Diyala dĭyă´lă, –´ə [key], river, c.275 mi (440 km) long, rising as the Sirvan River in NW Iran and flowing SW through the Zagros Mts. into E Iraq, where it enters the Tigris River S of Baghdad. The Diyala is unnavigable, but its valley is an important trade route between Iran and Iraq. A dam on the lower Diyala provides flood control and irrigates the area NE of Baghdad.
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