The length of the Great Zab has been variously estimated at and . Approximately of the river's course are 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. River modifications To date, one large dam has been partially constructed on the Great Zab: Iraq's Bekhme Dam and Turkey's 24 MW weir-controlled run-of-the-river Bağışlı Hydroelectric Power Plant. Five others have been planned in the