Either of two tributaries of the Tigris River .
The Great Zab rises in the mountains of Kurdistan and flows about 260 mi (420 km) south and southwest through southeastern Turkey and northern Iraq, joining the Tigris below Mosul, Iraq. The Little Zab flows about 230 mi (370 m) through northwestern Iran and northern Iraq before entering the Tigris about 50 mi (80 km) below the Great Zab. The two rivers double the flow of the Tigris during the floods of March and April.