The Orange River Gariep River, Groote River or Senqu River is the longest river in South Africa. It rises in the Drakensberg mountains in Lesotho, flowing westwards through South Africa to the Atlantic Ocean. The river forms part of the international borders between South Africa and Namibia and between South Africa and Lesotho, as well as several provincial borders within South Africa. Although the river does not pass through any major cities, it plays an important role in the South African economy by providing water for irrigation, as well as hydroelectric power. The river was named by Robert Jacob Gordon after the Dutch Royal House.
The Orange rises in the Drakensberg mountains along the border between South Africa and Lesotho, about 193 km (120 mi) west of the Indian Ocean and at an altitude of over 3,000 m. While in Lesotho, the river is known as the Senqu and parts of it freeze in winter, because of the high altitude there. The river then runs westwards through South Africa, forming the south-western boundary of the Free State province. In this section the river flows into first the Gariep Dam (the largest in the country), and on to the Vanderkloof Dam. From the border of Lesotho to below the Van der Kloof Dam the river bed is deeply incised. Further downstream the land is flatter, and the river is used extensively for irrigation.