At the source of the Orange River the rain fall is approximately 2000mm per annum and decreases as the river flows westward. At its mouth the rainfall is less than 50 mm per annum. Evaporation, on the other hand, increases in a westerly direction.
From Lesotho's border to below the VanderKloof Dam the river bed is deeply incised. Further downstream the land is low-lying with large areas under irrigation.
The Gariep Dam, near Colesberg, is the main storage structure within the Orange River. From here the water is supplied in two directions, namely westward along the Orange River (via hydro-electric power generators) to the Vanderkloof Dam, and southward through the Orange-Fish Tunnel to the Eastern Cape.
Irrigation, that turned thousands of hectares of arid land into highly productive agricultural land, was made possible by the construction of the Vanderkloof Dam below the Gariep Dam. Old established irrigation schemes such as those to be found at Buchuberg, Upington, Kakamas and Vioolsdrif have also benefitted because of the regulation of river flows.
Eskom operates hydro-electric power-stations at both the Gariep and the Vanderkloof Dams. The hydro-electric power station at the Vanderkloof Dam was the first power-generation station in South Africa situated entirely underground.
Orange River Facts
* The total catchment of the Orange River (including the Vaal) extends over 973 000 square km, i.e. about 77% of the land area of South Africa. Approx 366 000 square km (38%) however is situated outside the country in Lesotho, Botswana and Namibia.
* The river is 2,090 km long
* The Vaal River is its chief tributary.
* The lower Orange River flows through Namaqualand and the Ai-Ais Richtersveld National Park and in very dry years it does not reach the sea.
* At the mouth of the river are rich alluvial diamond beds.