都江堰 - Du Jiang Yan Irrigation Project

The Du Jian Yan Irrigation Project is situated 56km northwest of Chengdu, a 50-minute bus ride from Cha Dian Zi bus station in western Chengdu (茶店子西门公共汽车站) (on a direct, "deluxe" bus, for 15RMB a go). The project was constructed by the Shu Governor, Li Bing, in the 3rd century B.C. It is the world's largest example of a damless irrigation system and classed as a UNESCO World Heritage Site. The irrigation system serves to control and divert the flow of water for irrigation of the Chengdu Plain and consists of three components: the Fish Mouth dike (鱼嘴), the Flying Sand Spillway (飞沙堰) and the Bottleneck (宝瓶口).


Pay attention now, here comes the science bit: if I've understood the principle correctly (which, not being an eco-engineer, is far from likely), the Fish Mouth divides the Minjiang river into an outer and inner canal. The outer canal follows the course of the river and is shallower than the riverbed, while the deeper inner canal diverts faster flowing water along the face of the surrounding mountain. The inner canal serves to remove silt; the outer canal acts as an overflow when water levels are high so as to prevent flooding. The long, deep Bottleneck guides water from the inner canal into the irrigated area and was carved out of the face of the mountain, even before explosives were in common use, by repeatedly heating and cooling the rock, causing it to crack. Barriers made from bamboo and rocks, held together by rope, were used to protect the river banks.

Several temples and ancillary features were subsequently built on the site, and the walk back up the mountainside through the Songmao Road (松茂故道) is well worth it, giving some great views of the river and Du Jiang village (and the city, although that's not so nice......).