Introduction to Tongjiyan: Tongjiyan Arch Dam is the earliest arched dam in the world. It was originally built as a "wooden dam" (that is, "Xiao", for bamboo), and was changed to "Xiao" in the Southern Song Dynasty. The existing stone dam maintains the original ancient structural characteristics of the arch dam. The dam is 275 meters long, 25 meters wide at the bottom, and 2.5 meters high.