Tigris RiverRiver 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. 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.