Rising 3300m above sea level in the steep Maloti Mountains of eastern Lesotho, and flowing for some 2300km though an increasingly arid landscape until it reaches the Atlantic Ocean, the Orange River (or the Senqu, as it is known in Lesotho) is one of the largest river basins in the world. In South Africa itself, the Vaal River is considered to be the most important tributary, providing crucial water resources to the urban industrialized conglomeration of Johannesburg and Pretoria.