July–October. The average discharge of the Little Zab is per second, whereas the maximum recorded discharge is per second. Average annual discharge is . Because of its torrential nature, Medieval Arab geographers have described the Little Zab, and the Great Zab as well, as "demoniacally possessed". Watershed The drainage basin of the Little Zab covers ; from the location where the Dukan Dam has been constructed, it measures . The larger part of the basin (74%) is located within Iraqi borders; the remainder is in Iran. On the north, it is bordered by the Great Zab basin while on the south it is adjoined by the basins of the Adhaim and Diyala rivers. The parallel mountain ranges of the Zagros consist of limestone folds rising to elevations over . Water erosion has filled the Little Zab valley and the foothill zone south-west of the Zagros with layers of gravel, conglomerate, and sandstone. The Ranya Plain is the largest valley in the Little Zab drainage basin, and the second-largest in the Iraqi Zagros behind the Shahrazor. The Little Zab crosses very diverse climatic and ecological zones. Annual precipitation along the course of the river diminishes from over in the Iranian Zagros to less than at the confluence with the Tigris near Al Zab. Average temperatures follow a similar gradient, with the mountain valleys generally experiencing colder winters than the foothill zone, while summers in the latter are hotter. In the high Zagros, three different biomes can be distinguished. The tree line is at approximately ; above which herbs and shrubs predominate. The dominant vegetation between was an open oak forest (Quercus aegilops), but not much of this original vegetation remains. The river valleys are characterized by water-loving plants, and marshy areas were in the past – in the absence of drainage – prone to malaria. Although the foothill zone, especially the plain of Erbil, is heavily cultivated, patches of natural vegetation remain, with herbs in the genus Phlomis being very common. River modifications Two dams have been constructed on the Little Zab in Iraq while Iran is currently constructing one with two others planned. The two in Iraq are the Dukan Dam and the Dibis Dam. The Dukan Dam was constructed between 1957 and 1961 as a multi-purpose arch dam upstream from the town of Dukan. The dam's crest is above the riverbed ( amsl) and long. Its functions are to