How did Si-o-Se Pol function as both a bridge and a dam?

The Si-o-Se Pol, also known as the Allahverdi Khan Bridge, functions as both a bridge and a dam due to its specialized architectural design. While it serves as a vital transportation link across the Zayandeh River in Isfahan, Iran, its structure incorporates features that allow it to manage water flow.12

The bridge’s archways are equipped with sluice gates, which can be opened to regulate the river's level and allow excess water to pass through during periods of flooding. This dual functionality is supported by the bridge's robust construction, which utilizes sarooj—a traditional hydraulic mortar made of lime, clay, sand, and sometimes gypsum—that hardens upon contact with water, making it particularly well-suited for foundations exposed to river currents.2