Quick Answer: How Does Dujiangyan Divide the River?
Dujiangyan (都江堰, Dūjiāngyàn) diverts part of the Min River into irrigation channels on the Chengdu Plain. Three main structures work together to split the flow, release excess water and sediment, and limit what enters the irrigation network.[1][2][3]
- Fish Mouth (鱼嘴, Yúzuǐ): A pointed embankment divides the river into inner and outer channels.[1][3]
- Flying Sand Weir (飞沙堰, Fēishāyàn): A low overflow helps return surplus water and sediment to the outer channel.[2][3][4]
- Bottle-Neck Channel (宝瓶口, Bǎopíngkǒu): A narrow opening in rock controls the entrance to the irrigation channels.[1][2][3]
The three parts act together. The amount of water passing through each changes with the river’s flow.[3][4]
Look at a plan of the headworks and follow the branches in order. The first split does not send all of its inner-channel water straight onto fields. Some can spill back out before the remainder reaches the intake. That intermediate route is central to how the system works.[2][3][4]
Why the Qin Project Was Built Here
The Min River leaves the mountains and reaches the Sichuan Basin near Dujiangyan. The site lies upstream of much of the Chengdu Plain. Water can flow from this point into lower channels by gravity. The same river that provides irrigation can also carry high flows and sediment from the mountains.[1][3][4]
The original project is usually dated to around 256 BCE, when the state of Qin governed the region. It is associated with Li Bing (李冰, Lǐ Bīng). This belongs to the history of the Warring States period, before the later Qin empire. The date refers to the origin of the project, while the structures have changed through later centuries.[1][2]
What the Fish Mouth, Flying Sand Weir and Bottle-Neck Do
The Fish Mouth sits where the river divides. The inner branch carries water toward the intake. The outer branch carries water past the headworks. The shape of the upstream channel and the water level affect how much flow enters each branch. Both branches continue to carry moving water below the split.[1][3][4]
Farther downstream, the Flying Sand Weir gives water in the inner branch a way to return to the outer branch. When conditions favor overflow, water and sediment can pass over this low barrier. The Bottle-Neck Channel lies farther along the intake route. Its narrow opening limits the amount entering the irrigation system.[2][3][4]
Viewed together, the works give water more than one path before it reaches the plain. The Zhengguo Canal offers another Qin-era example of directing a river toward farmland in a different landscape.[1][2][3]
Where the Sediment Goes
Water and sediment do not necessarily follow the same proportions at a river split. Coarser particles move along the bed, and bends can guide that material toward a different part of the channel. Research on Dujiangyan examines how the river geometry and the overflow work together to send much of the bed material toward the outer branch.[3][4][5]
One published account of 1972 measurements puts about 72 percent of the bed load into the outer channel at the Fish Mouth, about 18 percent through the Flying Sand Weir and about 10 percent through the intake. Those figures describe that reported set of measurements. They are useful for seeing that there was more than one route for sediment, without treating them as a permanent division.[5]
How Dujiangyan Changed Over Time
Dujiangyan was modified and enlarged during later dynasties. The overall principle of diversion survived, while structures and materials changed. More recent works upstream also affect the flows that arrive at the headworks. Comparing an early description with today’s system therefore requires attention to date and to which part of the river is being described.[1][2][4]
The World Heritage inscription pairs the irrigation system with Mount Qingcheng. The inscription covers the irrigation system and mountain together. It does not imply every structure visible today is original. The history of Chinese suspension bridges uses the Anlan crossing at Dujiangyan to show a similar difference between a site’s history and later material evidence.[1][2]
FAQs About Dujiangyan Engineering
Does Dujiangyan use a dam?
The original diversion design split a flowing river without a high dam blocking it across its full width. Modern upstream works belong to the river system’s later history.[1][4]
Does the Fish Mouth split water in a fixed ratio?
The division changes with river flow and channel conditions. A quoted ratio should be read with its season, discharge or study conditions.[3][4]
Are the structures visible today all from the Qin period?
The project began in the Qin state period. The headworks were maintained, modified and enlarged in later centuries.[1][2]
Sources
[1] UNESCO World Heritage Centre. Mount Qingcheng and the Dujiangyan Irrigation System
[2] World Heritage nomination dossier. Mount Qingcheng and the Dujiangyan Irrigation System
[3] Journal of Hydro-Environment Research. Dujiangyan Irrigation System and Modern Hydraulic Science
[4] Sustainability. Numerical Investigation of Water and Sediment Diversions at Dujiangyan
[5] Journal of Mountain Science. Wisdom, Predicaments, and Challenges of the Dujiangyan Project