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Flooding May Offset Three Gorges Dam Impact on Environment, Expert Says

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Image of Three Gorges Dam, May 2006, courtesy NASA Earth Observatory

This post is part of a special National Geographic news series on global water issues.

Annual flooding behind the world’s largest hydro-electric dam, the Three Gorges Dam in China, will be unlike that of the Amazon River or anything else found in nature.

As the reservoir of Yangtze River water rises and falls by as much as 100 feet every six months there will be a profound impact on the landscape over time, many environmental experts worry.

Among the concerns: The reservoir will contain factory toxins and raw sewage and sediment might cause the water level to rise higher than planned, threatening to flood a large city upstream and possibly even send water spilling over the top of the dam.

But perhaps the flooding phenomenon can also be put to good use, according to a wetlands expert at Ohio State University.

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The city of Kaixian was relocated to make way for the reservoir behind the Three Gorges Dam in China.

Image courtesy of Ohio State University

“This annual variation in water levels will be unlike any other natural river system in the world. If done properly, ecological engineering can minimize some of the impact,” says William Mitsch, an environmental and natural resources professor at the university.

billmich[1].jpg“Nature is going to see something it’s never seen before,” said Mitsch, also director of the Wilma H. Schiermeier Olentangy River Wetland Research Park at Ohio State. “China’s farmers and merchants should take advantage of new agricultural and business opportunities that could help mitigate some effects of the annual flooding.”

Among the possibilities Mitsch sees in an ecosystem that has such an exaggerated change in flooding levels: introducing new agricultural practices during low water levels, creating terraced ponds and wetlands along the borders of the giant reservoir, and establishing food production businesses that capitalize on the changing water levels.

Photo of William Mitsch courtesy of Ohio State University

Mitsch described these opportunities as the lead author of a letter to the journal Science published today. The letter responds to an earlier Science article exploring the dam’s potential environmental effects, especially on aquatic life in and around the Yangtze River.

The level of water in the reservoir behind the dam will top off at 575 feet above sea level during the coming winter. The reservoir pool, covering abandoned cities, houses and farm fields formerly populated by an estimated 1.5 million people, will extend over 400 square miles. Then by summer the water level will drop 100 feet, and the cycle of flooding and receding water will repeat every year after that.

Flooding is expected to extend as much as 185 miles upstream. Almost 14,000 acres of land in the Pengxi River valley will be seasonally flooded, possibly for as long as six months each year, by the pooling behind the dam.

The region will become home to an entirely new ecosystem as an unprecedented amount of water covers what used to be dry land, Mitsch says.

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This is what a flooded area surrounding the Pengxi River might look like during low water levels in the reservoir region behind the Three Gorges Dam crossing the Yangtze River.

Image courtesy of Ohio State University

Farmers planting crops in the reservoir region will need to adjust, or ideally eliminate, the use of fertilizers during growing periods. Fertilizers would leave behind undesired nutrients that would promote the development of algae once standing water covers the crop fields.

“Fertilizers actually won’t be needed,” Mitsch said, because the sediment left behind by flooding will be rich in certain nutrients, especially phosphorus, which is beneficial to many kinds of crops.

Land users also could consider constructing cascading terraced ponds and wetlands along the border of the reservoir to retain water as it recedes and to reduce the loss of nutrients to the pulsing river system.

Perhaps the most commercially viable option would be expansive fish net systems that could be placed to capture fish at the end of the flooding season, as water levels diminish.

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Two space views of the Yangtze River, before the Three Gorges Dam was built (photo below) and after the dam was completed.

Images courtesy NASA Earth Observatory

Though the original Science article explored the dam’s possible harm to fish and mammals, Mitsch’s letter noted that mudflats at or near the river system in the summer may provide ideal habitat for shorebirds and other wading birds.

However, Mitsch cautioned, it’s really too early to tell whether the reservoir behind the dam will function in typical wetland fashion, acting as a buffer between land and water, filtering out chemicals in water that runs off from farm fields, roads, parking lots and other surfaces, and absorbing and storing carbon.

“It will be a large expanse of water, but for now we just know it will present an ecologically interesting situation,” Mitsch said. “More interesting will be to see how people adapt to it.” 

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This graphic shows the area likely to be most affected by fluctuating water levels in the reservoir region behind the Three Gorges Dam.

 Image courtesy of Ohio State University

More from National Geographic News:

China’s Three Gorges Dam, by the Numbers

Water in Dams, Reservoirs Preventing Sea-Level Rise