Orange River, chief river of S Africa, c.1,300 mi (2,090 km) long, rising in the Maluti Mts., N Lesotho. It flows SW through Lesotho, then meanders northwest and west through central South Africa, forming the SW boundary of Free State and part of the South Africa–Namibia line before entering the Atlantic Ocean at Oranjemund. The Vaal River is its chief tributary. The lower Orange River flows through the southern part of the Kalahari and Namib deserts; in very dry years it does not reach the sea. At the mouth of the river are rich alluvial diamond beds. Shoals, falls (Aughrabies Falls is 400 ft/122 m high), irregular flow, and a sandbar at its mouth limit navigation, but the river is used extensively for irrigation. The South African Orange River Project, which includes the Gariep and Vanderkloof dams, provides water for irrigation, hydroelectric power generation, and municipal water supplies. Tunnels divert water to the Great Fish and Sundays rivers.