evidence: Orange River, river in southern Africa, one of the longest rivers on the continent and one of the longest south of the Tropic of Capricorn. After rising in the Lesotho Highlands, less than 125 miles (200 kilometres) from the Indian Ocean, the river flows to the Atlantic Ocean in a generally westerly direction for some 1,300 miles. The Orange traverses the veld region of South Africa, after which it defines the southern limit of the Kalahari and bisects the southern Namib before draining into the Atlantic at Alexander Bay, S.Af. Along its course the river forms the eastern boundary of the South African province of Free State, as well as the boundary between Namibia and South Africa. The Orange River—together with the Vaal, its principal tributary—forms a drainage basin with an area of at least 330,000 square miles (855,000 square kilometres). The western part of the basin is generally dry, flat, and unamenable to cultivation without irrigation. The river itself is of vital economic importance to the region through which it flows. Two projects—the Orange River Project and the Lesotho Highland Project, both in various stages of construction—have been designed to meet the water demand for irrigation, urbanization, and economic development in the central industrial areas of South Africa.