dimension in Shushtar, since the Haydaris were pro-Arab and pro-monarchy while the Ne'matis were pro-Bakhtiyari and pro-constitutionalist. With the completion of the Trans-Persian Railway, Shushtar began to decline. The railway bypassed Shushtar in favor of Ahvaz, which took over Shushtar's commercial importance, and Shushtar's population decreased. Band-e Kaisar The Band-e Kaisar ("Caesar's dam") is believed by some to be a Roman built arch bridge [since Roman captured soldiers were used in its construction], and the first in the country to combine it with a dam. When the Sassanian Shah Shapur I defeated the Roman emperor Valerian, he is said to have ordered the captive Roman soldiers to build a large bridge and dam stretching over 500 metres. Lying deep in Persian territory, the structure which exhibits typical Roman building techniques became the most eastern Roman bridge and Roman dam. Its dual-purpose design exerted a profound influence on Iranian civil engineering and was instrumental in developing Sassanid water management techniques. While the traditional account is disputable, it's not implausible that Roman prisoners of war were involved in its construction. The approximately 500 m long overflow dam over the Karun, Iran's most effluent river, was the core structure of the Shushtar Historical Hydraulic System, a large irrigation complex from which Shushtar derived its agricultural productivity, and which has been designated World Heritage Site by the UNESCO in 2009. The arched superstructure carried across the important road between Pasargadae and the Sassanid capital Ctesiphon. Many times repaired in the Islamic period, the dam bridge fell out of use in the late 19th century, leading to the degeneration of the complex system of irrigation. Registration of ancient works in UNESCO World Heritage Ancient works of Shushtar, which were registered at the annual meeting of the UNESCO World Heritage Committee on June 26, 2009, under the title of Shushtar Historical Water System, as the tenth work of Iran in the UNESCO World Heritage List with number 1315. People and culture The people of Shushtar, called Shushtaris, maintain a unique cultural heritage stretching back to ancient times, and a Persian dialect called Shushtari dialect distinct to their group. The devoutness of Shushtar's people has led to it being nicknamed "Dar al-Mu'minin". Local tradition attributes certain customs to ancient Roman colonists, as well as the construction of the Band-e Kaisar and the introduction of brocade manufacturing technique. Climate Shushtar has a hot semi-arid climate (Köppen climate classification