evidence: Shaṭṭ Al-ʿArab, river in southeastern Iraq, formed by the confluence of the Tigris and Euphrates rivers at the town of Al-Qurnah. It flows southeastward for 120 miles (193 km) and passes the Iraqi port of Basra and the Iranian port of Ābādān before emptying into the Persian Gulf. For about the last half of its course the river forms the border between Iraq and Iran; it receives a tributary, the Kārūn River, from the eastern (Iranian) side. Its width increases from about 120 feet (37 metres) at Basra to 0.5 mile (0.8 km) at its mouth. Along the settled banks there are date palm groves, which are naturally irrigated by tidal action. The Kārūn River empties large quantities of silt into the Shaṭṭ Al-ʿArab, necessitating continuous dredging to keep the channel navigable for shallow-draft oceangoing vessels. The present river pattern probably is relatively recent, but its mode of formation is uncertain. The Tigris and Euphrates possibly once flowed to the Persian Gulf by a more westerly channel, while the Shaṭṭ Al-ʿArab’s present lower course may have been part of the Kārūn. In the 1980s the Shaṭṭ Al-ʿArab was the scene of prolonged and intense fighting between Iraq and Iran; the former had invaded the latter in 1980 after asserting Iraqi sovereignty over both banks of the river (see Iran-Iraq War). The exact demarcation of the boundary remained unresolved after the war.