Karun river, c.515 (829 km) long, rising in the Zagros Mountains, W Iran, and flowing S to the Shatt al Arab on the Iraqi border. The Karun is navigable to Ahvaz for shallow draft vessels; rapids prevent further upstream passage except during high water in April and May. The river was opened to foreign trade in 1888; but since the construction of a rail line during World War II between the river port of Khorramshahr, Ahvaz, and the main Iranian system, this route has lost importance.Karun has important tributaries, among them are the Dez River, with the conflux at Shushtar, and Kerkeh river coming from the north. At Shushtar there is a dam designed to irrigate an area of 500 sq mi (1,295 sq km); it is surmounted by a magnificent bridge (no longer in use), probably built in the 3d cent. for Shapur I of Persia by captured Roman soldiers.