Crater River

THE AMAZING VAAL

Ancient

You wouldn’t guess – if researchers didn’t tell you – but the Vaal River is one of our planet’s oldest rivers. It was born atop the Karoo landscape that covered much of South Africa from about 280 million years ago. The Vaal is also unique in being the only major river that crosses an impact crater – and it is the largest known crater, at that. Furthermore, the river and its predecessors in the basin, including glaciers, eroded this landscape, exposing its deep structure – and gold.

Thanks to erosion we can see into the heart of a complex crater, observing its formations far below the original crater surface. Those formations include the Witwatersrand rock strata which yielded the greatest gold fields in the history of the world.

Status

The Vaal was hardly mentioned in the 2005 declaration by Unesco that designated the impact landscape as a World Heritage Site. The river has played an integral role in the story of the Dome. It should be specially designated as an important feature in its own right.

Gold

Click to enlargeThe Vaal today drains much of the area of the original Witwatersrand basin, site of a vast inland lake where the gold was deposited nearly three billion years ago. Rivers carrying gold in powder form (“placer gold”) dropped their precious load along with pebbles to form the famous Wits ore conglomerate. Known as “banket”, the name was given to these beds from their resemblance to a pastry, known in Dutch as banket, resembling a nutty almond hard-bake.

Wilderness

Where we are based in the heart of the Vredefort Dome the river is an enchanting piece of wilderness, a habitat for otters, Fish Eagles, porcupines, Nile Monitor Lizards (leguaans), and many water birds. Dozens of channels wind between wooded islands. To canoe among these lush islands or raft down the feisty whitewater rapids is an experience unlike any other on this 1120km long river.

Click to enlarge

So close to Johannesburg, and yet far away in the public imagination (which regards the Vaal as dull), I often think the river here reminds me of Malawi. There, rivers pour off the plateau into the great Rift Valley Lake. Here, as the Vaal descends through the hills ringing the crater, it seems almost subtropical with dense vegetation and abundant bird-, animal- and fish-life.

We have the original meteorite blast, two  billion years ago, to thank for this rich and varied habitat.

Islands

Click for enlargementThe gigantic ball of fire that plunged into the Earth’s crust, just near where the town of Vredefort is today, melted the underlying rocks known as Basement Granite. The boiling mass, rising from as much as 70km deep down, rushed to the surface making a temporary upheaval dome  that quickly collapsed. As the granite cooled it cracked like cold porridge in a pot. The cracks remained in the structure, and subsequently, over two thousand million years, these cracks were exploited by moving ice and water to make the islands and channels we see today.

The time span is almost unimaginable. Aeons have gone buy with continents forming, splitting, drifting and crashing together. Through all of this the central interior of South Africa has remained relatively stable and untouched, preserving the crater features and allowing erosion to do its work.

Rivers

South Africa has had its cake and eaten it. Unless new veins of gold are found deep underground, we have already mined the major deposits out. Think about it though. It was rivers that carried the gold into the lake, and it’s been rivers (and ice caps) that have revealed the ancient buried Dome and its surrounding, fabulously rich, Wits system to us. It remains the Vaal that brings water to the industrial highveld and will be the key to future development.

On our river trips we get out of our boats to look at examples of the plentiful melt-rock, cast in puzzling patterns in the water-smoothed granite. These veins of false volcanic glass are a fingerprint of meteorite impacts. We can climb rugged koppies where boulders teeter seemingly ready to crash down into the river at any moment.

The Dome granites are steadily being leveled by water, weather and wind. There is no gold in the granite but gold is found in small quantities in the Dome Bergland. That’s the semicircle of upturned Wits strata that looms on the horizon as we paddle downriver.

Superposed

The modern river we know as the Vaal (“murky” or cloudy in Afrikaans) has the Sotho name Lekoa (Zulu: iLekwe), which apparently refers to the rather flat, featureless landscape through which the river flows during most of its course. But in the Dome structure there are mountains – part of the Witwatersrand. They were cast upwards and backwards,  leaving the strata standing like dinner plates in a rack. Here the Vaal forges its way through the mountains to cross the inner Dome and then make its way out, back into the rolling veld.

How the river  managed this is a remarkable story in itself. The river was born on the surface of the great Karoo plain, laid down by an inland sea from about 280 million year ago. This was only 10percent the age of the 2 billion year old Dome and its surrounding crater. But because the Karoo came later it covered the Dome features.

The river cut its down from above onto the much older, buried Dome, clearing much of the Karoo Supergroup but not all of it.  Large parts of the region still have the Karoo overlay and it is likely that some features of the crater are still hidden underneath. The Vaal is a “superposed” waterway that has sheared its way through the fairly soft Karoo rocks (sandstone, shale, coal) and in so doing exposed the Dome structure below.

Problems

Most South Africans regard the Vaal as a polluted industrial ditch but that is simply not true here. There are several reasons why organic pollutants from cities, factories and mines don’t often reach us. The Vaal barrage holds back a lot of messy gunk on the surface as colder water is released from below. Many reedbanks clean the water as do the rapids which oxygenate it, allowing bacteria to break down the organic waste.

Suspended solids however remain: chemicals and metals that make the water unfit for human or animal consumption. There are plastics, bottles and bits of wire tangled in the weeping willows, especially after floods. On the whole the river looks and smells clean but it is steadily losing its ability to dilute and decompose the waste we dump in it.

I often tell vistors that if you live near a river it will kindly bring to you all the junk our civilization produces, free of charge. We’ve had several floods at our place, Otters Haunt near Parys, and always try to do a cleanup of the riverbanks afterwards. We don’t go rafting in very high water or when there’s a surge of polluted runoff from farms or sewage works.

Future

The Vaal’s fate rests with the people of South Africa who need to understand its unique contribution to our commonwealth. It carries our vital water supply; but more than that it is the lifeblood of tourism and an absolutely key part of the Vredefort Dome story. Unless it is conserved, the poor state of the river could contribute to the deregistration of the Unesco Vredefort Dome World Heritage Site. There is still no finality to the government’s gazette plan to legislate protection for the Site.

Neglect will tear away the few existing safeguards for the river and its environment.

In 2010 a major flood freed eight large crocodiles from an enclosure upriver of us. Two were hunted and shot; the others disappeared but were not likely to survive the cold winters on the Vaal. Poor things. Their fate symbolized the fragility of conservation efforts.