Malebo Pool

Malebo Pool is the great lung in the middle of the Congo River — a lake-like widening where the river, after running broad and slow for a thousand kilometres, briefly slackens and spreads to about 16 mi across before plunging into the cataracts of the lower Congo. Stanley Pool on the old maps, it sits directly between Kinshasa and Brazzaville, the two capitals that face each other across its water and now hold something near twenty million people between them. Above is calm floodplain river; below is one of the steepest large-river descents on Earth. The pool is the hinge, and its fishes — sand-channel tilapias, the goby-like 'lionhead' cichlids, and a scatter of endemic catfishes — live in that narrow band between still water and the roar downstream.

Geography & hydrology

Malebo Pool forms where a rock sill across the bed of the Congo forces the river to expand directly above Kinshasa. The pool sits between the Douvre cliffs at its upstream end and the Kintambo rapids at its downstream end, a lentic interruption maybe 22 mi long and 16 mi wide that seldom exceeds 33 ft in depth. After that brief widening the whole river is funnelled back into a single channel and drops into the Lower Congo Rapids, where it falls roughly 919 ft over the 217 mi between the pool and Matadi — so the pool is the last stretch of quiet water before the most violent reach of the entire river (FEOW Ecoregion 548; Roberts & Stewart 1976).

The pool is split down the middle by Île Mbamou (Bamu Island), a large low island that divides the mainstream into two channels and carries the international boundary between the Republic of Congo on the Brazzaville side and the Democratic Republic of Congo on the Kinshasa side. Smaller islands, shifting sandbanks and shoals are scattered through the rest of the basin, and the margins grade into extensive palm and papyrus swamps. Mats of floating water-hyacinth (Eichhornia) drift across the open water and pile against the islands, a recurring feature of the pool noted in the early limnological surveys (Bailey 1986; FEOW Ecoregion 548).

The water is never truly still. Even where it spreads widest, the Congo keeps moving through the pool toward the rapids, so the basin behaves less like a lake than like a slow, very broad river reach. Discharge is enormous and seasonal in a doubled rhythm peculiar to the equator: because the Congo's northern and southern tributaries flood at different times of year, Malebo Pool sees two distinct high-water pulses — a smaller one from April to June and a larger one from October to January. Average flow at the pool runs on the order of 30,0 ft³/s at low water and 60,0 ft³/s in the flood season; in the exceptional rains of 1961 and 1962 it reached about 80,0 ft³/s. Water levels within the pool swing by roughly 10 ft over the year (FEOW Ecoregion 548; Bailey 1986).

That geometry has a deep history. Under the long-standing Cahen hypothesis, reported by Roberts and Stewart, the Congo Basin drained internally through much of the Miocene and Pliocene, very likely holding a vast interior lake, until a coastal Lower Guinean river cut back into the rim in the late Pliocene or early Pleistocene and captured the basin, opening the drop to the Atlantic. Malebo Pool sits right at that captured threshold — the place where the old interior basin spills out toward the sea (Roberts & Stewart 1976; FEOW Ecoregion 548).

Water & habitat

The dominant habitats of the pool are sand and current. Coarse sand moves constantly along the channels and builds the shoals and bars that the river reworks with each flood, while the deeper channels on either side of Île Mbamou carry the main flow. This is a floodplain-river environment in the FEOW major-habitat sense, with the open sandy pool fringed by swamp: palm stands, papyrus beds and seasonally inundated margins that the fishes use as nursery and refuge when the water is high (FEOW Ecoregion 548; Bailey 1986).

The seasonal regime sets the rhythm of the place. The two annual floods raise the water by about 10 ft and push it back into the marginal swamps and the mouths of affluent tributaries such as the N'sele, on the Kinshasa side, before it drains down again. During those pulses the floating hyacinth mats thicken and travel, sheltering small fishes and invertebrates and occasionally rafting them downstream. Between floods the channels run lower and the sandbanks re-emerge, concentrating fish into the deeper runs and the tributary mouths. The N'sele alone — one large affluent feeding the pool — was found to hold 148 fish species in 27 families, a measure of how much diversity these feeder streams add to the open water (Stiassny, Iyaba & Liyandja 2013).

Water chemistry across the pool follows the broader middle-Congo pattern: warm, soft, weakly mineralized and near-neutral to mildly acidic, drawn off the deeply leached Congo basin rather than from any mineral-rich uplands. The pool is not stratified in any lake-like way — the throughflow keeps it mixed — and dissolved oxygen stays workable in the moving channels, though backwaters choked with decaying hyacinth can go hypoxic. What makes the habitat distinctive for fishes is the contrast it straddles: lentic, sandy, swamp-fringed water in the pool itself, giving way within a few kilometres to the oxygen-charged, boulder-strewn torrents of the Kintambo rapids and the lower Congo below. A fish in Malebo Pool is living at the edge of two utterly different worlds, and the fauna sorts itself accordingly — broad-bodied open-water and sand species in the pool, and a set of current-adapted forms that push to its lower margin where the flow begins to bite (FEOW Ecoregion 548; Roberts & Stewart 1976).

The cichlid fauna

For all the pool's catfish endemism, its cichlids are what most aquarists know it for, and the headliners are the lionheads of the genus Steatocranus — squat, big-headed, reduced-swimbladder fish that have traded cruising the open water for hopping along the bottom in short goby-like darts, wedging into rock crevices and holding station against current. The buffalohead, Steatocranus casuarius, is the best-known: a stocky cichlid to about 4 in with the swollen nuchal hump that gives the group its common names, a cave-spawning algae-grazer of the pool and the adjoining rapids reach. The roster adds S. gibbiceps, similar in size and habit, and the slimmer, more elongate S. tinanti at only about 2.5 in. All three are rheophilic, most at home where the still pool gives way to broken water at its margins (FishBase, Pool Malebo species list; FishBase, Steatocranus casuarius).

The pool's other small rock-and-crevice cichlid is Lamprologus teugelsi, one of the riverine African Lamprologus — the same lineage that, in a separate radiation, seeded the lamprologines of Lake Tanganyika. Schelly and Stiassny described it in their 2004 revision of the Congo River Lamprologus from material taken at Malebo Pool and the lower Congo rapids; it is a small, shallow-bodied fish of rocks and hard substrate, told apart from its lower-river relatives by counts, osteology and colour. Their distribution maps showed large sampling gaps, a sign that even the named diversity here is incompletely mapped (Schelly & Stiassny 2004).

The larger, open-water cichlids span the usual African guilds. Coptodon congica is a substrate-spawning tilapia of around 10 in, a detritus-and-vegetation feeder of the demersal zone; Oreochromis lepidurus — the lower Congo tilapia — is a maternal mouthbrooder whose tight range runs down toward the river's mouth and which the IUCN assesses as Endangered, the most range-restricted of the pool's tilapiines. Tylochromis labrodon is a thick-lipped, sand-sifting cichlid of the bottom, reaching about 9 in and feeding low on the food chain. Pterochromis congicus, sole species of its genus, is a smaller demersal cichlid widespread through the central Congo and taken as food by local fishers. Rounding out the roster is Ctenochromis polli, a small haplochromine-type cichlid under 4 in — listed in FishBase as Haplochromis polli — found in the pool and its tributaries, its feeding studied in the Djoué on the Brazzaville side (FishBase, Pool Malebo species list; Tsoumou et al. 2019; Springer, Fishes of Angola on O. lepidurus).

Endemism here is concentrated in the catfishes rather than the cichlids. The FEOW account lists most of the ecoregion's endemics as catfishes — four amphiliids, a clariid and three mochokids, including mountain catfishes (Leptoglanis) and the upside-down catfish Atopochilus chabanaudi — several known only from their type localities. The pool's cichlids are largely shared with the wider middle and lower Congo, but the assemblage as a whole, poised between lentic pool and lower-river rapids, is distinctive and still thinly studied (FEOW Ecoregion 548; Schelly & Stiassny 2004).

Conservation

No African freshwater is under quite the pressure of Malebo Pool, because no comparable river reach has two capital cities on its banks. Kinshasa, on the southern shore, is one of the fastest-growing megacities on the planet, its metropolitan population now estimated around fifteen million; Brazzaville faces it across the water. Between them they pour largely untreated wastewater straight into the pool — neither city can transport or treat most of its sewage — and the tributaries draining Kinshasa carry a heavy burden of plastic, with city waste output put near 1,500 tonnes a day. The smaller feeder rivers run visibly clogged with bottles and debris, fouling the fishing grounds the riverside population depends on (IWA, Kinshasa & Brazzaville; reporting on Congo River plastic pollution).

The fishery is intense and largely artisanal, worked from pirogues across the pool and its tributaries with nets, traps and lines. The N'sele survey that catalogued 148 species framed the region as 'undergoing rapid environmental degradation,' and work on the pool's elephantfishes (Mormyridae) has wrestled with keeping a heavily fished resource sustainable so close to a city of millions. Sand and gravel extraction from the channels and bars — feeding Kinshasa's construction boom — disturbs the same sandy bottoms that Tylochromis labrodon and Coptodon congica feed and nest on, while clearing the marginal palm and papyrus swamps strips the nursery cover the flood pulse depends on (Stiassny, Iyaba & Liyandja 2013).

The largest shadow falls from downstream. The Congo's drop below the pool is the target of the Inga schemes: the existing Inga 1 and Inga 2 dams date to the 1970s and 1980s, and the proposed Grand Inga complex — Inga phases 3 through 8 — would, if built, more than double the capacity of China's Three Gorges and rank as the largest hydropower project on Earth. Conservation analyses warn that re-engineering the lower Congo's flow on that scale would reduce and steady the discharge, shift the dominant fish species, and convert free-flowing rapids into reservoir, with knock-on effects across a catchment of at least 700 fish species; earlier Inga construction has already been tied locally to a collapse in rapids fish diversity below the pool (International Rivers, Grand Inga; Global River Protection Coalition, Lower Congo).

The threats sort the fauna differently. The rheophilic specialists — the Steatocranus lionheads and Lamprologus teugelsi that need the oxygenated, rocky transition where pool meets rapids — are most exposed to any flow change from the Inga schemes downstream. The open-water tilapias and the sand-sifting Tylochromis are squeezed instead by dredging, swamp clearance and the pollution off the twin cities. Oreochromis lepidurus, already Endangered on its narrow lower-Congo range, has the least room to absorb any of it. The pool's fauna remains, in the FEOW assessors' own word, poorly explored, much of the baseline data decades old — so the region is changing faster than it is being documented (FEOW Ecoregion 548; International Rivers, Grand Inga).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Coptodon · 1

Ctenochromis · 1

Lamprologus · 1

Oreochromis · 1

Pterochromis · 1

Steatocranus · 3

Tylochromis · 1