The rivers Euphrates, Great Zab, and Little Zab acted as natural defenses for Mesopotamia.[4] Sasanian developing of irrigation systems in Mesopotamia further acted as water defense lines, notably the criss-crossing trunk canals in Khuzestan and the northern extension of the Nahrawan Canal, known as the Cut of Khusrau, which made the Sasanian capital Ctesiphon virtually impregnable in late Sasanian period.[5]