Shushtar is an ancient fortress city, approximately 92 km (57 mi) away from Ahvaz, the center of the province. Much of its past agricultural productivity derives from the Roman-built irrigation system which centered on the Band-e Kaisar, the first dam bridge in Iran. Shushtar, Historical Hydraulic System inscribes as a masterpiece of creative genius, can be traced back to Darius the Great in the 5th century B.C. This site is registered by UNESCO as the 10th Iranian Heritage Site. It involved the creation of two main diversion canals on the river Karun one of which, Gargar canal, is still in use providing water to the city of Shushtar via a series of tunnels that supply water to mills. It forms a spectacular cliff from which water cascades into a downstream basin. It then enters the plain situated south of the city where it has enabled the planting of orchards and farming over an area of 40000 ha. known as Mianab (paradise). The property has an ensemble of remarkable sites including the Salasel Castle, the operation center of entire hydraulic system, the tower where the water lever is measured, damns, bridges, basins and mills. It bears witness to the know-how of the Elamites and Mesopotamians as well as most recent Nabatian expertise and Roman building influence.