The Little Zab or Lower Zab (, al-Zāb al-Asfal; or ; , Zâb-e Kuchak; , Zāba Taḥtāya) is a river that originates in Iran and joins the Tigris just south of Al Zab in the Kurdistan region of Iraq. It is approximately long and drains an area of about . The river is fed by rainfall and snowmelt, resulting in a peak discharge in spring and low water in summer and early fall. Two dams have been built on the Little Zab, regulating the river flow, providing water for irrigation and generating hydroelectricity. The Zagros Mountains have been occupied since at least the Lower Palaeolithic, but the earliest archaeological site in the Little Zab basin, Barda Balka, dates to the Middle Palaeolithic. Human occupation of the Little Zab basin has been attested for every period since then.
Course
The Little Zab rises in the Zagros Mountains in Iran at an elevation of circa amsl. In its upper reaches, the course of the Little Zab is determined by the alignment of the major mountain chains that make up the Zagros. Thus, the river flows through valleys that are predominantly aligned along a northwest–southeast axis, parallel to the major mountain chains of the Zagros, only to change its direction abruptly where it cuts through these chains in narrow gorges. Along the way, it collects the waters coming down from the eastern face of the Qandil Mountains, which now form the border between Iran and Iraq. The Little Zab enters the Mesopotamian plain south of Dukan, where it first assumes a roughly westward course before turning to the southwest upstream from the town of Altun Kopru and uniting with the Tigris near the town of Al Zab. Most tributaries join the Little Zab upstream from Dukan, with the largest being the Baneh River and the Qala Chulan. A number of smaller streams joined the Little Zab in the Ranya Plain, which is now partly inundated by Lake Dukan.
Different estimates have been given for the length of the Little Zab: , and . For a short distance, the Little Zab forms the border between Iran and Iraq, and along its lower course it also constitutes the border between Erbil Governorate and Sulaymaniyah Governorate, and Erbil and Kirkuk Governorates. The river is fed by snowmelt and rainfall, resulting in a peak discharge in the period February–May. Low water levels are recorded for the period 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.