Initiated in the 4th year of Tianjian of Xiaoliang in the Southern Dynasty (A.D. 505) and rebuilt in the 1st year of Kaixi of Song Dynasty (1205), Tongji weir, situated in Songyin Stream, upstream of a tributary of Oujiang River in the southwest of Zhejiang Province, is the world's oldest arched water project mainly serving irrigation.

　　At first, Tongji Weir was built of wood. In Song Dynasty, He Dan, an official who resigned from office and went back to his hometown, requested the Emperor to rebuild Tongji Weir, and the rebuilt Tongji weir was changed to stone dam. The dam is 275m long, 25m wide and 2.5m high. The upstream rainwater collection area is about 3150 km2, the water diversion flow is 3 m3/s, and the whole dam is convex to the upstream about 120 degree arc. The arch dam can better withstand the flood force. Its construction time is earlier than that of Elqi arch dam built in the 16th century in Spain and Bondaldo arch dam built in 1612 in Italy.

　　In addition to the arch dam, this water project boasts of many characteristics. The first is to fully consider the natural elevation difference when selecting the dam site, so that the supporting irrigation channels can irrigate by gravity; the second is to use big pine as the dam foundation and cast stone dam with molten iron to ensure that the foundation is not rotten and the overall performance of the stone dam is strong. This is also one of the important reasons why the dam lasts for thousands of years. The third is to give consideration to both sand discharge and navigation. At the north end of the dam, there are two sand discharging gates with a clear width of 2m, which are deep to the bottom of the dam. The sand and stone washed down by the upstream flood will be automatically discharged to the downstream of the dam by rapid flows of these two gates. A ship lock with a clear width of 5m was built to ensure the navigation function in ancient times. Fourth, an overpass stone bridge for water diversion was built, commonly known as the "Three Hole bridge", to facilitate the mountain stream flood affecting the safety of the irrigation channels to pass through the bridge deck into the Oujiang River, and the irrigation water to flow under the bridge. Therefore, there is no mistake in flood diversion and irrigation. Fifthly, the irrigation water volume can be adjusted reasonably through 72 gates and 3 connected canals. From the song and Yuan Dynasties to the Qing Dynasty, Tongji Weir was maintained and renovated many times.

　　The historical records and rules of Tongji Weir have been well preserved. Since its launch, all dynasties had attached great importance to its maintenance and management with a set of self-contained and complete management methods. Among them, the earliest written "weir rule" appeared in the 7th year of Yuan of the Northern Song Dynasty. The earliest existing weir rule was formulated by Fan Chengda, governor of Chuzhou, in the 4th year of Qiandao in the Southern Song Dynasty (1168 A.D.), and was unique, scientific and comprehensive. It was imitated by later generations and used for more than 600 years. Now it is preserved in the Zhannan Sima Temple beside the dam. The rules cover the standard sizes of branch outlets, rotation irrigation system, annual maintenance and labor service, methods for apportioning labor and materials, methods for rewards and punishments in project inspection, etc. Until now, some rules are still in use.

　　Tongji Weir is now surrounded by breathtaking sceneries. The Yantou village where the river originates is cradled in mountains and covered by canopies of ancient camphor trees. The Wenchang Pavillion stands next to the “Three Hole Bridge”. To the west of the dam is the Zhannan Sima Temple, more famously known as the Dragon Temple. It keeps 16 stone tablets of the Song, Yuan, Ming and Qing dynasties and the Republic of China, recording the construction history as well as rules and maps of the dam. Tang Xianzu (a Dramatist and litterateur of Ming Dynasty in China)and other famous Chinese scholars and calligraphers left their works here.