In 224 CE, the Persian ruler Ardašir I defeated his Parthian overlord, king Artabanus IV. This event marked the beginning of the Sasanian Empire. Ardašir's castle and his palace near modern Firuzabad now became very important places, and the road to Istakhr and beyond had to be improved. The bridge across the Qara Aqaj river is said to have been built in these years. It is indeed likely that some kind of bridge existed.
However, the present bridge is built on arches, and the construction workers used mortar. Both suggest that Roman engineers were somehow involved. This is possible and even likely, because we know that many Romans were taken captive during the campaigns of Ardašir's son Shapur I. They certainly built the bridge at Shushtar, and may well have built the one at Kavar too. This would suggest a date after 260.
The fact that the arches were moderately pointed suggests repairs in the Early Islamic age, when the road from the sea along Firuzabad to Shiraz became more important. There were certainly repairs by the Safavid and Qajar kings, but the two central arches have collapsed.
The bridge, which can be seen about one kilometer south of modern Kavar, was about 125 meters long and had six arches. Made of natural stone and mortar, it stood about ten meters high and was four meters wide.
to Stefan Michels.