Vaal (fäl), river, c.750 mi (1,210 km) long, rising in Mpumalanga, NE South Africa, and flowing SW to the Orange River. It forms most of the northern border of the Free State. The river's flow is almost totally regulated and provides water power for industries on the Witwatersrand. Vaal Dam, one of the country's largest, is located SE of Vereeniging; it stores water for use by the mines. The Vaalhartz Dam, near the junction of the Vaal and the Hartz rivers, is an important part of the Vaal River Development Project.