The Great Bend of the Niger River: two separate rivers in the past.
The Great Bend is the most striking feature of the Niger River, in which it makes a 90-degree turn. It is the place were two once separate rivers joined. The changing course of the Niger River is the result of climate change in the Sahara.
Author: Kathelijne Bonne 2020
The Niger is 4100 km long and connects the interior of West Africa with the ocean. But this was not always so. The Niger as we know it came to existence when two separate streams connected to each other. This connection is the last stage in the Niger's long history. It bears witness to a time when the Sahara was green and fertile.
Why did the Sahara dry out and how did these two rivers link up? What was the impact on humanity?
Rivers are anything but static, most rivers undergo change throughout their history. This can have far-reaching consequences on the distribution and migration of living species, including humans.
The original source of this article is my article on the evolution of the Niger, published by the Geological Society of London.
The course of the Niger River
The Great Bend connects the Western and the Eastern Niger. The sources of the Western Niger, the upstream part of the river, lie in Guinea, only 300 kilometers from the Atlantic Ocean and at 800 m above sea level. From there, it flows away from the ocean, across the Sahel, to Timbuktu (Mali). After Timbuktu the Niger turns through the Great Bend. Beyond, the Eastern Niger flows through the Republic of Niger towards Nigeria in the southeast. It reaches the Atlantic Ocean via the Niger Delta.
In Mali, to the west of the Great Bend, a unique ecosystem of lakes, marshes and wetlands lies in the middle of the Sahel. This is the Inner Niger Delta. The Inner Delta is comparable to the Okavango Delta in Botswana.
In the Inner Niger Delta, the Niger branches off into an intricate network of smaller streams that irrigate the surrounding land. There are also many small lakes and ponds, such as Lake Debo. North of Lake Debo, the desert dunes of the Bara Erg begin. An erg is a sea of sand or dune field. The Niger's many small channels flow between the dunes of the Bara-erg. Then, near Timbuktu, the water merges into one wide rectilinear watercourse, this is where the Great Bend begins.
Separate river branch
At the beginning of the 20th century, scientists discovered how the Great Bend formed.
In the past, the western Niger was a separate river. The Great Bend did not exist. The Great Bend did not exist and there was no connection with the Eastern Niger, which ran from near Niamey to Nigeria.
So where did the western Niger flow to?
Salt in the desert
In the past, the western Niger flowed straight into the Azawad region, now part of the Sahara in northeastern Mali. This region wasn't always a desert. The western Niger discharged in large lakes in a green, humid savanna-like environment.
The Sahara was green.
How do we know? We know because there are many salt deposits in Mali. Salt precipitates as waterbodies evaporate. Hence, there is no salt without water.
Also, dried up river channels were discovered, bearing witness to the former connection between the western Niger and the lakes further north [i].
Let's learn more about the Green Sahara.
The ancient savanna consisted of grassy plains, trees, rivers and lakes. Early humans wandered through this area, as did many animal species that we now associate with East Africa, such as gazelles and giraffes. We know this thanks to rock paintings that are many thousands of years old. They can be found everywhere in the Sahara [ii].
The rock art tells a lot about the way of life and the migration routes of early humans.
Out of Africa
Humans originated in Africa. That is widely known and accepted. At some point, they began to spread across the continents.
But which route did they take to leave Africa?
Research on this topic is referred to as the Out-of-Africa theory. It was initially assumed that humans migrated northward, out of Africa, via the so-called Nile corridor, which is the green area seasonally flooded by the Nile. Also Yemen is amongst the possible outward routes. But perhaps humans also used other 'Out of Africa' routes, across the Sahara. And knowing that the Sahara was once green, that doesn't seem implausible [iii].
But why was the Sahara green?
The Sahara as a desert has existed for a very long time, perhaps seven million years [iv]. But that doesn't mean that it was dry all the time.
The Sahara has been subject to strong climate fluctuations. It alternated between phases of Desert Sahara and Green Sahara. During a desert phase, the desert expands far beyond its current boundaries. During a green phase the vegetation expands and sands retreat, until only a few dry, sandy areas remain.
What is the cause of these drastic climate changes?
The cause is astronomical. The movements of the Earth vary slightly every few thousand years. What movements does Earth make? It rotates around its axis; and that axis is slanted with respect to the ecliptic plane. Sometimes the tilt is greater than usual. The orbit of the earth, too, is sometimes more elliptical instead of circular.
Because of these variations, the sunlight on earth is distributed unevenly, through time. These variations, known to scientists as the Milanković parameters, therefore influence the irradiation, and therefore the climate.
These concepts are at the base of the Sahara pump theory, which describes the migration of biota and humans through the Sahara, driven by the climatic variations that cause the vegetation to expand and to retreat.
This is all extremely fascinating, but we got off-track. Let's get back to the main storyline. We learned that the western Niger was a separate river. This river flowed to a savanna-like area in the Sahara. It was green and humid.
What happened next?
Big sand dunes
Eighteen thousand years ago an important Green Sahara phase came to an end and a Desert Phase started [i].
The savannah became drier and drier. An extremely dry period started and desert sands expanded further than ever. This transition coincided with the peak of the last Ice Age that plagued other parts of the world. As you can see, climate events are interconnected globally.
Near Timbuktu, desert sand was blown in by strong trade winds. Large sand dunes piled upon each other. The dunes formed a natural dam, a barrier, which blocked the flow of the western Niger.
The water of the Niger stagnated south of the dune barrier and formed a lake. We will refer to this lake as Lake Bara. We choose the name Bara, because today this region is occupied by the dunes of the Bara Erg [v]. Lake Bara left behind deposits of salt and diatomite. That is hardened mud made up of the shells of diatoms (a kind of algae) [vi]. Without these deposits we would not know that this lake existed.
Then, eight to ten thousand years ago, the very last Green Phase of the Sahara started.
Connection between the two river branches
It rained and rained. The water level in Lake Bara was rising. At a given point, the lake filled completely, and water flowed over at the edge. But the dune barrier was still there. The water could not resume its original course to the northeast. Therefore, it flew over to the southeast, via Gao towards Niamey. This area happened to be the source area of the eastern Niger.
That way, the overflowing waters of Lake Bara connected the western with the eastern Niger.
As such, the Niger became a 4000-kilometer-long river connecting Guinea with Nigeria via the Sahara.
Six thousand years ago the climate in the Sahara started to dry again. It was the onset of the desert phase we are in today [vii]. The modern Sahara formed. Lake Bara dried up entirely and sand dunes formed instead. The Bara Erg now occupied this area. But the lake is not gone entirely. The remnant of Lake Bara is the smaller Lake Debo, at the northern edge of the swamps of the Inner Niger Delta.
Even though the area is extremely dry, there is always water in the Niger and in the Inner Delta, because the river's sources lie in tropical Guinea.
The enormous Niger River has had an important influence on the civilizations of West Africa, supplying freshwater to the Sahel. Thanks to the fertile Inner Delta, some of the earliest cities in sub-Saharan Africa could develop.
In the fifteenth century, Djenné, Timbuktu and Gao flourished under the Shongai Empire [viii]. Timbuktu is located north of the Great Bend and was a center for trade, where desert peoples and nomads exchanged goods with the larger settlements. It was connected to the rest of the known world through the Trans-Sahara trade, a network of caravan routes connecting the Sahel, West Africa and Europe, and also linking up with the Silk Road via Egypt.
Goods including gold, salt, ivory and slaves were transferred in Timbuktu from camel caravans on boats on the Niger. Today only salt is traded in this area.
Salt. Does that ring a bell? Indeed, it is the salt from the lakes that once lay in the Sahara.
As the story of the Niger shows, the changing course of a river system has far-reaching consequences, and is caused by climate change. For millions of years, the western Niger was a separate river, discharging into lakes in the Sahara. Along these watercourses and green areas, peoples and animals could spread across West Africa. And later, when the Sahara was already dry, West Africa's great civilizations developed near the Great Bend.
A blessing and a curse
Today the Niger is a river of extremes, a blessing and a curse. It is the only source of life in the developing countries Mali and Niger in the Sahel.
Downstream, in Nigeria, the Niger flows into the gigantic Niger Delta, one of the largest river deltas in the world. Because of its petroleum, the Niger Delta is a source of unprecedented wealth for a handful of rulers. Above all, unfortunately, exploitation causes environmental degradation and social disruption.
In the beginning of this article we said that this story of the Niger is only the last stage of a much longer history. We'll get back to it in the next article. We will explain why there are whale skeletons under the desert sun.
Go back to all Articles.
This article is taken from my publication on the Niger River:
Bonne K. 2014 Reconstruction of the evolution of the Niger River and implications for sediment supply to the Equatorial Atlantic margin of Africa during the Cretaceous and the Cenozoic. Geological Society of London Special Publications 386.
[i] Jacobberger, P. 1981. Geomorphology of the upper Inland Niger Delta. Journal of Arid Environments, 13, 95-112.
[ii] Dunne, J., Evershed, R., Salque, M. et al. First dairying in green Saharan Africa in the fifth millennium bc . Nature 486, 390-394 (2012) doi:10.1038/nature11186
[iii] Drake N.A., Blench R.M., Armitage S.J. et al. 2011 Ancient watercourses and biogeography of the Sahara explain the peopling of the desert. PNAS, 108(2).
[iv]Zhang Z., Ramstein G., Schuster M., et al. 2014 Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene. Nature, 513, 401-404
[v] Makaske B., de Vries E., Tainter J.A. & McIntosh R.J. 2007 Aeolian and fluviolacustrine landforms and prehistoric human occupation on a tectonically influenced floodplain margin, the Méma, central Mali. Netherlands Journal of Geosciences, Geologie en Mijnbouw, 86(3), 241-256.
[vi] Bridges, E. M. 1990. World Geomorphology. Cambridge University Press, Cambridge.
[vii] Kröpelin S., Verschuren D., Lezine A.-M. et al. 2008 Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320(5877):765-8
[viii] Haywood, J. 2000, Atlas of World History. Metro Books.
Article by Kathelijne Bonne, geologist and soil scientist. Editor of GondwanaTalks.