We like to think of the Delta as a living organism, and like a living organism it grows and shrinks and changes shape. Channels which have flowed for years become gradually blocked with sand and plants and as the waters rise they suddenly rush off down a hippo path and a new channel is formed. And sometimes as a result of this areas that have been dryer for decades suddenly become the wet and full of life. This dynamic nature is best displayed by the mystical Lake Ngami, which when first measured by David Livingstone was 80km long and 20km wide and then for decades during the 20th century was totally dry, right up until the 1990s when suddenly the waters from the swamp began flowing back towards the lake.
Continuing with the analogy of comparing the Delta to a living organism, If you think of the channels in the delta as the arteries of this gigantic living organism, then its heart is 1 250 km away in the Angola highlands. This distant heart beats but once a year when Angola receives three times the amount of rainfall that Botswana does. Sending a giant pulse of water coursing across Africa which eventually drains its lifeblood into the waiting arteries of the Okavango. This is referred to as the flood but unlike in the rest of the world where a flood has negative connotations, in the Okavango the flood is always welcome!
Although the summer rains fall in Angola in January, they take a whole month to travel the first 1 000 km of the Okavango River. And then they take a further four months to filter through the plants and numerous channels of the final 250 kms of the Delta. As a result the flood is at its biggest sometime between June and August, during Botswana’s dry winter months. And the delta swells to three times its permanent size, attracting animals from miles around and creating one of Africa’s greatest concentrations of wildlife.
At its widest point in a big flood year the seasonal swamp stretches to 150 km across from east to west. And one of the factors that leads to the ever changing nature of the Delta is the flatness of the area. To give you an idea of how flat it is, if you were to take a cross section of the Delta at its widest point, along this cross section you would find that the height variation from the mean over that 150km is less than 2m. So a little sand deposition can cause major changes.
Where The Water Goes
Every year 90 million litres of water flow into the delta and although one would think that the water was simply swallowed up by the thirsty sands of the desert, an incredible 60% of this water leaves into the atmosphere via transpiration, in other words via the leaves of the plants rather than via evaporation. Amazingly only 2% of the water soaks into the desert and about the same amount flows out at the southern end of the delta. Little wonder then that Aurel Schultz, one of the first Europeans to clap eyes on the swamps, asked “Where all this water goes is a mystery?”
With all of that water leaving the Delta into the atmosphere the Delta is unable to flush itself of the minerals carried in by the river and the water should become salty and uninhabitable like the Dead Sea. But remarkably it hasn’t and there are two reasons for this. Firstly, the water itself has a very low salt content, and secondly as so much of the water is leaving via the plants rather than evaporation the salt from that water collects around the roots of the plants rather than being left behind in the water.
The catchment area of the Okavango is on Kalahari sands and for the people who live along the river and around the Delta this is a mixed blessing. The drawback is that the farmers don’t get a good silt load enriching their lands during flooding since there are very few nutrients in the waters of the flood. In fact your average urban tap water contains three times as many nutrients as the waters of the Delta! But the upside is that because the sands are so barren the Delta has not ended up as a lifeless salt pan, but rather as the host of a rich diversity of flora and fauna. And in turn those plants keep transpiring and inadvertently taking the salts out of the water and thus saving themselves and the Delta from a salty end.
If you are ever fortunate enough to fly into the Delta you will see thousands of little islands, many of which have bare white patches in their centre. These islands grow each year because of the salts that are deposited on them as the water is drawn up by the trees on the island. As the water enters the roots it leaves some of the salts behind, these salts concentrate in the sands of the island, causing the islands to grow a little each day. It is for this same reason that the islands have bare, white patches in their centres. Those parts of the island have become too salty to support plants, aside from the odd salt resistant palm tree. The sand near the edges of the islands has yet to become that salty and the trees and grasses can still grow there.
Approximately 70% of the islands in the delta began as termite mounds. Termite mounds are the soil homes that termites or flying ants, as they are more commonly known, construct. Some termites build a mound of earth and then a tree takes root. And from those small beginnings these islands have grown.
The plants of the Delta play an extremely important roll provide cohesion for the sand. The banks or levees of a river normally have a high mud content and this combines with the sand in the river’s load to continuously build up the river banks. Here in the Delta, because of the clean waters of the Okavango, there is almost no mud and the river’s load is comprised almost entirely of sand. It is the plants that capture the sand, acting as the glue and making up for the lack of mud and in the process creating further islands on which more plants can take root.
This process is important in the formation of the other type of islands that you see in the swamps, linear islands. They are long and thin and often curved like a gently meandering river. The reason for that is that they are actually the natural banks of old river channels. Which over time have become blocked up by plant growth and sand deposition, resulting in the river changing course and the old river levees becoming islands. Due to the flatness of the Delta, and the large tonnage of sand flowing in to it from the Okavango River, the floor of the delta is slowly but constantly rising. And were channels are today islands will be tomorrow and new channels may wash away these existing islands.
These changes can be rather dramatic for those living off the river. And a modern drama is about to unfold as a channel from which a village in the northern part of the Delta gets its water and food is about to silt up. And the water will soon follow a different course into the Delta. This doesn’t sound too dramatic until you realise that the villagers will suddenly find themselves walking 12km to get to the water from which they derive their livelihood.
The largest Island in the Delta is Chief’s Island and it wasn’t formed by either of these processes but rather by a fault line which uplifted this part of the Delta. Historically the island was reserved as an exclusive hunting area for the chief, hence its modern name. Chief’s Island is over 70km long and 15km wide and provides the core area for much of the Delta’s resident wildlife, especially as the waters rise and dry land shrinks. So here we have an irony common in many parts of the world, the hunting grounds of yesterday are the core conservation areas of today.
There are an estimated 200 000 large mammals in and around the Okavango Delta. Many of these animals live in the Delta but the majority move in and out. They leave with the summer rains to find renewed fields of grass to graze on and trees to browse. And then as winter approaches and the countryside dries up they make their way back to swamps. This leads to some of the most incredible sightings as large numbers of prey and predators get squashed into a smaller and smaller area. And certain areas of the Delta provide some of the best predator action that you can see anywhere in the world.
The most populous large mammal in the Delta is the lechwe. There are more than 60 000 lechwe and perhaps you will be lucky enough to see or hear a herd of these swamp dwelling antelope dashing through the shallow waters of the swamps. They are a little larger than impala and like the impala only the males have horns. The lechwe have adapted to live in marshy areas and occur throughout these swamps and those in Zambia. They live off aquatic plants and their elongated hooves and a water repellent substance on their legs enable them to move rapidly through knee deep water. And like the better known Waterbuck they also take to water when threatened by predators.
Aside from lechwe you can also see large herds of buffalo and elephants in the Delta, both of which total about 30 000. And together with the other animals, birds, plants and the beautiful environment, they make the Okavango Delta a truly remarkable destination.
One of the most common questions is what about global warming. The ecosystem seems so fragile that the slightest change might take it away, so surely global warming will make the swamps disappear?
The answer is we don’t know. Three different investigations have returned three different findings. But evidence suggests an 80 year weather cycle and we are about to enter a wet phase which suggests the swamps won’t be drying up soon. However one thing we do know is that things will change, the swamps are only 10 000 years old and they will disappear. Hopefully they will move as evidence suggests the existence of other, prehistoric inland deltas in the area. But for now just feel privileged that you lived at the same time as the Okavango Delta.