Dujiangyan, Waterworks Without a Dam

Dujiangyan has no dam.

It is not that the dam broke. Nor was a dam ever taken down. For more than 2,200 years, this waterworks has tamed a great river rushing down from the high mountains, day and night. It splits the water, sweeps away the sand and leads the water into the fields. Yet none of its three main parts is a wall across the stream.

To understand this strange thing, one has to stand at the very spot where the Min River leaves the mountains.

The Min River comes down from the mountains to the northwest. At that stretch, the river has only just escaped a rocky gorge when the land suddenly opens into a wide plain, the Chengdu Plain. From there to the city of Chengdu, it is only about 50 kilometers. Yet over that distance the ground drops by 273 meters. The water comes rushing down from up there. In the rainy season it bursts its banks and drowns the fields. In the dry season it shrinks away and leaves the fields cracked. Two hardships a year, every single year.

Around 256 BC, a man named Li Bing was sent to govern this land of Shu. He stood and watched the river for a long time. Then he made a decision that engineers still tip their hats to today. He would not stop the water. He would divide it.

The first task was to build a stone point right in the middle of the riverbed, its sharp tip turned upstream. Seen from above, that stone point looks just like the open mouth of a fish taking in the water. So people simply called it Yuzui, which means fish mouth.

When the water reaches this spot, the stone point splits it in two. One part goes into the inner channel, which is narrow and deep and meant for watering the fields. The other part goes into the outer channel, which is wide and shallow and meant for carrying off floods and flushing out sand and gravel.

The clever part is that the two channels were dug to different depths. In the low-water season the level is low, and the current follows the deeper bed. So more than six parts in ten of the water flows into the inner channel by itself, enough for the fields to drink. In the flood season the water rises high and the current runs straight ahead. Then the share entering the inner channel drops, at times to less than four parts in ten. All the rest of that wild water pours into the outer channel.

The river shares itself out by season, mostly thanks to the shape of the bed that people dug long ago.

But dividing the water was only half the job. A river coming down from the mountains always carries sand and gravel on its back. Many irrigation works around the world have died because sand filled their ditches and then buried their water gates.

So farther downstream, Li Bing had a stone bank built to join the two channels. This bank was kept low on purpose. Water at its normal level does not spill over it, but as soon as the water rises high, it spills straight across. People call it Feishayan, which means the stone bank that makes sand fly.

At that spot the riverbed curves. When the water in the inner channel rises too high, it runs into the bend and swirls around. In that swirl, the heavy sand and gravel are flung outward. They clear the low stone bank and fall back into the outer channel. More than nine parts in ten of the sand and gravel that slips into the inner channel is thrown back out this way. In the end, less than eight percent of the whole river's silt makes it to the water intake. The fields receive water that is almost crystal clear.

As for that water intake, it had to be cut right through a mountain.

Between the inner channel and the Chengdu Plain stood Mount Yulei, a single solid mass of rock. To bring water into the fields, a gap had to be opened through it. In those days there were no explosives and no drills. There were only stone hammers, iron chisels and human strength.

Li Bing came up with a way. Workers piled firewood against the rock face and set it alight. They let it burn until the surface of the rock glowed hot. Just when the rock was at its hottest, they threw cold water on it. Struck by the sudden cold, the rock shrank, cracked loudly, and split off in slabs. The workers chiseled away the cracked part and cleared the rubble. The next day they piled up wood again, lit it again, and doused it again.

Day after day, season after season. Smoke, fire and steam rose thick over the river. The sound of splitting rock mixed with the sound of running water. It went on like that for eight long years, until the day the mountain finally opened a passage.

The opening was narrow, with two sheer rock walls, like the neck of a bottle. People named it Baopingkou, which means the mouth of the precious bottle.

That narrowness was on purpose too. However high the water outside might rise, only a certain amount can pass through the gap into the plain. The excess piles up in front of the opening and swirls. Then it spills over the low stone bank of Feishayan by itself and returns to the outer river. The fish mouth divides the water, the stone bank throws out the sand, and the narrow gap measures the water. These three parts fit together like the three gears of a machine that needs not a single drop of fuel.

Of course, this machine of stone and water still needs people to care for it. Every year, in the low-water season, the people of the area go down into the riverbed. They dredge out the mud and sand that have settled and rebuild the places where the stones have shifted. They have done this almost without a break since the waterworks was built. Each generation passes the next a short saying: dig the riverbed deep, keep the stone bank low. Deep to hold the water, and low so the sand can still fly out.

Today, the Chengdu Plain still drinks from those same three parts. The stone point shaped like a fish mouth still splits the river in two. The low stone bank still throws the sand back. The narrow mountain gap still pours water into the fields. This region has long been called a land given by heaven, because every season brings enough water and every field brings a good harvest. Yet that gift of heaven really began with a very human decision: not to fight the water.

On the riverbank at the foot of Mount Yulei, later generations built a temple to Li Bing. Alongside him it honors a son named Erlang. The story of that son is folk tradition passed down by word of mouth, not something the history books record for certain. Still, for generations, the people of the area have burned incense for both father and son.

There is something familiar in all of this.

People in Vietnam do the very same thing, only on a smaller scale. A ditch leads water from a stream to the fields. A small weir set across a creek raises the water just enough, then lets it flow along the ditch on its own. The terraced fields in the northern mountains hold water level by level. When an upper level is full, it spills down by itself to the one below. The dike along a river works the same way. It does not block the river. It only stands beside it and shows the water the way.

Then there is the Vietnamese legend of Sơn Tinh and Thủy Tinh. Sơn Tinh is the mountain god of that old legend. However high the water rose, the mountain rose just as high, and the two sides went on testing their strength without end. But the ditches and small weirs of Vietnam, and the three stone works on the Min River, all chose a different way. No one tests their strength against the water. People open a path for it, then step aside and watch it choose its own course.

As for the dam of Dujiangyan, the answer is still the same as at the start: there is no dam at all. There is only a stone point shaped like a fish mouth in the middle of the stream, and a low stone bank that is always wet. And there is a narrow mountain gap that people opened more than 2,200 years ago, after burning fires against the rock for eight years. The water still flows through it tonight, as it has flowed through it every night.