Zab (zäb, zăb), name applied to the two principal tributaries of the Tigris River. The Great Zab, 265 mi (426 km) long, rises in SE Turkey and flows generally S through Iraq to the Tigris. The Little Zab, 250 mi (402 km) long, rises in NW Iran and flows SW through Iraq to the Tigris. Both rivers are extensively used for irrigation, flood control on the Tigris, and hydroelectricity. Bakhma Dam on the Great Zab and Dokan Dam on the Little Zab are among the world's highest dams.