Watershed
Estimates of the drainage basin of the Great Zab vary widely – from a low 25,810 square kilometres (9,970 sq mi) to a high figure of 40,300 square kilometres (15,600 sq mi). 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 3,000 metres (9,800 ft) 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 1,000 millimetres (39 in) prevails. From there, the river flows into the foothill zone of the Zagros, where rainfall drops to less than 300 millimetres (12 in) 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 ecozones: the area above the treeline at 1,800 metres (5,900 ft) where shrubs and herbs dominate, the area between 1,800 and 610 metres (5,900 and 2,000 ft) 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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