The Zambezi River has recently been declared the oldest river in the world, celebrating its 280 millionth birthday, so was flowing 30 million years before the Triassic, when the first dinosaurs appeared. How can we say that?
A small caveat…
Geology is one of the most fascinating subjects in the world. To a layman it can seem like an incredibly complicated subject — and it is! — but there is much of value in understanding some basics relating to the stone and dirt beneath our feet.
Understanding the evolution and development of the Zambezi river system is deeply complicated and new research constantly changes what we know.
What is presented here is an amalgamation of the latest findings, simplified with as little of the inevitable jargon as possible.
Tell us a little about the Zambezi.
Today it is Africa’s fourth-largest river and remains one of the finest and least spoilt rivers in the world. Its source rises from a tiny spring in north-western Zambia and flows for 2,700 kms into the Indian Ocean at its delta on the Mozambique coast.
Its basin, if you include all the river’s tributaries, covers most of central and southern Africa; an area of some 1.3 million square kilometres — larger than the Sahara Desert.
Eight countries in the region are directly linked into this vast river system: Zambia, Angola, Namibia, Botswana, Zimbabwe, Malawi, Tanzania and Mozambique.
Within its catchment, the geological and palaeontological heritage of this part of the planet is almost without comparison.
Current biodiversity — the species of birds, plants, animals, insects, fish, birds, etc — within this area is ancient and deserves much more protection, research and appreciation than is currently the case.
How old is the river?
Newly published research has dated the beginnings of the river to 280 million years ago!
What is particularly fascinating about this latest research is geological study now establishes the ancient Proto-Zambezi (i.e. the river in its earliest phase) as the Earth’s oldest-known river and, perhaps confusingly to us today, that this ancient Proto-Zambezi ran in exactly the opposite direction, west across Africa into the Karoo, compared to what it does today, flowing east into the Indian Ocean.
The Proto-Zambezi was formed as Gondwana’s ice sheets begin to melt as the super-continent drifted northwards.
Water flowing downhill formed part of the river we would now call the Middle Zambezi and the Luangwa. The river flowed west at this time because of the huge mountains — bigger than the Himalayas — in the centre-east portion of the super-continent (see Figure A).
What’s the evidence for this?
There is geological, hydrological, landscape formation studies, as well as a study of the distribution of various lifeforms, especially fish, living in the river (see below). One can date the spread of such species via cutting edge genetic studies.
The Proto-Zambezi river originated in the drainage system that was established as vast Dwyka ice-sheets were melting and becoming smaller across the super-continent of Gondwana (see Figure A).
Gondwana was formed when the cratons that now form the continents of Africa, South America, Antarctica, India and Australia were welded together thanks to plate tectonics.
As Figure B shows, at this time the immense highlands of the Trans-Gondwana mountains directed the westerly flow of glacial meltwaters into the interior of Gondwana.
With its catchment entirely contained within Gondwana, this massive river sustained a vast, inland sea centred on modern‑day Botswana.
So how did the geology affect the rivers?
This geological record of the ancestral Proto‑Zambezi follows much of the same route as the modern river.
In particular, the evidence reveals that precursors of the modern Luangwa and Middle Zambezi already existed in the early Permian, some 300 million years ago.
This geological record in the Luangwa and Middle Zambezi valleys has survived despite the complete reversal of the Zambezi’s flow caused by Gondwana’s breakup, which forged the African continent.
Around 200 million years ago, for a period of 100 million years, a flowing mass of lava began to emerge from within the earth — the Stormberg lavas.
Fluid enough to flow over the land and fill hollows, troughs and valleys, it left behind an almost horizontal aspect, trapping sandstones beneath.
Eventually an area of at least 2,000,000 square kilometres were covered by these various lava sheets to a depth of perhaps nearly 1000 metres deep at Victoria Falls and up to 9000 metres elsewhere!
This basalt was subjected to great stress due to cooling and regional tectonic activity.
This stress caused deep fracturing and lines of weakness, and it is these that the growing Zambezi river would come to exploit in its course via river capture.
Explain river capture.
When a river is extending its channel upstream by headward erosion ( the lengthening of a river’s course by erosion backwards from its source), it may come into contact with the headwaters of a river which is less vigorous.
The headwaters from the minor river may be diverted into the more rapidly eroding channel. This is more or less how the Zambezi has grown into the massive river we know today.
At one time, most of the tributaries of the Zambezi river flowed south into the Limpopo River, but have all been captured.
As Gondwana broke up and as the continents drifted, river by river, flow in the Proto‑Zambezi’s catchment was redirected eastwards into the Indian Ocean, away from the previous lake inland where Botswana lies today.
The Zambezi captured rivers arising to the north – the Kafue, then the upper Zambezi and lower Kwando.
This capture resulted in the loss of water to the palaeo-lake Makgadikgadi and it shrank in size.
And the evidence from fish?
Something quite fun to consider is the fact that we can also trace the changing river systems in this area thanks to the spread of a rather important creature to Zimbabwe’s tourism industry.
The first known ancestors of the tigerfishes, Hydrocynus, are known in North Africa dated to between 35-21 million years ago.
The evidence from DNA studies reveals our local tigerfish, the Zambezi tigerfish, H. vittatus evolved within the confines of the Zambezi River where it became isolated from neighbouring populations in central Africa within the past 2-3 million years.
The species is believed to have originated in the Congo Basin; and thereafter it dispersed through south- central Africa southwards as far as southern Mozambique, and Mpumalanga and Kwa-Zulu-Natal in South Africa. T
he Kasai, a north-flowing Congo tributary, captured an easterly flowing headwater of the Upper Zambezi. Thereafter tigerfish dispersed far and wide south of the Southern Equatorial Divide.
It only colonised the Middle Zambezi below the Batoka Gorges within the past 200,000 years.
What next?
The next river to be captured is the Okavango which will see the eventual end of the magnificent Delta.
Once the Okavango River is captured an entire drainage network that once formed part of the Limpopo Rover system will have shifted to the Zambezi.
Geologically recent captures of the Chobe, Kafue and Upper Zambezi testify to the relative youth of the (modern) river’s shape.
The Great Equatorial Divide on Africa’s high plateaux comprises the Zambezi’s modern watershed, and its present position testifies to repeated re-shuffles of links between long lived rivers. To summarise, scientists now estimate the modern Zambezi basin to be barely 250 thousand years old.
This article is based on the work published by: Cotterill, F.P.D., A.E. Moore & R.M. Key. 2016. On the Antiquity of the Zambezi River, its Rifts, and its Wilderness. African Fisherman 172, 31-35.
