Paraguay

Geography & hydrology

The Río Paraguay is the fifth-largest river in South America and the biggest tributary of the Paraná, running about 2,342 mi from the Chapada dos Parecis in Mato Grosso to its confluence with the Paraná at the southern tip of the ecoregion. FEOW treats the whole catchment as Ecoregion 343, drawing its northern edge along the divide with the Amazon on the Chapada dos Parecis and dos Guimarães, its eastern edge against the Paraná and Araguaia, and its western edge at the main channel itself; the left-bank tributaries — the Cuiabá, Taquari, Miranda, Aquidauana, Negro, Apá, Ypané and others — do most of the draining (FEOW Ecoregion 343; Chernoff et al. 2001).

What makes the basin unusual is its near-total lack of gradient. The Pantanal sits in a vast structural depression filled with Quaternary sediment, most of it sandy, with elevations between about 50 and 656 ft and a slope of only 0.5–1 in per kilometer north to south and 2.5–3 in per kilometer east to west. Rather than a single trough, the floodplain is built from coalescing alluvial fans — inland deltas where rivers like the Taquari spill out, shift course and rebuild — so the water has no tidy path to the sea. The entire Pantanal drains through one outlet, the Río Paraguay, and that bottleneck is why the flood backs up and lingers (FEOW Ecoregion 343; Por 1995; Heckman 1998).

The flood pulse here runs on a strange clock. Rain falls mostly between October and April, but because the plain drains so slowly, the high water arrives late and out of phase with the rain that caused it: maximum flooding occurs as early as February in the northern sub-regions and as late as June in the south, and it can take roughly six months for a pulse to travel from the headwaters down to the lower Paraguay. At the Ladário gauge near Corumbá, where the river has been measured for over a century, the annual stage swings by 13–16 ft. Satellite and field work between 1979 and 1987 found the inundated area ranging from about 11,0 mi² at low water to 110,0 mi² at peak, averaging near 53,0 mi² — roughly 80% of the Pantanal under water in the biggest years. Only the Paraguay and the Cuiabá keep a steady flow year-round; everything else expands and contracts (FEOW Ecoregion 343; Hamilton et al. 1996; Bao et al. 2017).

Water & habitat

Chemically the Paraguay is gentle water. The main channel runs slightly acidic, around pH 6.0–6.5, with low conductivity (60–100 µS/cm), warm temperatures of 75–81 °F, and dissolved oxygen that is usually under 6 mg/L even before the floodplain gets involved. This is soft, weakly mineralized, tannin-influenced water shaded by gallery forest and fringed with floating meadows — anchored water hyacinth (Eichhornia azurea), water lettuce, the floating ferns Salvinia, and pickerelweed forming mats along the banks and across the baías (FEOW Ecoregion 343; Chernoff et al. 2001).

The defining event of the year is the flood pulse, and its most consequential side effect is oxygen. When the rising river spreads onto the plain, its water meets months of accumulated dead grass and leaf litter, and the decomposition of all that organic matter strips the oxygen out. Pantaneiros call the result the decoada (or dequada): a slug of deoxygenated, acidified, often blackened water that moves down the system and can kill fish by the thousand. Hamilton and colleagues documented one such anoxic event spilling off the floodplain into the Paraguay itself, a biogeochemical pulse strong enough to register in the main channel. It is not a malfunction; it is the engine of the system, because the same decomposition that pulls down the oxygen also releases the nutrients that drive the next burst of growth (Hamilton et al. 1997; Calheiros & Hamilton 1998).

For fishes the year therefore has two faces. On the rising and high water, the flood opens an enormous nursery — inundated grassland and flooded forest where young fish, invertebrates and aquatic plants explode in number, and where migratory species move out of the rivers to feed and spawn. On the falling water, that habitat collapses back into the channels. Once-continuous sheets of water break into shallow baías, oxbows and isolated pools; fish are concentrated into shrinking volumes that warm up, lose oxygen and draw predators and fishers alike. Some residents are built for the worst of it — the South American lungfish (Lepidosiren paradoxa) breathes air and can wait out a drying pool, and small armored catfishes gulp air at the surface — while annual killifishes simply die and leave drought-resistant eggs in the mud. The cichlids fall in between, riding the flood out and packing into the deeper refuges when it goes (FEOW Ecoregion 343; Lowe-McConnell 1987; Bao et al. 2017).

The cichlid fauna

The Paraguay basin is species-rich — more than 80 fishes are endemic to the ecoregion in 14 families, most of them characins, loricariid catfishes and rivulid killies — and endemism runs higher in the shield headwaters than out on the floodplain. The cichlids are a smaller slice of that total, but they are among the most familiar fishes of the system to anyone who keeps South American species, because so much of the slow, vegetated, soft-water habitat they prefer is exactly what the Pantanal manufactures (FEOW Ecoregion 343; Willink et al. 2000).

The signature eartheater is Gymnogeophagus balzanii, the Paraguay or humphead eartheater, a substrate-sifting cichlid that turns up away from the fast main channels in sluggish tributaries, creeks and floodplain baías. Males grow a pronounced forehead hump and reach around 8 in; the species is a delayed mouthbrooder, and it is one of the more cold-tolerant Neotropical cichlids, since the southern reaches of the La Plata system it belongs to cool sharply in winter. The pike cichlids are well represented: Crenicichla lepidota is one of the most widespread cichlids in the whole La Plata drainage, an ambush predator of weedy margins; the stockier C. semifasciata (sometimes split into its own genus, Batrachops) hunts among submerged structure; and C. ploegi is a more recently described member of the Paraguay–Paraná pike-cichlid fauna. The chunky, deep-bodied Chaetobranchopsis australis filters plankton and fine detritus from the slack water of lagoons and backwaters (FishBase; Seriously Fish; Piálek et al. on La Plata Crenicichla).

The mid-sized open-water cichlids are headed by Cichlasoma dimerus, a hardy acara of the Paraguay and lower Paraná that has become a standard laboratory model for cichlid reproduction and behavior, and by Bujurquina vittata, the banded acara, a biparental substrate-spawner of the Paraguay and middle Paraná drainages. Mesonauta festivus, the festive cichlid, hangs in vegetation near the surface, often angled head-down among stems. The smaller end of the roster belongs to dwarfs: Laetacara dorsigera, the red-breasted smiling acara, and four Apistogramma — A. borellii (the cool-tolerant umbrella apisto common in Paraguayan and Argentine waters), A. trifasciata, A. commbrae and A. inconspicua — small cichlids of leaf litter, marginal vegetation and shallow creeks. None is a rapids fish; collectively they read as a roll-call of the Pantanal's quiet water (FishBase; Seriously Fish; Staeck & Linke).

Two things are worth keeping straight about this fauna. First, it is a La Plata fauna, not an Amazonian one — the Pantanal was a closed basin before it linked to the Paraná in the Holocene, and the upper and lower Paraguay still differ in their fish composition as a result. Second, the cichlids share the water with the basin's economic giants: the migratory sábalo (Prochilodus lineatus) and the big pseudoplatystoma catfishes that run upstream to spawn on the early flood, the movements that the whole fishery is built around (FEOW Ecoregion 343; Resende 2003).

Conservation

The thing that makes the Pantanal work — a flat plain that floods and drains on its own schedule — is also what makes it fragile, because almost any change to the timing or volume of water reaches everywhere. The largest looming threat is the Hidrovía Paraguay–Paraná, a long-running plan to turn the river into an industrial barge corridor for soy, grain and minerals by dredging and straightening hundreds of kilometers of channel. The concern is not only the dredging footprint, which overlaps protected areas and Indigenous lands, but the hydrology: cutting and deepening the channel speeds drainage and disconnects the river from its floodplain, pulling the plug on the very flood pulse the wetland depends on. More than forty scientists have warned that the project could push the system toward 'the end of an entire biome,' and reviewers have long cautioned that even piecemeal works — a 'tyranny of small decisions' — can degrade the Pantanal without any single dramatic dam (Gottgens et al. 2001; Nature-based Solutions Initiative 2024).

Upstream, the headwater plateaus that feed the Pantanal are being converted to cattle pasture and row-crop agriculture. Clearing the uplands accelerates erosion, and the Taquari fan is the textbook case: sediment washed off deforested ground has choked and avulsed the river, drowning large areas of pasture and ranch land under permanent water while leaving the channel itself a shifting mess. Small hydroelectric plants on the upland rivers chop up the flow and block the migration routes of the sábalo and the big catfishes, which need an unobstructed run from the rivers up onto the early-season floodplain to spawn. Soy and pasture also bring agrochemicals and fertilizer into water that is naturally low in nutrients and oxygen (FEOW Ecoregion 343; Resende 2003; Bao et al. 2017).

Then there is fire. In 2020 the Pantanal had its driest year in roughly half a century; the Paraguay River ran far below normal, the plain that usually floods stayed dry, and fire — most of it set deliberately to clear pasture and land — ran out of control. By December nearly 4.5 million hectares had burned, around 30% of the entire biome and more than twenty times the area lost in the previous two decades combined; some conservation units lost over 90% of their area. Analyses tie the catastrophe to human ignition near roads and waterways acting on top of an extreme drought, with weakened environmental enforcement letting the burning spread. For the fishes the chain is indirect but real: drought shrinks the dry-season refuges and intensifies the decoada, fire strips the riparian vegetation and floating meadows that shade and feed the floodplain, and ash and erosion load the water afterward. A wetland whose fish fauna is tuned to a reliable annual flood is poorly equipped for a future of deeper droughts and a re-plumbed river (Mongabay 2020; de Magalhães Neto & Evangelista 2022; FEOW Ecoregion 343).

Sources

1. Paraguay — Freshwater Ecoregions of the World (FEOW Ecoregion 343; boundaries, hydrology, flood regime, endemic fishes)
2. A biological assessment of the aquatic ecosystems of the Pantanal, Mato Grosso do Sul, Brasil (Willink, Chernoff, Alonso et al., Conservation International RAP 18, 2000)
3. An anoxic event and other biogeochemical effects of the Pantanal wetland on the Paraguay River (Hamilton, Sippel, Calheiros & Melack, Limnology and Oceanography 42(2):257-272, 1997)
4. The Paraguay–Paraná Hidrovía: Protecting the Pantanal with Lessons from the Past (Gottgens et al., BioScience 51(4):301-308, 2001)
5. The end of an entire biome? World's largest wetland, the Pantanal, is being threatened by the Hidrovia Paraguay-Paraná (Science of the Total Environment, 2024)
6. Scientists warn of threat to Pantanal wetlands from the Hidrovia Paraguay-Paraná waterway (Nature-based Solutions Initiative, University of Oxford)
7. 'Devastating' fires engulf Brazilian Pantanal wetlands — again (Mongabay, 2020; ~4.5 Mha / 30% of biome burned, 2020 drought)
8. Human Activity Behind the Unprecedented 2020 Wildfire in Brazilian Wetlands (Pantanal) (de Magalhães Neto & Evangelista, Frontiers in Environmental Science 10:888578, 2022)
9. Gymnogeophagus balzanii (Paraguay eartheater) — distribution, habitat and biology (Seriously Fish)
10. Bujurquina vittata, banded acara — Paraguay and middle Paraná distribution (FishBase)