Dujiang Yan (The Dujiang River Dyke) is a great ancient irrigation project situated about 50 kilometers northwest of Cheng Du City, Sichuan Province.

The Minjiang River is a major tributary of the Yangtze River in its upper reaches. Before Dujiang Yan was built, when torrential water rushed down the mountains in summer, the Minjiang River overflowed its banks and caused serious floods on the Cheng Du Plain. When the rainfall was not plentiful, the flow of water in the Minjiang River would decrease, aggravating the drought on the western Sichuan Plain.

In 256BC, Li Bing was appointed governor of Sichuan. To conquer the floods of the Minjiang River, he trekked upstream until he reached the area of the present-day Aba Tibetan Autonomous Prefecture. He got a lot of hydrological data on the Minjiang River. Then he began to build Dujiang Yan.

Dujiang Yan system consists of the Fish Mouth, the Sand-flying Dyke and the Precious Bottle Neck.

The Fish Mouth is a diversion dam built of pebbles. It divides the Minjiang River into the inner river and the outer river. The water of the inner river flows through the Precious Bottle Neck to irrigate 200,000 hectares of farmland around Cheng Du. The outer river was designed mainly to release floodwater. The Sand-flying Dyke between the Fish Mouth and the Precious Bottle Neck is an overflow dyke. When there is too much water in the inner river, the surplus water brims over the Sand-flying Dyke and flows into the outer river.

The Precious Bottle Neck, dug out of a mountain, is the conduit through which the inner river water flows to the irrigated area.

After Dujiang Yan was completed, Li Bing composed the rhyme easy for memory: "Dredge the riverbed when the water is deep and build low dykes when the water is low." It contains the guiding principle of dredging the waterways in ancient China.

Dujiang Yan serves the purposes of navigation, flood control, log floating and urban water supply. The trinity of the Fish Mouth, the Sand-flying Dyke and the Precious Bottle Neck keeps an effective control of water flowing into the inner river. In case of drought, Dujiang Yan channels water into the fields. If waterlogging occurs, the floodwater can be diverted. With the completion of Dujiang Yan, both floods and droughts have been brought under control and famine years have gone. The Cheng Du Plain becomes "a land of abundance", with an irrigation area of 10 million mu (1 mu =1/15 hectare).

Dujiang Yan, a great water conservancy project built in ancient times, has been in use for more than 2,000 years. It is a miracle in the world history of water conservancy.

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