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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.

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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.

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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.

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