Amazonas Guiana Shield

Geography & hydrology

FEOW Ecoregion 315 takes in the mid and upper basins of the rivers that drain the southern face of the Guiana Shield, from the upper Rio Arapari in Amapá in the east across to the Rio Uraricoera in Roraima in the west. It spans parts of Brazil, Guyana and Suriname and is bounded against the Orinoco basin to the north by the Serra Pacaraima and Serra Parima. The main rivers are the Jari, Paru, Trombetas, Nhamundá and Jatapú, plus the upper Rio Branco above Caracaraí and the headwaters of the Demeni and Padauiri. All of them run roughly south into the Amazon along its left, or north, bank — which is what separates this fauna from the Andean-fed whitewaters and the south-bank shield rivers like the Xingu and Tapajós (FEOW Ecoregion 315; Petry & Hales).

The rock underneath is what gives the region its character. These are ancient crystalline uplands, mostly sandstone and quartzite, and in the north they rise into the Tumucumaque highlands, an undulating, mountainous country reaching about 2,328 ft. The northwestern rivers, the Uraricoera above all, cut deep valleys and run fast and steep. Further south the land drops into the low alluvial plains of the Amazon proper, where recent lacustrine and marine deposits replace the hard shield terrain. The transition shows up in the vegetation as much as the rock: lowland moist forest and igapó along most of the rivers, várzea fringing the lower Rio Branco, and — north of Caracaraí — the open Lavrado savanna, a grassland riddled with seasonal lagoons and gallery forest, Mauritia palms tracking the watercourses (FEOW Ecoregion 315; Petry & Hales).

The Rio Branco is the regional oddity. It is a whitewater river surrounded by clear and blackwaters, but it is a mild sort of whitewater: mean annual discharge around 2,2953 ft³/s and electrical conductivity of just 20–30 µS/cm, far less turbid than the Andean-charged Amazon because the shield soils it drains are old and already leached. The Trombetas, by contrast, is the second-smallest of the Amazon's tributary basins and a clearwater river through and through, rising in the Tumucumaque uplands and dropping over a string of cataracts before it reaches a broad, lake-filled lower floodplain. Its flood season runs March to August; the dry season, September to February, pulls so much water out of the mid and upper basin that tributaries shrink to a trickle. That seasonal swing — connect, then isolate — is the metronome the whole fish community lives by (FEOW Ecoregion 315; Petry & Hales; Araújo et al., 2026).

Water & habitat

What ties these rivers together is the shield itself. Because they rise on deeply weathered crystalline basement rather than young, erodible mountains, the clearwater and blackwater rivers of Ecoregion 315 carry very little suspended sediment and few dissolved nutrients. In the headwater streams of the southern shield, visibility can reach 16 ft — water clear enough to collect fish by diving, which is exactly how recent rapids surveys have worked. The clearwaters run transparent and green-tinged; the blackwaters, draining sandy campinarana soils and leaf litter, run tea-dark, acidic and even more nutrient-poor. The Rio Branco sits between the two, technically a whitewater but with a conductivity (20–30 µS/cm) that would not look out of place in a clearwater system (FEOW Ecoregion 315; Petry & Hales; Camargo et al., Acta Amazonica).

The defining habitat is the rapid. Where these rivers leave the uplands they run over rocky channels with steep gradients and sandy-to-rocky beds, throwing up waterfalls and cataracts — the Cachoeira Porteira on the Trombetas is one of the last large natural rapid systems left in the whole Amazon basin. Upstream of such falls the rivers are fast and broken; downstream they slacken, widen and braid into floodplains stitched with lakes. A single river therefore offers a stack of microhabitats: scoured bedrock and boulder fields in the current, sand and gravel bars, sunken wood, leaf-packed margins, and the still, warm water of cut-off floodplain lakes. Rheophilic fishes — the current-lovers — wedge into rock crevices in the fastest water, where Teleocichla wajapi, for one, holds station between rock slabs and along the bank where the flow eases, on a bed of pebbles and boulders (Araújo et al., 2026; FishBase, Teleocichla wajapi).

Water chemistry shifts with the rock each sub-basin drains and with the season. The shield rivers are generally soft, weakly conductive, near-neutral to acidic and warm; the blackwater tributaries run more acidic and oxygen-poor than the clearwaters. When the dry season strands the upper Trombetas tributaries, isolated pools heat up and lose oxygen, while the lower floodplain lakes hold their own communities. Come the March flood the system reconnects, the rapids drown, and fish move out across inundated forest and lake. That annual filling and draining, layered over the clarity and hard substrate of the shield, is the setting the regional fauna has specialized into (FEOW Ecoregion 315; Petry & Hales; Camargo et al., Acta Amazonica; Araújo et al., 2026).

The cichlid fauna

FEOW counts close to 40 fish species endemic to Ecoregion 315, in a dozen families, with no endemic genera — a fauna that reads as a distinctive Guiana Shield selection from the wider Amazon pool rather than an isolated radiation. Among the genera carrying multiple endemics are the cichlid lineages Apistogramma and Crenicichla, alongside the loricariid catfishes Hypostomus and Harttia and the tetra genus Hyphessobrycon. The cichlids span the full habitat range of these rivers, from rock-wedged rapids specialists to open-water predators to the dwarf species of quiet margins (FEOW Ecoregion 315; Petry & Hales).

The rapids end of that spectrum is the most specialized. Teleocichla is a genus of slim, bottom-hugging cichlids built for fast water, and the ecoregion holds Teleocichla wajapi, described in 2015 from the Rio Jari and its tributaries the Iratapuru and Pacanari in Amapá — a small fish, under 3.5 in, that lives in clearwater rapids and is still ranked Data Deficient for want of study. A baseline rapids survey of the Trombetas, working largely by diving, recently extended the known range of Teleocichla into that river as well, recording 53 species in a single fast-water assemblage and finding Cichlidae the second-richest family after the armored catfishes. The same survey turned up a range extension for the rheophilic loricariid genus Hypancistrus, underscoring how poorly the shield rapids have been sampled — the Trombetas alone has some 414 fish species on record, about 4% of them endemic (Araújo et al., 2026; FishBase, Teleocichla wajapi).

Beyond the rapids the roster fills out across the slower habitats. A rapid assessment of the southern shield tributaries — the Nhamundá, Cuminapanema, Curuá, Jari, Mapuera, Cuminá and Paru, run across five protected areas — logged 286 fish species and roughly 20 cichlids, drawn from Aequidens, Apistogramma, Crenicichla, Geophagus, Krobia, Mesonauta, Satanoperca and others. Those genera frame the atlas roster: the sand-sifting eartheater Geophagus and the acara Aequidens tubicen working the sandy beds; the dwarf Apistogramma and the small Krobia among leaf litter and submerged wood; the pike cichlids Crenicichla — here the marbled C. marmorata, the big-eyed C. macrophthalma's relatives, C. cincta, C. hummelincki, C. pydanielae, C. tigrina — hunting from cover in current and channel. Larger predators close the list: the peacock bass Cichla monoculus and the oscar Astronotus ocellatus in pools and floodplain lakes, the chocolate cichlid Hypselecara temporalis and the angelfish Pterophyllum scalare among flooded margins. As a group the cichlids are only part of a fish fauna dominated, in the rapids, by loricariid plecos — but it is the same maze of rock, sand, wood and seasonal lake that sorts each cichlid into its niche (Camargo et al., Acta Amazonica; Araújo et al., 2026; FEOW Ecoregion 315).

Conservation

Two industries press hardest on these rivers, and both put metals into the water. On the Rio Branco system in Roraima, illegal gold mining has driven a long-running contamination crisis centered on the Uraricoera, which runs through Yanomami land. Miners use mercury to bind gold; roughly a fifth is dumped straight into the rivers and most of the rest goes up in smoke when the amalgam is burned. In the water, microbes turn it into methylmercury, the organic form that climbs the food chain — and fish, the main protein for riverside and Indigenous communities, are the principal route by which it reaches people. By the late 2010s thousands of prospectors were working the Uraricoera from dozens of barges, consuming tens of kilograms of mercury a month, and studies have documented worrying mercury burdens in Yanomami communities near the mines, with the Uraricoera and Mucajaí among the most affected reaches of the Branco basin (Vega et al., SciELO Ambiente & Sociedade; Mongabay, 2016).

Far downstream on the Trombetas, the issue is bauxite. Mineração Rio do Norte has run Brazil's largest bauxite mine since the late 1970s inside the Saracá-Taquera National Forest near Oriximiná, at Porto Trombetas. For its first decade — 1979 to 1989 — the operation discharged tailings straight into Lago Batata, dumping on the order of 24 million tonnes of red residue and burying parts of the lakebed under meters of sludge; bauxite tailings carry trace arsenic, cadmium and mercury, and the pollution reached connected creeks and waterways. The lake has been a long-term subject of recovery research since, but the episode smothered fishing grounds used by quilombola communities and reshaped one of the floodplain lakes that the lower Trombetas fauna depends on (Mongabay, 2020).

Against that sits a real conservation apparatus. The lower Trombetas holds the Rio Trombetas Biological Reserve, created in 1979 over roughly 385,000 ha to protect, among other things, major Amazon river-turtle nesting beaches, and the wider region is laced with FLOTA state forests and the large ESEC Grão-Pará and REBIO Maicuru reserves that the recent fish surveys worked within. The bigger worry is that the fauna is so thinly known: FEOW rates taxonomic exploration here as only fair in the large rivers and poor in the headwaters, and the rapids — where the most specialized cichlids and plecos live — have barely been inventoried. The Cachoeira Porteira is among the last unmodified large rapids in the Amazon, which makes the shield's clear, fast water both a refuge and an obvious target for the next round of dams, dredges and deforestation before anyone has finished counting what lives in it (FEOW Ecoregion 315; Petry & Hales; Araújo et al., 2026; Camargo et al., Acta Amazonica).

Sources

1. Amazonas Guiana Shield — Freshwater Ecoregions of the World (FEOW Ecoregion 315; Petry & Hales: boundaries, hydrology, Rio Branco/Trombetas, endemic fishes)
2. Baseline fish inventory of the Trombetas River rapids, a drainage of Southeastern of the Guiana Shield, Brazil (Araújo et al., Papéis Avulsos de Zoologia 66, 2026; 53 species, Teleocichla & Hypancistrus range extensions)
3. Rapid assessment of the ichthyofauna of the southern Guiana Shield tributaries of the Amazonas River (Acta Amazonica / SciELO; 286 species across five protected areas, ~20 cichlids)
4. Checklist of the Freshwater Fishes of the Guiana Shield (Vari, Ferraris, Radosavljevic & Funk, Bull. Biol. Soc. Washington 17, 2009)
5. Teleocichla wajapi — FishBase (distribution in rio Jari, Iratapuru and Pacanari, Amapá; clearwater rapids habitat; size; IUCN Data Deficient)
6. Mercury-Based Mining in Yanomami Indigenous Lands and Accumulation in Fish — Uraricoera/Branco basin (Ambiente & Sociedade / SciELO)
7. Illegal gold mining causing mercury contamination in Indigenous groups — Yanomami and Ye'kuana, Roraima (Mongabay, 2016)
8. MRN bauxite mine leaves legacy of pollution, poverty in Brazilian Amazon — Trombetas / Saracá-Taquera / Lago Batata tailings (Mongabay, 2020)
9. Teleocichla wajapi, a new species of cichlid from the rio Jari, Brazil (Varella & Ploeg / Zenodo record of the original description)