Chinese Empire

The Dujiangyan Irrigation System: 2,200 Years of Qin Engineering

The Dujiangyan (都江堰) irrigation system, on the Min River near Chengdu in Sichuan, is the oldest and most ingenious large-scale water management system in the world still in continuous operation. Designed in 256 BCE by the Qin engineer Li Bing and his son, it has been diverting water from the Min River onto the Chengdu plain — a fertile agricultural region of about 5,300 square kilometres — for more than 2,200 years, without ever ceasing to function. The system is a masterwork of pre-modern civil engineering and a model of how to manage a major river without a dam. Its context within the broader tradition of Chinese engineering is surveyed in our overview, and the canal and bridge engineering of the empire in engineering marvels of imperial China. The Grand Canal, the other great Chinese water engineering project, is treated in our dedicated article.

The Problem: Flood and Drought on the Chengdu Plain

The Chengdu plain, surrounded on three sides by mountains, is one of the most fertile agricultural regions in China. The Min River, which flows down from the mountains of western Sichuan and joins the Yangzi at Yibin, is the principal source of water for the plain. Before the Dujiangyan project, the Min was a destructive river. In the dry winter and spring, its flow was too low to irrigate the plain, and farmers suffered from drought. In the summer monsoon, the river would flood catastrophically. The alternation of flood and drought had prevented the plain from reaching its full agricultural potential.

The state of Qin, in the third century BCE, was engaged in a long campaign to conquer the rival states of the central plain. The Chengdu plain, with its rich agricultural potential, was a crucial strategic prize. The king of Qin, Zhaoxiang (r. 325–251 BCE), ordered Li Bing to design a permanent water management system that would tame the Min River and allow continuous irrigation. Li Bing had been governor of Shu (the Qin-controlled part of Sichuan) for some years, and he had a thorough knowledge of the river.

The Three Elements of the System

The genius of the Dujiangyan system is that it requires no dam. Instead, it works by dividing the Min River into an inner channel that carries irrigation water onto the plain, and an outer channel that carries the flood waters and the bulk of the silt. The division is achieved by three principal elements: the fish-mouth levee, the flying sand weir, and the bottle-neck channel.

The fish-mouth levee (魚嘴, yuzui) is a long, low diversion structure in the middle of the Min River at the head of the plain. Named for its shape (the rounded, snout-like upstream end resembles a fish’s mouth), it divides the Min into two channels: the inner channel (内江, nei jiang), which carries the irrigation water onto the plain, and the outer channel (外江, wai jiang), which carries the main flow and the flood water back into the original riverbed.

The fish-mouth levee works on a simple but effective principle. The Min River carries a much larger volume of water during the summer flood than in the winter dry season, and the proportion of water that the levee allows into the inner channel changes automatically with the flow. In the dry season, when the river is low, most of the water is diverted into the inner channel. In the flood season, the levee blocks the excess water and the bulk of the silt, allowing it to continue down the outer channel. The result is a year-round, almost self-regulating supply of irrigation water to the plain, without any active control.

The flying sand weir (飛沙堰, feishayan) is a low spillway across the inner channel, just downstream of the fish-mouth levee. Its purpose is to flush out the silt and excess water that enter the inner channel during the flood season. The weir is built at a precise level: high enough to maintain a normal flow into the irrigation system, but low enough that, when the river is in flood, the excess water and silt spill over the weir and back into the outer channel. The name “flying sand” comes from the observation that during a major flood, the silt-laden water actually appears to fly over the weir, throwing the heavier sand and gravel into the outer channel. The inner channel is automatically self-flushing during floods.

The bottle-neck channel (寶瓶口, baopingkou) is a narrow cut through a ridge of rock at the foot of the hills, connecting the inner channel with the irrigation network that distributes water over the Chengdu plain. The channel was originally cut by Li Bing’s workers, who used fire and cold water to crack the rock and then chiselled out the opening. The name “bottle-neck” comes from the shape of the cut: narrow at the upstream end and widening as it descends the slope of the plain. The bottle-neck channel is the control point of the entire system. Its narrow opening regulates the flow of water into the irrigation network, preventing the inner channel from carrying too much water onto the plain in a flood and too little in a drought. The geometry of the cut ensures that the water flow is roughly constant regardless of the river’s flow.

Li Bing’s Engineering and the Chinese Tradition of Hydrology

Li Bing left a famous six-character maxim for his successors: “深淘灘，低作堰” (shēn táo tān, dī zuò yàn) — “Dredge the channels deep, and keep the weirs low.” The first tells the maintenance crews to dredge the inner channel regularly to keep it from silting up; the second tells them to keep the flying sand weir low, so that it will flush effectively in a flood.

Li Bing also left the Three Stone Men (三石人) — three stone figures placed in the inner channel that served as water-level markers, with marks at the levels corresponding to “too low”, “just right”, and “too high”. The system was so effective that the Chengdu plain became one of the most productive agricultural regions in China within a few decades, and it remained so for the next two thousand years.

The Dujiangyan system is, in many ways, the high point of the Chinese tradition of hydraulic engineering. The Chinese approach to water management did not usually rely on the construction of large dams but on the careful management of existing rivers by means of low levees, weirs, and diversions. Dujiangyan is the purest expression of this approach. The same approach was used on a smaller scale in hundreds of other places throughout imperial China, and was also the basis of the Grand Canal (treated in our article on the construction of the Grand Canal).

The System in the Imperial Period and Today

The Dujiangyan system was continuously maintained and rebuilt throughout the imperial period. The Han dynasty added new canals to the irrigation network. The Tang rebuilt the fish-mouth levee in stone, and the Song added further extensions. The Ming carried out a major reconstruction under the official Fan Chengda in the late fifteenth century, and added a series of iron bonds to anchor the fish-mouth levee more firmly. The Qing carried out further repairs, and the system continued to irrigate the Chengdu plain throughout the imperial period.

After the system was completed, Li Bing and his son were honoured as gods, and a Temple of Li Bing (二王廟, “Temple of the Two Kings”) was built on the hillside above the bottle-neck channel. The temple has been rebuilt many times, and remains an important pilgrimage site. In the modern era, the Dujiangyan system has been supplemented by modern dams, reservoirs, and hydroelectric projects, but the original Qin system is still in operation. The Dujiangyan irrigation system was inscribed as a UNESCO World Heritage Site in 2000.