South Africa's longest and major river.Rising in the Drakensberg in Lesotho (where it is known as the Senqu) it flows westward for 2200km, where it finally reaches the Atlantic Ocean at Alexander Bay.At the source of the Orange River in Lesotho the rainfall is 2000mm per annum. It decreases westwards so that at its mouth, rainfall is less than 50 mm per annum. Evaporation also increases in a westerly direction.The total catchment area of the Orange River (including that of a major tributary, the Vaal) is 973 000 km2 (which is 77% of the land area of the entire South Africa: 1 268 535 km2). Approximately 366 000 km2 (38%) are however situated outside the country in Lesotho, Botswana and Namibia.The Orange feeds the Gariep dam, which at 6 billion cubic metres, is the country's largest reservoir. This dam, near Colesberg, is the main storage structure on the Orange River. From there 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.