Tigris, an important river of Turkey in Asia; rises in the mountains of Kurdistan, flows SE. to Diarbekir, E. to Til (where it receives the Bitlis), and hence SE. through a flat and arid country, till, after a course of 1100 m., it unites with the Euphrates to form the Shat-el-Arab, which debouches into the Persian Gulf 90 m. lower; is navigable for 500 m. to Bagdad; on its banks are the ruins of Nineveh, Seleucia, and Ctesiphon.