The river is fed by rainfall and snowmelt, resulting in a peak discharge in the spring and low water in the summer and early fall.

Water erosion has filled the Little Zab valley and the foothill zone south-west of the Zagros with layers of gravel, conglomerate, and sandstone.

[15] 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.

[17][18][19] 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.

Its functions are to regulate the flow of the Little Zab, to store water for irrigation in its reservoir (Lake Dukan) and to provide hydroelectric power.

The earliest evidence for human occupation in this region comes from the Middle Palaeolithic site of Barda Balka, where Late Acheulean stone tools have been found.

[33] Archaeological research elsewhere in the Zagros confirms the importance of this area to early human hunter-gatherers – including groups of Neanderthals as evidenced by the finds in Shanidar Cave in the Great Zab basin.

[40][41] The archaeological fieldwork in the Ranya Plain showed that this area was occupied during the Ubaid, Uruk and Ninevite V periods – roughly from the middle 6th to the mid-3rd millennium BCE.

[43] The region enters history at the end of the 3rd millennium BCE, when Erbil is mentioned as Urbilum by king Shulgi of the Ur III dynasty.

[44] From that time onward, the Little Zab basin became increasingly entangled in the affairs of the successive Mesopotamian empires that sought control over the Zagros Mountains.

In the early second millennium BCE, king Shamshi-Adad of Upper Mesopotamia waged war to the land of Qabra, which was probably located along the lower course of the Little Zab, and installed garrisons in the conquered towns.

The archive of clay tablets found at Tell Shemshara (ancient Shusharra) shows that the local governor switched allegiance and became a vassal of Shamshi-Adad.

[45] During the 14th century BCE, the region was part of the Mitannian kingdom, with sites like Nuzi and Tell al-Fakhar, south of the Little Zab, yielding clay tablet archives for this period.

[48] On the Ortelius Theatrum Orbis Terrarum maps of Turkish Empire and Persian Kingdom it is listed as Noue aque fl.