Madeira

The Madeira is the Amazon's largest tributary by discharge, a muddy Andean river that gathers half of Bolivia, a slice of Peru and a wide band of western Brazil before pouring some 26,1903 ft³/s into the Amazon. It is built from two different worlds: glacial-fed whitewater off the young Andes, hauling more sediment than almost any river on Earth, and the clear and black tributaries of the old Brazilian Shield to its right bank. Where those waters meet the hard shield rock between the Bolivian border and Porto Velho, the river broke into a 360-km staircase of cataracts — Teotônio, Santo Antônio and others — that for millennia sorted its fishes and funneled the longest fish migration on the planet. Two run-of-river dams have now drowned that staircase, and the fauna is paying for it.

Geography & hydrology

The Madeira drains about 1.3 million km², roughly 19% of the whole Amazon basin and an area larger than any other Amazon tributary catchment. Close to half of that lies in Bolivia, around 40% in Brazil and the rest in Peru, with headwaters reaching the southernmost Andes within the Amazon system near Cochabamba. The trunk forms where the whitewater Beni and Mamoré rivers join near the Bolivia–Brazil frontier; those two alone supply most of the Madeira's flow, with long-term means on the order of 9,000 and 8,1312 ft³/s. Below the confluence the river runs north and east for some 1,62 mi to meet the Amazon downstream of Manaus, delivering around 26,1903 ft³/s at the mouth — about 15% of the Amazon's freshwater (FEOW Ecoregion 321; AMAZON WATERS / Aguas Amazónicas).

What sets the Madeira apart from the lowland Amazon it feeds is the shield. Between the Bolivian border and the city of Porto Velho the river crosses the buried crystalline basement of the Brazilian Shield, and the soft Andean valley fill gives way to hard rock. The result was a long reach of rapids and waterfalls — Teotônio, Santo Antônio and a chain of lesser cataracts spread across roughly 360 river-kilometers — that historically split the basin in two. Above the falls lay the encased upper Madeira and its Bolivian and Peruvian tributaries; below them the river spread into a broad, meandering floodplain on its way to the Amazon. The cataracts were not just scenery. Teotônio and Santo Antônio were the first hurdles a migrating fish met on its way upriver, and they marked the boundary between the high, confined river and the lowland floodplains downstream (FEOW Ecoregion 321; Aguas Amazónicas; Santos et al., Biota Neotropica).

The flood pulse drives everything. Water begins to climb late in the year and peaks around March or April, with extensive forest along the floodplain submerged for four to six months at a time. On the shield-draining right bank the picture is different again: rivers such as the Ji-Paraná (Machado), Aripuanã and Abacaxis rise on weathered crystalline uplands — the Serra dos Pacaás Novos tops 1,262 ft — and run clear or tea-dark rather than muddy, so that the mainstem and its tributaries can sit side by side with sharply different chemistry. The Madeira is, in effect, a seam between two geologies, and its fishes are sorted along that seam (FEOW Ecoregion 321; AMAZON WATERS / Aguas Amazónicas).

Water & habitat

Harald Sioli's 1967 scheme put the Madeira squarely among the whitewater rivers, and few rivers wear the label better. Its color is a pale yellow-brown of suspended Andean silt and clay, the same fine material that builds the várzea floodplains downstream. Measured at the HYBAM gauging stations, the river delivers on the order of 426 million tonnes of suspended sediment a year to the Amazon, while the Andean catchment that feeds it produces an estimated 640 million tonnes annually — figures that place the Madeira among the two largest sediment sources in the entire Amazon system. The load is not steady; fine-sediment concentrations have shifted with rainfall over recent decades, but the river's defining trait remains its turbidity (HYBAM / Vauchel et al., J. Hydrology 2017; Moquet et al., Biogeochemistry 2011).

Chemically the whitewater mainstem is the opposite of the soft, acidic blackwaters that drain old shields. Andean weathering loads it with dissolved ions; calcium and bicarbonate dominate, concentrations climb in the dry season as the diluting flood recedes, and pH sits near or above neutral with comparatively high conductivity. Light barely penetrates the silt, so primary production in the channel is limited and much of the food web runs on material washed in from the floodplain and the forest. That is reflected in the fishes near the rapids: a survey of a stream just below Santo Antônio Falls found an assemblage dominated by detritus-feeders — curimatids and allied genera made up roughly 60% of individuals — living in tea-stained, oxygen-modest water with a mean pH around 6.1 (Moquet et al., 2011; Santos et al., Biota Neotropica).

The habitat that mattered most, ecologically, was the rapids themselves. Fast water broken over shield bedrock created oxygenated channels, scoured rock, sand bars and deep plunge pools, and the falls acted as a coarse filter that let some species through and held others back. Off the muddy trunk, the clearwater shield tributaries on the right bank add a second kind of habitat — transparent, lower in nutrients, lined in places with white-sand igapó forest — so the broader Madeira corridor holds whitewater channel, clear tributary, blackwater backwater and seasonally flooded forest within a short distance of one another. The annual flood knits them together and then pulls them apart, opening floodplain lakes and forest to fish at high water and stranding them as the river drops (FEOW Ecoregion 321; Santos et al., Biota Neotropica).

The cichlid fauna

The Madeira sub-basin is one of the richest on Earth for freshwater fishes — roughly a thousand species have been catalogued from it, on the order of a third of all the fishes known from the Amazon — and cichlids are a familiar, widespread part of that fauna rather than a narrow endemic radiation. The atlas roster for the Madeira leans toward the species that range across the whitewater trunk and its Bolivian and Peruvian headwaters, the part of the basin where the shield cataracts historically sorted the fauna (AMAZON WATERS / Aguas Amazónicas; FEOW Ecoregion 321).

The headline cichlid is the peacock bass Cichla pleiozona, the Bolivian tucunaré, a stout ambush predator of the upper Madeira and one of the most heavily caught and eaten fishes in the region. It is the species that turns up again and again in fisheries and contamination studies of the river, which makes it a useful sentinel for the whole system. Alongside it the roster carries the basin's larger generalist, the oscar Astronotus crassipinnis — a robust, slow-water cichlid of floodplain lakes and quiet margins across the southern Amazon and into the Bolivian lowlands.

The rest of the roster is built from the smaller acaras and dwarf cichlids that work the river's edges and tributaries. The genus Bujurquina — mouthbrooding 'acaras' of shaded forest streams and clear, tinted creeks — is represented by two species tied to the Madeira's southwestern headwaters: Bujurquina cordemadi, described by Kullander in 1986 from the Madre de Dios drainage in Peru, and Bujurquina eurhinus, described from the Bolivian Amazon and associated with the Mamoré and Iténez (Guaporé) systems that build the upper Madeira. Three Apistogramma round out the list, all dwarf cichlids of leaf-littered, slow tributary water: Apistogramma linkei, A. luelingi — named from Bolivia in the 1970s — and A. rubrolineata, a red-striped species of the southwestern Amazon. None of these is a rapids specialist of the kind the Xingu produced; they are creek-and-margin fishes whose fortunes ride on the floodplain and the tributaries rather than the cataracts.

That said, the cataract reach itself was a genuine center of cichlid character. FEOW's delineation of this ecoregion singles out Crenicichla and Aequidens among the genera with endemic species here, the pike cichlids in particular favoring the broken, fast water that the falls created. The 18 fish species the ecoregion counts as endemics are concentrated in characoids and catfishes, but the cichlids' role in the Madeira has always been less about endemism than about abundance — they are part of the standing fauna of nearly every habitat the river offers, from the silty trunk to the clear shield creeks (FEOW Ecoregion 321; Kullander 1986; Aguas Amazónicas).

Conservation

The defining event in the Madeira's recent history is the loss of its cataracts. Two large run-of-river dams near Porto Velho — Santo Antônio, operating from 2012, and Jirau, from 2013 — were built across the shield rapids, drowning Teotônio and the staircase of falls behind reservoirs covering several hundred square kilometers, with backwater reaching into Bolivia and upstream flooding estimated to have cleared more than 36,000 hectares of forest. The dams were sold as low-impact because their reservoirs are small, but the rapids they replaced were the heart of the river's ecology (Fearnside, Environmental Science & Policy 2014; MAAP / Mongabay 2016).

The heaviest blow fell on migration. The Madeira carried the longest freshwater fish migration known — the dourada or gilded catfish, Brachyplatystoma rousseauxii, whose larvae drift to the Amazon estuary and whose adults swim back to spawn in Andean headwaters, a round trip of 8,000–11,0 mi over a lifetime. Otolith work by Marília Hauser and colleagues showed that before the dams roughly 80% of Madeira dourada had migrated all the way to the Andes to spawn; afterward, nearly all sampled fish were confined to the stretch where they were caught, and dourada in the upstream spawning grounds fell to about a tenth of pre-dam abundance. The fish-passage channel at Santo Antônio — the first tried in the Amazon, adapted from temperate designs — has not delivered passage for these long-distance migrants (Hauser et al.; Duponchelle et al., via Mongabay 2019).

The collapse extends well beyond one catfish. Work led by Carolina Doria and colleagues documented catch declines of up to 90% at some sites among the 65 species fished in the dams' zone of influence, with several communities reporting that their grounds had simply stopped producing; staples such as tambaqui, pirarucu, pirapitinga and the dourada largely vanished from local catches, and fishers describe hauls that once filled a canoe now amounting to a handful of fish. A 2025 effort enlisted some 120 fishers to log daily catch and map those losses (Doria et al.; Mongabay 2025).

Mercury is the second front. The cataract reach has a long history of gold mining, and illegal dredges still work the river, releasing mercury that bacteria convert to methylmercury in flooded soils and reservoir sediments. A 35-year record from the upper Madeira found total mercury in the carnivorous tucunaré, Cichla pleiozona — a roster species — climbing about fivefold since the late 1980s, with post-2016 concentrations frequently topping 3 mg/kg and exceeding Brazil's legal limit for predatory fish, while the detritivorous curimatã stayed low. The same studies implicate deforestation as much as mining in mobilizing mercury from soils. The drowned rapids, broken migrations, crashing catches and rising mercury together show how a river engineered for power can come undone — and the science was available before the concrete was poured (Mongabay 2024; Fearnside 2014).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Apistogramma · 3

Astronotus · 1

Bujurquina · 2

Cichla · 1