Karun (kärōnˈ) [key], river, c.450 mi (720 km) long, rising in the Zagros Mts., 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. 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.