Upper Zab

The Great Zab or Upper Zab (; or ; ; ) is an approximately long river flowing through Turkey and Iraq. It rises in Turkey near Lake Van and joins the Tigris in Iraq south of Mosul. During its course, the river collects water from many tributaries and the drainage basin of the Great Zab covers approximately . The river and its tributaries are primarily fed by rainfall and snowmelt – as a result of which discharge fluctuates highly throughout the year. At least six dams have been planned on the Great Zab and its tributaries, but construction of only one, the Bekhme Dam, has commenced but was halted after the Gulf War.

The Zagros Mountains have been occupied since at least the Lower Palaeolithic, and Neanderthal occupation of the Great Zab basin has been testified at the archaeological site of Shanidar Cave. Historical records for the region are available from the end of the third millennium BCE onward. In the Neo-Assyrian period, the Great Zab provided water for irrigation for the lands around the capital city of Nimrud. The Battle of the Zab – which ended the Umayyad Caliphate – took place near a tributary of the Great Zab, and the valleys of the river provided shelter for refugees from the Mongol conquest of Iraq. During the 19th and 20th centuries, the Great Zab basin saw frequent uprisings of local Kurdish tribes striving for autonomy.

Course

The Great Zab rises in Turkey in the mountainous region east of Lake Van at an elevation of approximately 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, both part of the Kurdistan Region. 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 and will have a large portion of it inundated with water if the project is completed. 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 and . Approximately of the river's course is located within Iraq. The average discharge of the Great Zab is per second, but peak discharges of up to per second have been recorded. The average annual discharge is . Because of its torrential nature, Medieval Arab geographers have described the Great Zab – together with the Little Zab – as "demoniacally possessed".

Watershed

Estimates of the drainage basin of the Great Zab vary widely – from a low to a high figure of . Approximately 62 percent of the basin is located in Iraq; the remainder is in Turkey. To the south, the Great Zab basin borders on that of the Little Zab while on the east it adjoins the Tigris basin. The Zagros consists of parallel limestone folds rising to elevations of over amsl. The valleys – including that of the Great Zab – and the south-western foothill zone are filled with gravel, conglomerate, and sandstone; the result of water erosion. The Amadiya valley within the Great Zab drainage basin is the third-largest valley in the Iraqi Zagros, after the Shahrazor and the Ranya Plain.

The Great Zab rises in the highlands of the Zagros Mountains, where a climate with cold winter and annual precipitation in excess of prevails. From there, the river flows into the foothill zone of the Zagros, where rainfall drops to less than per year at the confluence with the Tigris. Average summer temperature in the foothill zone are generally higher in the foothill zone than in the mountains. The high Zagros is characterized by three different biomes: the area above the treeline at where shrubs and herbs dominate, the area between that was in the past dominated by open oak forest (''Quercus aegilops''), and the wetter and sometimes marshy river valleys. Other trees besides oak that can be found in the forested zone including juniper at higher elevations; ash, hawthorn, maple and walnut at intermediate elevations; and pistachio and olive trees in lower, drier areas. In the foothill zone, many areas are now cultivated, but there remain small patches of natural vegetation dominated by herbs of the genus ''Phlomis

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