It is formed by the confluence of the East Fork Vermillion River and West Fork Vermillion River. The East Fork, approximately 55 mi (90 km) long, rises in Lake Whitewood in Kingsbury County on the Coteau des Prairies. The West Fork, approximately 50 mi (80 km) long, rises in Miner County. Both forks flow south, roughly parallel, joining east of Parker. The combined river flows south and joins the Missouri 5 mi (8 km) south of Vermillion.