Course
The Great Zab rises in Turkey in the mountainous region east of Lake Van at an elevation of approximately 3,000 metres (9,800 ft) amsl and joins the Tigris on its left bank in Iraq. In Turkey, the Great Zab traverses the provinces of Van and Hakkâri, whereas in Iraq it flows through Duhok Governorate and Erbil Governorate. Together with the Tigris, the Great Zab forms the boundary between Erbil Governorate and Ninawa Governorate. In its upper reaches, the Great Zab flows through steep, rocky gorges. The stretch between Amadiya and the Bekhme Gorge – where the Bekhme Dam remains unfinished – has been called the Sapna Valley. Numerous mountain streams and wadis join the Great Zab on its right and left banks. The Great Zab receives most of its waters from the left-bank tributaries; the Rubar-i-Shin, Rukuchuk, Rubar-i-Ruwandiz, Rubat Mawaran and Bastura Chai.
The length of the Great Zab has been variously estimated at 392 kilometres (244 mi) and 473 kilometres (294 mi). Approximately 300 kilometres (190 mi) of the river's course are located within Iraq. The average discharge of the Great Zab is 808 cubic metres (28,500 cu ft) per second, but peak discharges of up to 1,320 cubic metres (47,000 cu ft) per second have been recorded. The average annual discharge is 13.2 cubic kilometres (3.2 cu mi). Because of its torrential nature, Medieval Arab geographers have described the Great Zab – together with the Little Zab – as "demoniacally possessed".
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