/ 19 January 2022

Science’s burning questions about the Okavango River Basin are answered

View Of A Small River In The Gomoti Plains Area, A Community
View of a small river in the Gomoti Plains area, a community run concession, on the edge of the Gomoti river system southeast of the Okavango Delta, Botswana. (Photo by Wolfgang Kaehler/LightRocket via Getty Images)

The Wild Bird Trust is implementing the National Geographic Okavango Wilderness Project (NGOWP), which is championing a new model for conservation. It not only protects one of Southern Africa’s most important water sources, but also empowers its people. 

Water is the lifeline, and this water knows no borders. The Okavango River twists and turns through Angola and Namibia until it arrives in Botswana, where it spreads into one inland wetland called the Okavango Delta, which eventually ends at the Makgadikgadi saltpans where it disappears into the deep Kalahari sands. The delta, and the watersheds and rivers that feed it, comprise the fragile water reservoir for the inhabitants of three countries in Southern Africa. Millions of birds. Thousands of elephants and hippos.

The project’s first podcast series, Guardians of the River, which was written and recorded by Cat Jaffee of House of Pod, recently won a Jackson Wild Award in the podcast category, and Best Narrative Nonfiction Podcast Award at the 2021 Tribeca Film Festival. In the eight-part series, Jaffee unpacks the delicate and intrinsic relationship humans and wildlife have with the river as the team explore the source lakes in Angola’s Highlands all the way to the spreading waters of the Delta. 

She highlights, through many voices, the potential value this scientific research has for the protection of the Okavango Basin when used in combination with the knowledge of the local people. 

Guardians of the River takes a snapshot of the work of the wilderness project, which was completed over six years, starting in May 2015 when the project leader, Steve Boyes launched the Source to Sand Megatransect of the source of the Cuito River. This became the topic of the award-winning National Geographic documentary film Into the Okavango

Since then, many expeditions have been launched and led by the wilderness project’s research director, Rainer von Brandis, to gather data about the region’s biodiversity, water and wildlife migration corridors. As a result, they are beginning to find answers to many of science’s most burning questions in relation to the Okavango River Basin.

How was the Okavango Delta formed?

The three great rivers that are today the Zambezi, the Kwando and the Okavango used to flow together across Botswana, joining the Limpopo valley and flowing east out to sea with the Limpopo River. Over geological time the combined Zambezi/Kwando/Okavango River broke away from the Limpopo and, being surrounded by Kalahari sands, the water accumulated as a great lake over the current day Makgadikgadi Pans and Mababe depressions. Within these pans and depressions vast amounts of salts were deposited, forming Botswana’s salt pans.

After this, at the southwestern most extent of the Rift Valley, tectonic movement first stole the Zambezi River and then the Kwando River away from the Okavango system to flow east down the current day Zambezi Valley. Without the additional flows the great lakes dried up. The faults that captured the Zambezi and Kwando rivers also affected the Okavango. The land to the south of the Gumare fault — the northern end of what is today the Okavango Delta — tilted downwards while the land to the north of the Thamalakane fault —– the southern end of the Okavango — tilted upwards. This tilt resulted in a reduced gradient and water flowing down the Okavango fanned out seeking the easiest route becoming the “alluvial fan” we know today as the Okavango Delta.

How did the Okavango Delta’s islands, channels and floodplains form?

The surface of the Okavango Delta is interwoven with thousands of ever-changing channels that feed water to floodplains and everywhere there are islands of all shapes and sizes.

The Okavango River transports sediment made up of sand and silts; the sands are the heavier sediments and can settle out in the faster flowing channels, organic silts are much finer and only settle in slow flowing waters in the floodplains adjacent to the channels forming vast beds of peat. Both the channels and the floodplains build up with deposited sediments. 

During a drought year the peat beds dry out and burn away with fires, the sand deposited in the channel remains, now the highest point in the landscape. When the water returns, it flows to the low points creating long meandering islands where the channel used to be.

Along comes an unlikely actor on this ever-changing wetland. During droughts termites colonise nutrient rich floodplains. Their mounds are capped with tall “cooling towers” that become a highpoint in the landscape; an attractive vantage for birds and animals, where they perch and deposit seeds in their droppings. 

In the nutrient rich soil of the termite mounds, those seeds grow into trees, shrubs and bushes whose roots aid in trapping more sediment and slowly an island is formed. This process continues over centuries, and in time many islands will be formed, and others will be eroded away.

Sedimentation and termites are responsible for the ever-changing landscape which is so characteristic of the Okavango Delta.

What keeps the Okavango Delta’s waters flowing all year around?

The Okavango Delta is fed by two main rivers, both of which have their headwaters in the Angolan highlands.

To the west is the Cubango River, a fast, cascading, flushing river which comes down off the Angolan Plano-Alto with a relatively steep gradient. It is the Cubango River that provides the “pulses” and the annual inundation of water into the delta. It is fed by hundreds of tributaries across a large area, at the source of each are tiny springs.  The delta has an area of 15 000 km2, but after the annual flooding of the Cubango River it increases in size up to 22 000 km2, bringing new life and rejuvenation during the delta’s winter months.

To the east, the Cuito River is also fed by tiny springs but here they form source lakes; 11 main source lakes feed the greater Cuito River, a meandering, gentle gradient river with steady flow all year round. For a long time, scientists wondered how this was possible, and then the wilderness project team discovered the answer — ancient, stratified peat deposits. The source lakes and many of the streams that flow out of them sit on layers of peat deposited over centuries. Peat is a rare and unique find for this part of Africa; it holds up to 25 times its own weight in water, much like a mountain glacier or a giant sponge. Peat is also a huge carbon sink, it’s potential for carbon capture is one of the questions that is still under research.

The combination of the steady flow from the Cuito and the voluminous, flushing inundation of water from the Cubango feeds the Okavango Delta.

What biodiversity would scientists discover in the Angolan catchment of the river?

The Angolan source lakes have had little to no interference from modern developments, because of landmines, war and the sheer remoteness of the place. An endless sea of Miombo woodland and forest cloaks the catchments of the source lakes and these upland streams. These trees can survive in nutrient-poor sands, where other plants might wither. In this nutritionally limited landscape, survival isn’t only difficult for plants, but also for animals and humans. The biodiversity of the source lakes, upland streams and floodplains is species rich and has highly specialised endemics.

During the repeated surveys, scientists collected 14 141 insect specimens and of these there are 21 new species to science across the genera of caddisflies, mayflies, beetles, damselflies and dragonflies. The surveys have identified at least 31 bat species, including the relatively rare Ruppell’s bat and butterfly bat. As aerial predators, bats are very important in controlling insect populations; and the fruit-eating species are crucial for forest regeneration, because bats disperse seeds.

From the 25 000 specimens collected in Angola so far, 48 species are new to science with a further 81 potentially new species. These discoveries underscore the importance of the upper Okavango catchment in Angola, whose rivers and source lakes sustain the Okavango Delta, one of Earth’s last wildernesses.

To find out more about the project and to listen to Guardians of the River, visit https://www.wildbirdtrust.com/guardians-of-the-river.