都江堰 Dūjiāngyàn — Dujiangyan

Two thousand two hundred years, and no dam

Dujiangyan is about sixty kilometres from Chengdu and is, by a reasonable measure, the oldest piece of infrastructure on earth still doing the job it was built for. Construction began around 256 BC under Li Bing, governor of the Qin state's Shu commandery, and the works still irrigate something over half a million hectares of the Chengdu plain today. What makes it remarkable is what it does not have. The Min river came down out of the mountains carrying enormous seasonal volumes and silt, flooding the plain in summer and leaving it dry in winter. The obvious answer is a dam, and Li Bing did not build one. Instead the works split the river: an artificial island called 鱼嘴, the fish mouth, divides the flow into an inner channel for irrigation and an outer one that carries the excess, and the proportions shift by themselves with the water level, so that more goes to the fields in the dry season and more to the flood channel in the wet. A weir called 飞沙堰 lets surplus water and silt spill back out, which is why the system does not clog. And a channel cut through the rock at 宝瓶口, the bottle neck, limits how much can enter the plain at all. The whole thing works by terrain and gradient, with no moving parts and nothing to fail. The original construction materials were bamboo baskets filled with river stones — 竹笼 — and wooden tripods, which could be repaired annually by ordinary labour. That was part of the design: a system maintainable by the people who benefited from it, rather than one requiring the state to be competent forever. It has been repaired continuously for two millennia, survived the 2008 Wenchuan earthquake whose epicentre was close by, and is a UNESCO World Heritage site. It is hard not to read this against lesson 64's Three Gorges dam, and Chinese engineers and commentators do exactly that — sometimes as a straightforward contrast between working with a river and blocking it, sometimes to point out that the comparison is unfair, since Dujiangyan diverts a mountain tributary for irrigation while Three Gorges is a flood-control and power project on the main stem of one of the world's largest rivers. Both points are made seriously. What is not in dispute is that 都江堰 has functioned for longer than any comparable work anywhere, and that its principle — let the water do the sorting — is genuinely elegant. For a visitor: it is an easy half-day from Chengdu by high-speed train, best combined with Qingcheng Shan next door, one of the founding mountains of Daoism and a pleasant walk. Go for the engineering rather than the temples, stand on the bridge over 鱼嘴, and watch the river divide itself.