Bushtracks’ specialist guide and guest blogger David Bristow explains the geo-science behind the seemingly erratic water flow of Botswana’s Okavango Delta recently named the 1000th UNESCO World Heritage Site.
Ever since the time Archimedes invented the water screw and the Romans built their aqueducts, people have known that open water can flow in only one direction and that is downhill.
So imagine how surprised were explorers Charles Andersson and David Livingstone when, on reaching the confusing waterways of the Okavango wetlands in northeastern Botswana, they found rivers and channels that seeming went this way or that at will. The two explorers were so puzzled by the seemingly erratic, and oftentimes anti-gravitational, flow of the waters of Ngamiland (as this region is known), Livingstone insisted in his famous diary: “A river cannot flow backwards, or uphill!”
And yet that is exactly what they do, backwards at any rate if not actually uphill. The waterways of Ngamiland have posed a hydrological conundrum for centuries, from the time their erratic flow was first noted. Channels like the Khwai which drains the Okavango to the east, the Selinda Spillway which connects the Delta to the lesser Linyanti wetlands and the Savuti Channel that drains Lake Zibadianja, have at times been seen to flow east, or west, sometimes in great flood and at others not at all.
The Savuti Channel, Okavango Delta, Botswana
Over the years theories have abounded with each arrival of the annual flood waters from the Angolan highlands around mid-wintertime (the Okavango is located about dead center in the arid Great Kalahari Basin, the largest sand lens on our planet, a place where it seldom rains, and when it does, it is in short, violent summer thunderstorms). It was only with the advent of modern geo-science that the mechanism driving this enigmatic system was understood.
Beneath the vast layer of sand the granitic bedrock is crisscrossed by a network of hairline cracks – the tail-end of the Great Rift Valley. Every time there is a tremor deep within the bowels of the Earth, it sends vibrations down these hairline cracks. It is usually imperceptible to anything but modern scientific sensing instruments, but sometimes just enough to cause the wetlands and waterways of this pancake-flat country to reconsider their options. Botswana might well be the flattest country on Earth: over nearly 300 kilometers from north to south, the land on which the Delta lies drops a scant 45 meters.
Hippo family in the Okavango Delta, Botswana
When Livingstone first saw the area in 1851/2 all the channels of the Okavango were flowing at full flood. A few decades later the famous hunter FC Selous recorded that the Selinda and Savuti channels were all but dry. In the mid-1990s they flowed strongly, but towards the end of the century they both began to dry and indeed to flow backwards to their sources. Lake Zabidianja, at the fulcrum of these two sinuous waterways, began to dry to the point that large pods of hippos were fossiled alive in the glutinous ooze. By the turn of the millennium the once-large lake was a dry bowl where zebras and wildebeest liked to take dust baths.
Canoeing at Kwando, Okavango Delta, Botswana
When the most violent earthquake in modern times ripped apart an island in Indonesia and sent shock waves around the world around Christmas time in 2004, a ghost tremor of that cataclysm rippled along the east coast of Africa and down the Rift Valley. Those hair-line cracks beneath Ngamiland quivered, the sand rippled and shifted, ever so subtly no-one noticed. But the water the Okavango and Kwando Rivers quivered and fluttered, and in the Chobe, also in the Linyanti wetlands and the Okavango Delta. And it began its slow journey down the Selinda Spillway, into Lake Zabidianja and down the Savuti Channel for the first time in nearly 40 years.
Who knows when the seemingly solid ground beneath Ngamiland will again be disturbed by seismic shifts, or when the channels of the Okavango will stop flowing, or change direction entirely? But at least now we know how and why.