|
Contrary to popular belief, the Orange River was not named after the reddish
orange colour of its silt-laden water. It was in fact named in 1779 by Colonel
Robert Gordon, the commander of the garrison of the Dutch East India Company
(Cape Town) during a reconnaissance into the interior, in honour of the Dutch
House of Orange.
Click on image for full size picture
The Orange River basin is the largest river basin in South Africa with a total
catchment area in the order of 1 000 000 km2 of which almost
600 000 km2 is inside the Republic with the remainder in Lesotho,
Botswana and Namibia. The effective catchment area is difficult to determine
since it includes many pan areas and also several large tributaries which rarely
contribute to flows in the main river channel.
The Orange River, (called the
Senqu River in Lesotho), originates high in the Lesotho Highlands some
3 300 m above sea level where the average annual precipitation can exceed
1 800 mm, with a corresponding average annual potential evaporation of 1 100 mm.
The river stretches 2 300 km from the source to
Alexander Bay where the
average annual precipitation drops to below 50 mm while the average annual
potential evaporation rises to over 3 000 mm.
According to various sources the average natural run-off from the total basin is
more than 12 000 million m3/a. This represents the average river flow
that would occur if there were no developments of any nature in the catchment.
This value can, however, be very misleading since the basin is now heavily
developed with the result that the current average annual run-off reaching the
river mouth at Alexander Bay is less than half of the natural run-off.
There are now three main storage reservoirs on the Orange River, namely
Gariep Dam and
Vanderkloof Dam on the Orange
River inside South Africa and the recently completed
Katse dam in Lesotho on the
Senqu River. The Gariep Dam forms the largest reservoir in South Africa with a
capacity in excess of 5 000 million m3 while Vanderkloof Dam forms
the second largest reservoir with a storage of over 3 200 million m3.
Although the storage of the Katse reservoir is lower at a modest 1 950 million m3,
it is the highest dam in the Southern Hemisphere with a height of approximately
185 m above foundation.
The Vanderkloof Dam is currently the last main storage structure on the Orange
River and effectively controls the flow of water along the 1 400 km stretch of
river between the dam and Alexander Bay on the Atlantic Ocean.
The banks of the Orange River downstream of Vanderkloof Dam are heavily
developed in many areas, principally for irrigation purposes. Both the Gariep
and Vanderkloof dams are used to regulate the river flow for irrigation as well
as to produce hydro-electricity during peak demand periods. Very little Orange
River water is used for domestic or industrial purposes with the exception of
that used in the Vaal River basin.
From a national viewpoint, the Orange River basin is by far the most important
river basin in South Africa and includes the
Vaal River basin which is the
largest and most important tributary of the Orange River. The Vaal River in turn
supplies water to the industrial heartland of southern Africa, including the
Greater Pretoria and Johannesburg areas. The industrial areas supported from the
Vaal River produce more than 50% of South Africa's wealth as well as more than
80% of the country's electricity requirements - more than 50% of all the
electricity generated in Africa. From the Vaal River water is also supplied to
some of the largest Gold and Platinum mines in the world as well as many of the
world's largest coal reserves. No less than six of the nine provincial regions
in South Africa are affected by the Orange River basin to some degree and some
of the largest and most ambitious water projects to be undertaken in Africa are
situated in the Orange River basin.
Click on image for full size picture
As a result of various catchment developments particularly in the Gauteng area,
flows from the Vaal River rarely make a significant contribution to the flow in
the Orange River except during flood events when the inflows from the Vaal River
can be large. Under normal circumstances the Vaal River is managed in such a way
as to avoid any spillage into the Orange River downstream of Douglas. As a
result of this operating rule, the silt laden Orange River water can often be
seen backing up into the green coloured water of the Vaal River.
Click on image for full size picture
|