The Tashan Weir lies at the intersecting point of the Zhangxi and Yinxi brooks, which run along the Tashan and Miaoshan mountains in Yinxian County, Zhejiang Province. 

Built in 833 during the Tang Dynasty (618-907), Tashan Weir, at 134.4 x 4.8 x 10 meters, was one of the greatest irrigation works in ancient China. The top of the weir was built using stone bars two to three meters long, four meters wide and 0.2-0.35 meters thick. Both sides of the weir have 36 steps. Three sluice gates -- the Wujin Gate, the Jidu Gate and the Xingchun Gate -- were built over the Nantang River to the east of the weir as supplement facilities dividing the river. A complete irrigation system was thus formed using the Tashan Weir as its pivot.

Other supplemental projects were built to the west and north of the weir, including a flood bay to block seawater, store freshwater and drain flooded fields; and the Guanchi Pier and the Huisha Sluice to increase water levels, deposit mud and prevent silts.

For more than 1,000 years, the Tashan Weir has played an important role in China's water conservancy history. It not only irrigates over 200,000 mu (1 mu=1/5 of a hectare) of farmland, but also channels water into Ningbo City for daily consumption. After many renovations since the Tang Dynasty, the weir is well preserved.