Tigris (tĪˈgrĭs) [key], river of SW Asia, c.1,150 mi (1,850 km) long, rising in the Taurus Mts., E Turkey, and flowing SE through Iraq to join the Euphrates River, with which it forms the Shatt al Arab. It flows swiftly and receives many tributaries, including the Diyala, originating in the Zagros Mts., and the Great and Little Zab. The lower Tigris is connected to the Euphrates by semipermanent natural channels and by ancient canals. Much of the marshland along the lower Tigris was drained in the early 1990s; restoration began in 2003. Dams across the river divert water for irrigation.
The Tigris is subject to sudden, devastating floods, and the Wadi Ath Tharthar Scheme, Iraq's largest flood-control project, protects Baghdad and vicinity from floods in addition to irrigating c.770,000 acres (311,600 hectares) of land. New large-scale projects in Turkey aimed at increasing irrigated land area there threaten to cause problems in countries downstream that rely on the river's resources. The Tigris is navigable to Baghdad for shallow-draft vessels; above Baghdad, rafts carry much of the trade to Mosul. Its importance as a trade artery has declined with improved road and rail connections. Basra, at the junction of the Tigris and Euphrates, is Iraq's chief port.
In antiquity, some of the great cities of Mesopotamia, including Nineveh, Ctesiphon, and Seleucia, stood on the banks of the Tigris, and the river served as an important transportation route. The Tigris floodplain was cultivated by irrigation from the earliest times; the Sumerians dug a canal from the Tigris to Lagash c.2400 B.C. The Tigris is called the Hiddekil in the Bible.