOS

About the Area

The Brazilian Shield is the pre-cambrian geologic formation lying primarily south of the Amazon River. The ancient sedimentary rocks of the Brazilian Shield are well weathered, and the tributaries draining them tend to be nutrient-poor ‘clearwater’ rivers, which appear transparent.

Hundreds of rivers and streams flow through this area on their way to the Amazon. Despite the low nutrient content of the water, these rivers are believed to contain exceptional levels of aquatic diversity, with a large number of migratory fish species.

As the rivers and streams near the Amazon, they swell and pick up speed. At one point, the mighty Madeira River rushes through 320km (200 miles) of deep, swirling rapids.
Size:
2,500,000 sq. km (1,000,000 sq. miles)

Habitat type:
Large River Headwaters

Geographic Location:
Central South America: Brazil, Bolivia, and Paraguay

Conservation Status:
Vulnerable

Local Species

Along with the Upper Amazon Rivers and Streams, this ecoregion includes species from several fish families.

Fish groups with high numbers of species representation include Characins (e.g. tetras, piranhas, pacus), Catfish (e.g. doradids, loricariids, pimelodids), Gymnotoids (‘Electric eels’), Osteoglossids (relic ‘bony-tongues’), and Cichlids. Other aquatic species are also expected to show high levels of diversity.

Featured species

 / ©: WWF / Zig KOCH
Electric eel (Electrophorus electricus), Juruena River, Juruena National Park, Brazil.
© WWF / Zig KOCH
Electric Eel (Electrophorus electricus)

This fish is not a true eel, but a giant member of the knifefish order with an eel-like body. When fully grown, it can be as thick as a human thigh, and is one of the largest freshwater fishes in South America. It has an elongated square body with a flattened head, and an overall dark grayish green colour shifting to yellowish on the bottom. The electric eel can grow up to 2.5m (8.2 ft) in length and weigh up to 20kg (44 lb). It has a continuous fin that runs along its underside, but no fins on its back.

Electric eels have poor eyesight, and find their way by using weak pulses of electricity. However, using modified muscles, or electric organs, that run the entire length of their bodies, they can also produce sudden jolts of up to 600 volts – powerful enough to kill other fishes and potentially lethal to humans, making it a top predator in its range.

Juvenile eels feed on invertebrates, while adult eels feed on fish and small mammals. First-born hatchlings will even prey on other eggs and embryos from later batches.

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Threats

This ecoregion comprises the most altered part of the Amazon basin. Threats include petroleum and mineral exploration, logging, construction of dams, agricultural expansion, and pasture development.

Overexploitation of aquatic species, including hunting of aquatic mammals like the giant river otter pose additional threats.

WWF’s work

WWF-Brazil acts nationwide with the mission of contributing to a Brazilian society that conserves its natural environment, harmonizing human activity with the preservation of biodiversity and the sustainable use of natural resources.

WWF has played a unique role in the conservation of the Amazon region for over 40 years, developing scientific knowledge, experience, and key partnerships with local, regional, and international players. By delivering tangible, significant results, WWF has laid the groundwork for delivering even greater results in the years ahead.

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