Zambezi River ecosystem

Category: Inland Aquatic Biomes.

Geographic Location: Africa.

Summary: The fourth-longest river in Africa, the Zambezi is the site of the world-famous Victoria Falls, one of the largest waterfalls in the world.

The source of the Zambezi River is in the furthest-northwest corner of Zambia, where it borders Angola to the west and the Democratic Republic of the Congo to the north. The elevation here is about 4,900 feet (1,500 meters). The river flows south through Angola for about 145 miles (230 kilometers), before crossing the border back into Zambia. After traversing Zambian territory in a southward direction, it then turns east and forms the international boundaries between both Zambia and Namibia and Zambia and Zimbabwe. Entering Mozambique at Luangwa, it flows southeast across Mozambican territory into the Indian Ocean. The Zambezi, at 2,200 miles (3,540 kilometers) in length, drains the entire region of south-central Africa, an area of about 540,000 square miles (1.4 million square kilometers).

Because of their vast ecological differences, the Zambezi River system can be divided into three separate sections: the Upper, Middle, and the Lower Zambezi. Each possesses different geographical features, landscapes, history, and biome characteristics.

Major Segments

The Upper Zambezi contains the source of the Zambezi River, which originates from a marshy wetland region. Near the source is the divide between the Zambezi and Congo River drainages; the two river systems do not connect. After exiting Angola and re-entering Zambia, the river is approximately 1,300 feet (400 meters) wide, and fast-flowing during the rainy season. The beginning of floodplains is marked by the Zambezi’s first major tributaries, the Kabompo River and the Lungue River. As the river travels the remaining stretch through Zambia, it flows over a thick mantle of sand that contributed to the formation of the Barotse Floodplain.

After the floodplain, the Zambezi goes through the Caprivi Swamps along the Botswana border, where it briefly forms the border between four countries (Botswana, Zimbabwe, Namibia, and Zambia). After emerging from the swampy areas, the Chobe River joins the Zambezi and the river goes through the Katambora rapids. As it travels toward the Middle Zambezi, the river is set just over 3,000 feet (900 meters) above sea level on the basalt sheet of the south-central African plateau. The separation of the Upper and Middle Zambezi sections is marked by an area that cuts across the river bed; this forms Victoria Falls.

The Middle Zambezi is commonly defined as the area between Victoria Falls and the Lupata Gorge. This section of the river forms the border between Zambia and Zimbabwe. It is also very popular for tourism, as it contains many sandy beaches and clear waters, and several sections that are international attractions for whitewater rafting and kayaking. On the Zimbabwe side, the area is also home to the Zambezi National Park. The Middle Zambezi has seen the most human modification out of the Zambezi’s three sections. After passing through Victoria Falls, the river cuts through gorges of basalt rock between 650 to 800 feet (200 to 250 meters) high.

The river then emerges from the basaltic plateau and turns eastward to enter one of the largest human-made lakes, Kariba Lake. In 1959, the Kariba Dam was constructed and enabled hydroelectric power to be generated. Today, the dam provides electricity for much of Zambia and Zimbabwe. However, years of erosion have whittled away the base of the dam, leaving it in danger of a catastrophic failure. In 2018, work began to resolve the erosion problem and fix cracks in the dam.

The river is again joined by a major tributary, the Kafue River, and then by the Luangwa River. It then enters Lake Cahora Bassa; this water body was created in 1974 when the Cahora Bassa Dam was constructed. Downstream from here, the Zambezi is fairly shallow in many places in the dry season. The river stretches to a wide range, except in one area that is confined between high hills; this is known as the Lupata Gorge, which marks the division between the Middle and Lower Zambezi.

The Lower Zambezi is the area from the Lupata Gorge through the remainder of the river. Just 200 miles (320 kilometers) from the mouth of the Zambezi, the Lupata Gorge stretches only 650 feet (200 meters) wide and flows on top of a sandy river bed. During the rainy seasons, streams come together and form fast-flowing waters here. Aside from the Lupata Gorge, the Lower Zambezi mainly spans a range of 3–5 miles (5–8 kilometers) in width. As the Zambezi River approaches the Indian Ocean, it splits into several branches to form a wide delta. Before entering the ocean, the Zambezi waters flow through four principal mouths, each obstructed by sandbars: the Milambe, Kongone, Luabo and Timbwe Deltas. Prior to the construction of the Kariba and Cahora Bassa dams, this delta was about half as wide as it is today.

Biome Divisions

Just as the Zambezi River is commonly considered in sections, its basin can also be divided into four main biomes. The river basin flows through the Congolian biome, Zambezian biome, a montane biome, and a coastal biome. Starting at the headwaters, the Congolian biome is associated with northwestern Zambia and northeastern Angola. The Congolian biome is moister and warmer in climate than the rest of the basin. This biome is considered tropical, is high in rainfall, and is without a marked dry season. It is also linked to the Congo Basin.

The Zambezian biome is subtropical and covers the majority of the basin, up to 95 percent. This biome contains woodland, grassland, swamp, and lakes. The climate is marked by a dry season and is primarily based on seasonal changes. Within the Zambezian biome, there can also be a subdivision made between the drier and moister areas.

The montane biome is temperate, with some dry-season precipitation; it is cooler and wetter overall. It is part of the Eastern Arc Mountains; the species in this biome are similar to those in nearby mountains.

Finally, the coastal biome is tropical with a mild, stable climate. The coastal biome encompasses the small part of the Zambezi basin affected by coastal climate influences—mainly the delta area. It is marked by a dry season, but temperatures do not greatly fluctuate. The habitats here include dry forest, woodlands, and grasslands; most species found here are widespread along the east African coast.

Flora and Fauna

The Zambezi hosts a diverse range of plant and animal species. There are estimates of between 6,000 and 7,000 species of flowering plants, more than 200 species of mammals, 700 species of birds, 290 species of reptiles and amphibians, at least 190 species of freshwater fish, 210 species of dragonflies, 1,100 species of butterflies, and 100 species of freshwater mollusks.

The river supports large populations of many animals. Hippopotamus and crocodile are abundant along most of the calm stretches of the river. Monitor lizards are found in many places. There are four Important Bird Areas in the basin, including the Lower Zambezi National Park. Species present in large numbers include heron, pelican, egret, trumpeter hornbill, Meyer’s parrot, narina trogon, and the African fish eagle. Cormorant, openbill stork, spur-winged goose, common pratincole, caspian plover, whiskered tern and African skimmer are found in the wetlands.

Woodlands in the basin also support many large mammals, such as spot-necked otter, buffalo, reedbuck, eland, zebra, giraffe, and elephant. These animal population numbers have declined, however, since the 1970s.

The Zambezi also supports diverse species of fish, some of which are endemic to the river (found nowhere else). Catfish, carp, and eel species are common. The bull shark, sometimes known as the Zambezi Shark, is found around the world. It normally inhabits coastal waters, but has been found far inland in many large rivers including the Zambezi.

Human Impact

Within such a widespread ecoregion, there are several possible threats to the Zambezi River’s ecosystems. The Zambezi provides water for more than 32 million people who live in its watershed. Much of the water used is for agriculture. Poor land management practices, practiced mainly by indigenous farmers of the basin, have contributed excessive sedimentation through extreme soil erosion throughout much of the area. In urban areas, sewage effluent is a major cause of water pollution, as inadequate water treatment facilities allow for the release of untreated sewage into the river. This has resulted in eutrophication of the river water, and has facilitated the spread of diseases such as cholera, typhoid, and dysentery.

In recent years, groups and organizations have been recruited or formed to protect the fairly intact Zambezi River, such as the Nature Conservancy, Ramsar Convention on Wetlands, and the transfrontier Okavango-Zambezi Conservation Park. Land- and water-use policy, industrial development, maintaining migratory routes, and preparing for climate change are all issues being addressed relative to the Zambezi and its watershed.

Bibliography

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Munjoma, Leonissah. “Benefit Sharing in Integrated Water Resources Management.” Zambezi 7, no. 2 (2006).

Purchase, G. K., C. Mateke, and D. Purchase. A Review of the Status and Distribution of Carnivores, and Levels of Human Carnivore Conflict, in the Protected Areas and Surrounds of the Zambezi Basin. Bulawayo: Zambezi Society, 2007.

Serpell, Namwali. "Learning From the Kariba Dam." The New York Times, 22 Jul. 2020, www.nytimes.com/interactive/2020/07/22/magazine/zambia-kariba-dam.html. Accessed 8 Aug. 2022.

Timberlake, J. Biodiversity of the Zambezi Basin Wetlands. Harare, Zimbabwe: International Union for Conservation of Nature, Regional Office for Southern Africa, 1998.

Tweddle, Denis. “Overview of the Zambezi River System: Its History, Fish Fauna, Fisheries, and Conservation.” Aquatic Ecosystem Health & Management 13, no. 3 (2010).

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