river in northeastern Northern Territory, Australia, rising about 45 miles (70 km) south of Anthony Lagoon, along the scarp that marks the northern edge of the Barkly Tableland, and flowing northwest for 150 miles (240 km) across rugged country to Port McArthur on the Gulf of Carpentaria. Swamp and jungle border its lower course, which is navigable by barge for 40 miles (64 km). The river, fed by Tooganginie Creek and the Kilgour and Clyde rivers, often floods in the summer. It was visited in 1845 by the explorer Ludwig Leichhardt (Leichhardt, Ludwig), who named it after James and William McArthur (Macarthur), sheep ranchers of Camden, N.S.W.