The Vaal River is the largest tributary of the Orange River in South Africa. The river has its source near Breyten in Mpumalanga province, east of Johannesburg and about 30 kilometres (19 mi) north of Ermelo and only about 240 kilometres (150 mi) from the Indian Ocean. It then flows westwards to its confluence with the Orange River southwest of Kimberley in the Northern Cape. It is 1,458 kilometres (906 mi) long, and forms the border between Mpumalanga, Gauteng and North West Province on its north bank, and the Free State on its south.