northern tributary of the Orange River, South Africa. Rising at Sterkfontein Beacon near Breyten, in Mpumalanga province, it flows 750 miles (1,210 km) southwest to its confluence with the Orange near Douglas; the Vaal's middle section forms most of the Free State's northern provincial boundary. It is basically a plateau river and occupies a shallow bed. Most of the year its flow is minimal, but the winter months can create the muddy torrent for which the Vaal (“Gray-brown”) is named. The river's flow is regulated by the Vaal Dam, 23 miles (37 km) upstream of Vereeniging, where water is diverted into the Hartz Valley irrigation scheme. The river's major tributaries—the Klip, Wilge, Vals, Vet, and Riet rivers—enter on its left bank. The Vaal is fully developed economically, its waters used for the domestic and industrial needs of the Witwatersrand.