The Zambezi & Okavango

The Zambezi and Okavango are the great floodplain rivers of south-central Africa — warm, shallow, sand-bedded waters that rise in the rainy highlands of Angola and Zambia and breathe in and out with a seasonal flood pulse. Each year the rains push a wave of water down onto vast, flat grasslands: the Barotse floodplain of western Zambia, the Kafue Flats, and — most famously — the Okavango Delta of Botswana, a 5,800-square-mile (about 15,0 mi²) inland delta that empties not into the sea but into the sands of the Kalahari, where the water simply evaporates and sinks away. There is no rift, no thousand-meter abyss, no permanent thermocline here. Instead the story is the flood: water spreads thin and clear over the plains, fish follow it out to spawn and feed, and the plains drain again. Out of that rhythm has come a distinctive Zambezian cichlid fauna — the big serranochromine predators, the snail-crushers, the floodplain breams and tilapias — and a fishery, and a wilderness, shared across six countries.

Setting & origin

The Upper Zambezi begins in the high, wet miombo country where Zambia, Angola and the Democratic Republic of Congo meet, then runs south and west across a remarkably flat landscape floored by deep Kalahari sands. Along the way it floods two of Africa's great wetlands. The first is the Barotse (Bulozi) floodplain of western Zambia — the heartland of the Lozi people — which at full flood stretches roughly 150 miles (about 149 mi) from Lukulu to Nangweshi, up to 22 miles (22 mi) wide, drowning on the order of 2,900 square miles (about 7,311 mi²) of grassland (FEOW; Timberlake, Biodiversity of the Zambezi Basin). The second is the Kafue Flats, on the Zambezi's great northern tributary the Kafue, another shallow seasonal plain famous for its lechwe and its fishery. At Victoria Falls — Mosi-oa-Tunya, 'the smoke that thunders' — the river drops about 320 feet (322 ft) into a basalt gorge. That waterfall is not just scenery: it is a hard biogeographic wall, an impassable barrier that separates the species-rich Upper Zambezi above from the Middle and Lower Zambezi below (FEOW). It is why the lower Zambezi, and the entirely separate Rift lake systems of Malawi and Tanganyika, belong to a different fish world from the one described here.

The Okavango is the Zambezi's strange, beautiful neighbor. It rises in the same Angolan highlands as the Cubango (Kavango) and Cuito rivers, runs along the Namibia–Angola border, and then, in northern Botswana, fans out into the Okavango Delta — an alluvial fan often called the world's largest inland delta. It is endorheic: the river never reaches the ocean. Instead its water spreads across a panhandle and fan of channels, lagoons and floodplains covering some 5,800 square miles (about 15,0 mi², swelling toward 8,500 mi²/22,0 mi² in the biggest flood years) before it is lost to evaporation, transpiration and the Kalahari sands, with only a trickle reaching Lake Ngami or the Boteti in wet years. The two systems are not fully isolated: in big floods the Selinda (Magwegqana) Spillway can link the Okavango to the Kwando–Linyanti–Chobe and on toward the Zambezi, which helps explain why they share so much of their fish fauna (Tweddle 2010).

These are profoundly transboundary waters. The Zambezi basin is shared by eight countries — Angola, Botswana, Malawi, Mozambique, Namibia, Tanzania, Zambia and Zimbabwe — and the Upper Zambezi floodplains alone touch Zambia, Angola, Namibia's Caprivi (Zambezi) Strip and Botswana. Since 2014 the Zambezi Watercourse Commission (ZAMCOM), headquartered in Harare, has coordinated the eight riparian states. The Okavango is shared by just three — Angola, Botswana and Namibia — which since 1994 have managed it jointly through the Permanent Okavango River Basin Water Commission (OKACOM). Two enormous dams sit on the system: Kariba, on the Middle Zambezi below the falls, and the Itezhi-Tezhi and Kafue Gorge dams on the Kafue.

Temperature & flow

These are warm, subtropical waters that turn cold at night and in the dry winter. In the Okavango, water temperature swings seasonally from around 61 °F to the low 90s °F (about 61–90 °F), and because the water is rarely more than about 13 feet (13 ft) deep, the column is well mixed and nearly uniform top to bottom — there is no permanent thermocline and no cold deep layer (Okavango fish-physiology studies; van der Waal). The Zambezi floodplains run in the same warm band, with shaded papyrus channels cooler than open, sun-baked floodplain lagoons.

Flow, not depth, is the master variable. Everything here is driven by a single seasonal flood pulse. Rain falls on the Angolan and Zambian highlands from roughly November to March; the resulting flood wave then takes months to travel downstream, so the plains far to the south flood in the dry season, out of phase with the local rains. On the Upper Zambezi the river rises and falls by an average of about 17 feet (17 ft) between low and flood stage, spilling out of its channel to inundate the Barotse plain (FEOW). The flood that fills the Okavango Delta is even more dramatically lagged: the Angolan rains of summer arrive in the panhandle around May and the floodwater creeps across the fan to peak in the far south around June, July and August — the very depths of Botswana's dry season — so the desert blooms when the sky is cloudless. Discharge is huge but variable: the long-term mean flow of the Zambezi at Victoria Falls is on the order of 1,328 ft³/s, ranging from severe drought years near 1280 ft³/s to floods well above 3,000–4,0 ft³/s (Zambezi River Authority). Into the Okavango, the Angolan rivers deliver on the order of 10–11 billion cubic meters a year, almost all of which never leaves the basin. The fish are tuned to this rhythm: the rising flood is the signal to move out onto the plains to spawn and feed, and the falling flood concentrates them — and the fishery — back into the channels and lagoons.

Water chemistry & clarity

The defining chemical fact of this region is how soft, dilute and clean the water is — a direct consequence of rivers running over nutrient-poor Kalahari sand rather than rich volcanic or sedimentary rock. The Okavango is the extreme case. In its main channels electrical conductivity is astonishingly low — commonly 30–100 µS/cm, and as little as ~23 µS/cm in the panhandle — among the lowest of any large African wetland, with total dissolved solids averaging only about 40 mg/L (Mmualefe & Torto, Water SA 2011; Okavango fish-physiology studies). pH in the channels runs slightly acidic to neutral, around 6.5–7.0, across the wider Delta spanning roughly 6.1–9.4, drifting alkaline only downstream near settlements and cattle. Nutrient levels are so low — total nitrogen and phosphorus generally a fraction of a milligram per liter — that the Delta is classed as hyperoligotrophic: it essentially cannot eutrophy. Much of that purity is biological housekeeping. The papyrus (Cyperus papyrus) and reed beds that line the channels act as a vast living filter, taking up nutrients and trapping sediment, while the salts carried in by the river are concentrated under the Delta's countless islands and precipitated out there rather than building up in the open water — which is how a closed, evaporating basin stays fresh.

The payoff is clarity. Okavango channel water is famously transparent — clear enough to see fish and sand grains on the bed — quite unlike the tea-stained or silty water of many tropical rivers; in the seasonal swamps, tannins from decaying vegetation can stain it darker. The Upper Zambezi and its floodplains are similarly soft and lightly mineralized — 'nutrient-poor and unproductive' rivers running over Kalahari sand (FEOW) — though they carry more suspended sediment than the Okavango, especially as the flood first spills onto the plains and stirs up the season's accumulated organic matter. Aquarists who keep upper-Zambezi and Okavango cichlids accordingly aim for soft, slightly acidic-to-neutral water — closer to West African blackwater than to the hard, alkaline conditions of the Rift lakes.

Habitats

Because this is floodplain country, its habitats are defined by water that comes and goes. The seasonal floodplain is the signature: flat grassland that is dry pasture for half the year and shallow lake for the other half. When the flood arrives it opens up an enormous, food-rich, warm, shallow nursery — newly drowned grass, insects, and detritus — and fish pour out of the permanent channels to exploit it, spawning in the warm shallows so that fry hatch into a temporary paradise. As the flood recedes, that habitat shrinks and then vanishes, stranding and concentrating fish on their way back to the river. Threading through the plains are the permanent channels and the papyrus-fringed lagoons and oxbows (in the Okavango, the lediba) that hold water year-round and serve as the dry-season refuge. The Okavango adds a habitat the Zambezi plains lack at scale: a permanent swamp — the upper panhandle and central fan, a labyrinth of papyrus, reeds, water lilies, floating islands and deep clear channels that never dries out.

The flip side of all this warm, shallow, vegetation-choked water is oxygen. In the Okavango's seasonal swamps, the annual flood drives severe, prolonged hypoxia: as the water spreads over masses of dead and living plant matter, decomposition strips out the oxygen, and dissolved-oxygen levels can sit near or below 0.5 mg/L for weeks at a stretch and stay low for three to five months after the flood front passes (Okavango fish-physiology studies, e.g. work at Guma Lagoon). Fish here live a 'tough way of life' and are adapted to it — through enlarged gills, raised hemoglobin, air-gulping at the surface, and seeking out the thin oxygenated film at the air-water interface. These low-oxygen floodplains, not the clear deep channels, are where much of the system's biology actually plays out.

The cichlids

The Zambezi and Okavango share a single, distinctive Southern-African (Zambezian) cichlid fauna — modest in number next to the hundreds of species in the Rift lakes, but full of character and ecologically central to the rivers. Its stars are the serranochromines, a lineage of haplochromine cichlids that reaches its greatest diversity in exactly these waters — the upper/middle Zambezi, Okavango and adjacent basins. Serranochromis are the largemouths or 'nembwe': big, robust, ambush piscivores — the river's pike-equivalents — that lie among reeds and papyrus and hunt other fish; the Upper Zambezi drainage alone holds roughly six largemouth Serranochromis species (including S. robustus, S. angusticeps, S. macrocephalus, S. longimanus, S. thumbergi and the recently described S. altus). Their sister group, Sargochromis, are the smallmouths — smaller, blunter-toothed cichlids built to crush snails and other hard-shelled invertebrates, with about three species (such as S. giardi and S. carlottae) in the Upper Zambezi. A small ambush predator, Pharyngochromis acuticeps, rounds out the group.

Alongside the serranochromines are the workhorse breams and tilapias that anchor the floodplain fishery. The substrate-spawning 'happy breams' — Coptodon rendalli (redbreast tilapia, a macrophyte-grazer) and Tilapia sparrmanii (banded tilapia) — share the plains with the mouthbrooding Oreochromis: O. andersonii, the three-spot tilapia, and O. macrochir, the greenhead or longfin tilapia, both prized food fish native to the Upper Zambezi, Kafue and Okavango. Smaller still is Pseudocrenilabrus philander, the southern mouthbrooder, an abundant, adaptable little dwarf cichlid found right across the Zambezi and Okavango systems and a favorite of aquarists. Across the whole Upper Zambezi–Okavango assemblage there are on the order of 30-plus cichlid species; the Okavango Delta alone supports about 71 fish species of all families, with the Cichlidae the richest family in its lagoons (Tweddle et al. 2003). Crucially, the flood pulse shapes how these fish live: most are floodplain spawners that ride the rising water out onto the grass, and the timing and height of the flood — not water depth or temperature stratification — is what governs a good year from a bad one.

People & pressures

These floodplains have fed people for centuries. The Barotse plain, the Kafue Flats and the Okavango Delta all support important artisanal and subsistence fisheries built around the flood: nets, traps, weirs and basket fishing follow the fish as they move on and off the plains, and the seasonal floodplains — not the deep permanent channels — produce most of the catch. The Lozi flood calendar of the Barotse, centered on the Kuomboka ceremony when the king moves from the flooding plain to higher ground, is a cultural expression of the same hydrology the fish obey. The Okavango Delta adds another value entirely: as one of Africa's last great wetland wildernesses it is a Ramsar Wetland of International Importance and, since 2014, a UNESCO World Heritage Site, the foundation of Botswana's high-value wildlife tourism.

The pressures are real and mostly come from upstream and outside. On the Zambezi, big dams — Kariba on the Middle Zambezi and Itezhi-Tezhi plus the Kafue Gorge dams on the Kafue — have re-plumbed the flood pulse that the downstream fisheries and wetlands depend on, smoothing the natural flood-and-drought rhythm of the Kafue Flats and altering downstream habitat. The Okavango's threats are different: the Delta itself sits in sparsely populated, well-protected Botswana, but its water is born in Angola and crosses Namibia, and both upstream countries are planning agricultural irrigation, water abstraction (even for distant cities such as Windhoek) and potential hydropower as their populations rebuild after Angola's long civil war. Any large diversion of the Angolan inflow would directly shrink the flood that makes the Delta. On the water itself, invasive species are a growing concern — the floating fern Salvinia molesta chokes channels, and the introduced Nile tilapia (Oreochromis niloticus) threatens to hybridize with and displace native Oreochromis, contributing to two native tilapias being assessed as Endangered. Climate change overlays it all, with warming raising evaporation and making the flood — the one thing the whole system runs on — more variable from year to year (OKACOM; UNESCO World Heritage; National Geographic).

Sources

Last reviewed 2026-06-06.

How to cite

Aquarist Atlas (2026). The Zambezi & Okavango. Aquarist Atlas. https://www.aquaristatlas.com/water/zambezi-okavango/

Setting & origin

Temperature & flow

Water chemistry & clarity

Habitats

The cichlids

People & pressures

Sources

Cichlids recorded here

Astatotilapia

Chetia

Coptodon

Hemichromis

Oreochromis

Pharyngochromis

Pseudocrenilabrus

Sargochromis

Serranochromis

Thoracochromis

Tilapia