Shatt al Arab, a river in southeastern Iraq. It is formed by the junction of the Tigris and Euphrates rivers and flows about 115 miles (185 km) to the head of the Persian Gulf. The river's lower course, which is navigable by oceangoing ships, marks a section of the boundary between Iraq and Iran. Major cities on the Shatt al Arab are Basra, the chief port of Iraq; and Abadan, Iran, one of the world's largest petroleum-refining centers.