Guaporé-Iténez

On the map the Guaporé is a border: for roughly 559 mi it separates Brazil from Bolivia, running northwest off the Mato Grosso plateau to meet the Mamoré and form the Madeira, the Amazon's largest tributary. What sets it apart from its neighbors is where its water comes from. Every other major arm of the Madeira drains the Andes and runs thick with sediment; the Guaporé — the Río Iténez to Bolivians — rises on the worn Precambrian shield instead, so it runs clear and tea-soft over sand and rock. Along its course it spills into the Llanos de Moxos, one of the largest seasonal wetlands on Earth, where a flat-as-a-table savanna floods and drains every year and a quiet, Corydoras-rich fish fauna has evolved in near-isolation.

Geography & hydrology

The Guaporé-Iténez basin is the southwestern corner of the Amazon, the only part of the giant Madeira system that drains the Brazilian Shield rather than the Andes. The river starts on the Mato Grosso plateau in Brazil and runs northwest toward the Río Mamoré, gathering tributaries off the Serra dos Pacaás Novos and the Chapada dos Parecis on its right bank. FEOW draws the ecoregion (number 319) to take in the whole drainage upstream of Costa Marques, including the Río San Itonamas, bounded north by those serras, east by the divide with the Paraguay basin, west by the divide with the Mamoré, and south by the Parapetí at the edge of the Chaco. Most of the northern basin is a low peneplain of Tertiary sediments lying between about 100 and 820 ft above sea level; only in the far southwest, where the Parapetí heads into the Andean foothills, does the land climb toward 3,984 ft (FEOW Ecoregion 319).

The mainstem is modest by Amazon standards. Mean annual discharge is around 2,328 ft³/s — about a quarter of the Mamoré's 8,1312 ft³/s that joins it from the west — and the gradient across the lowlands is almost nothing, which is the key to how the basin behaves. In the east the Huanchaca (Caparuú) plateau rises out of the plain as a block of Precambrian sandstone and quartzite, its escarpments cut by canyons and waterfalls; much of that highland sits inside Bolivia's Noel Kempff Mercado National Park, about 1.5 million hectares straddling the Paraguá and the upper Iténez. Between the highlands and the river lies the Llanos de Moxos, the Beni savannas — a mosaic of seasonal grassland, wetland, gallery forest and forest islands that ranks among the largest tropical floodplains anywhere (FEOW Ecoregion 319; Hamilton et al., 2002).

The whole system runs on a single annual flood pulse. Rain falls heavily from roughly November through April and drains slowly off ground with almost no relief, so water backs up across the Moxos for months before it works its way out through the Guaporé and Mamoré. At the peak, shallow sheet-flooding spreads over tens of thousands of square kilometers of savanna; by the late dry season much of that has fallen back to a network of permanent channels, oxbow lakes, subsidence lakes and marshes. The Moxos is studded with shallow, flat-bottomed lakes — Lago San Luis, Lago Huachi, Lago Concepción — that have no real counterpart elsewhere in the basin. For the fishes, this rising and falling sheet of water is the calendar: it opens the floodplain as feeding and spawning habitat, then strands and concentrates fish in shrinking pools as it withdraws (Hamilton et al., 2002; Lowe-McConnell, 1987).

Water & habitat

Limnologically the Guaporé is a clearwater river, and it owes that to its bedrock. Where the Andean tributaries of the Madeira carry suspended silt scraped off young mountains, the Iténez drains a shield that has been weathered flat for hundreds of millions of years and has little fine sediment left to give. The water comes off it transparent and lightly stained, moving sand rather than mud, building beaches and bars among rock outcrops. Its chemistry is soft and close to neutral: FEOW gives a mainstem pH of about 6.3 to 6.8, and the dissolved-ion load is low, the signature of a river fed by old, leached rock instead of fresh Andean weathering (FEOW Ecoregion 319).

That base chemistry is not uniform across the basin, because the land it crosses is not uniform. Tributaries draining the cerrado and the Huanchaca sandstones run clear and acidic; sluggish backwaters and forest-shaded creeks in the Moxos pick up tannins and tip toward blackwater, cooler and more darkly stained; and the broad savanna lakes, sitting in the sun and full of aquatic plants, swing through the day on their own oxygen and temperature cycles. The seasonally flooded forests and savannas that line the Guaporé are the productive heart of the system — when the flood comes up, fish move out of the channels into inundated grass and woodland to feed on fruit, seeds, invertebrates and detritus, the same floodplain-feeding pattern Rosemary Lowe-McConnell documented across tropical river fisheries (FEOW Ecoregion 319; Lowe-McConnell, 1987).

Habitat structure tracks the substrate. In the clearwater reaches the bed is sand and exposed rock, with submerged wood and leaf litter where the channel runs under forest; these are the open, well-lit, sandy-bottomed places favored by eartheaters and sand-sifting catfishes. In the Moxos the picture is the opposite — soft-bottomed, weed-choked, slack water in oxbows and marshes, where dwarf cichlids and small characins shelter among roots, leaf litter and emergent plants. Stagnant dry-season pools lose oxygen and warm up, which is part of why so many of the basin's fishes — the knifefishes, the air-gulping species — carry adaptations for getting by in poorly oxygenated water. The combination of clear shield water, extreme seasonality and a vast, low-gradient floodplain is exactly the template the Guaporé's fauna is built around (FEOW Ecoregion 319; Hamilton et al., 2002; Lowe-McConnell, 1987).

The cichlid fauna

Around 260 fish species are known from the Guaporé-Iténez, and roughly 25 of them are found nowhere else — a respectable level of endemism for a basin of this size. More than half of those endemics are armored catfishes in the genus Corydoras, the small, plated bottom-dwellers prized by aquarists; the basin is something of a Corydoras hotspot, and one of its oddest endemics, the blind catfish Phreatobius sanguijuela, turned up in a single well and seems to live in groundwater. The cichlids are not the most endemic-rich part of the fauna, but they are among the most conspicuous, and the atlas roster for this water area runs across the basin's habitats from open sandy channels to shaded floodplain pools (FEOW Ecoregion 319).

The dwarf cichlids of the genus Apistogramma are the heart of the floodplain assemblage, and the Guaporé holds an unusual concentration of them. The roster carries Apistogramma trifasciata, the three-striped dwarf cichlid that ranges from the Guaporé into the Paraguay system; A. commbrae; A. linkei; A. erythrura; and A. sororcula, which was described in 2016 specifically from the Guaporé drainage in Bolivia (Beni) and Brazil (Mato Grosso). These are fishes of the quiet margins — leaf litter, root tangles and shallow weedy water in the oxbows and creeks of the Moxos — where males hold small territories and females guard cavities. Sharing that slackwater habitat is the checkerboard dwarf Crenicara latruncularium, a slender, sand-and-litter cichlid of the same gentle backwaters.

The larger cichlids belong to the open river. The pike cichlids are represented by Crenicichla lepidota, a widespread ambush predator of vegetated edges and rocky margins, and by the bigger Crenicichla adspersa, both lie-in-wait hunters that hang in cover and dart at passing fish and invertebrates. Out on the sand, the eartheater Geophagus megasema works the bottom in the classic Geophagus way, taking up mouthfuls of substrate and sieving out invertebrates and organic matter through the gills. The roster also carries the broad-bodied, generalist acaras and their kin that fill quieter Amazon habitats: the green acara Aequidens viridis, an adaptable cichlid of slow margins and floodplain pools; Cichlasoma boliviense, a hardy savanna-lake and backwater cichlid named for the basin it lives in; and Heros spurius, a deep-bodied, disc-shaped severum-type cichlid that hangs among submerged wood and roots in slower water. This is a recognizably upper-Madeira fauna — the same broad cast of genera you meet across the southwestern Amazon — but filtered through the basin's shield chemistry and its long history of seasonal isolation behind the Moxos (FEOW Ecoregion 319; Britzke et al., 2016 on Apistogramma sororcula).

Conservation

The Guaporé sits on the southern frontier of the Amazon, which is also the leading edge of the 'arc of deforestation' — the band of forest loss that has pushed north and west out of Mato Grosso and Rondônia for four decades. On the Brazilian side the dominant pressure is land conversion: cattle pasture and mechanized agriculture replacing forest and cerrado, which strips the gallery forests that shade and feed the tributaries, loads the clear shield water with sediment and nutrients, and erodes the floodplain connectivity the fishes depend on. The basin's great asset — clear, low-sediment water — is precisely what siltation from cleared, plowed and grazed ground destroys (Mongabay, 2018; FEOW Ecoregion 319).

On the Bolivian side the threat is different in kind. The Llanos de Moxos is open savanna already, and the pressure there is the intensification of cattle ranching and the alteration of the floodplain's water — the seasonal flood that the whole fauna is timed to. Across the wider Bolivian Amazon a boom in small-scale alluvial gold mining has driven mercury into the rivers; miners use mercury to amalgamate gold, and it ends up in the water, in sediment, and — concentrated up the food chain — in the predatory fish that people eat. Studies of the Madeira system have repeatedly found elevated mercury in fish and in the hair of riverside and Indigenous communities, and the same dynamic now plays out on the Guaporé and its Rondônia tributaries, where mining has expanded in recent years (IUCN NL; Mongabay, 2023; Malm et al., Madeira basin).

Downstream, the Madeira itself has been transformed. The Santo Antônio and Jirau dams, built near Porto Velho in the 2000s and 2010s, turned a long stretch of the upper Madeira into run-of-the-river reservoirs and cut the migratory corridor that long-distance characins — the Brycon, Colossoma, Leporinus and others that move up out of the Guaporé-Mamoré lowlands to spawn — have used for millennia. A dam on the lower Madeira is a wall across the front door of the entire Guaporé-Iténez basin. Set against all this is one of the strongest protected areas in the Amazon: Noel Kempff Mercado National Park, roughly 1.5 million hectares of largely intact forest, savanna, cerrado and wetland on the Huanchaca plateau and the upper Iténez, which shields a major slice of the basin's headwaters and its endemic fauna. The basin is, for now, far less altered than the Madeira's Andean arms — but it is being approached from every side at once (IUCN/UNESCO Noel Kempff evaluation; CASA, Madeira dams; Survival International; Mongabay, 2018).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Aequidens · 1

Apistogramma · 5

Cichlasoma · 1

Crenicara · 1

Crenicichla · 1

Geophagus · 1

Heros · 1

Lugubria · 1