By Julian Kane
Giant dams are costly engineering marvels that are built for expected benefits to humans or their political leaders, but they also have negative effects which sometimes outweigh the positive ones. The Three-Gorges Dam under construction on China's Yangtze River is a case in point. It will be one of the largest dams ever built; 1.3 miles wide and 607 feet high, and its reservoir will be 370 miles long.
The dam's positive effects are: It would prevent the recurrent catastrophic floods that have devastated China's major agricultural breadbasket for several millennia by providing a steady flow-velocity and discharge-volume of water along the Yangtze's 800 miles below the dam to Shanghai and the sea. Its reservoir will provide needed water for farms and cities during drought periods. Its 26 turbines would generate 18,200 megawatts of electrical energy, the equivalent of 18 nuclear power plants -- more than any other dam in the world. The reservoir's shoreline would spawn a recreational boating and fishing industry, and less dredging of silt would have to be done in Shanghai's harbor.
The dam's negative effects are: Roughly 2 million people would be displaced from their villages, and farms would be submerged (along with thousands of archeological sites) under the reservoir's water and silt. The Yangtze's tremendous silt load (530 million tons/year, the fourth largest of all world rivers) would end up in the reservoir behind the dam, which would steadily decrease its water-storage and electrical-generation capacity and which could render the dam useless within 150 years (unless expensive silt-dredging and disposal is done continuously). Chungking's major harbor, 1,200 miles upstream from the sea and now kept silt-free by the Yangtze's fast current, could (unless dredged vigorously) lose its marine shipping when it fills with delta sediments at the head of the new reservoir's placid waters.
China's coastline will undergo severe erosion if deprived of its normal Yangtze deltaic sediments which counteract the shore losses to typhoons and nor'easters. Industrial wastes and 250 million gallons/year of raw sewage now dumped in the river and carried to sea would accumulate in the reservoir and turn it into an unsightly, unhealthy, hypoxic, stinking mess.
Failure of the Three-Gorges Dam would release a monstrous flood, greater than most pre-dam ones, that would destroy the Gezhou Dam 30 miles downstream via a domino effect; while drowning millions of people on the fertile floodplain and in Shanghai. A dam rupture could occur as a result of warfare, sabotage or earthquake. In 1961, Xinfengjian Dam near Canton was severely damaged by a 6.1-magnitude quake caused by weight stresses of its concrete and reservoir water. Finally, the reservoir would cover the breathtaking vistas that have inspired great Chinese writers and artists for millennia -- the gorges' spectacular cliffs and waterfalls, and the beautiful birds and wildlife living there.
Some of the negative effects could be avoided (while retaining many of the positive ones) by building several small check dams on the Yangtze's tributaries instead of one huge dam directly upstream of a small one -- but this scenario, despite its practicality, is as likely to happen as a request to the pharaohs thousands of years ago would have been to build smaller pyramids.
Indeed, modern Egypt's giant Aswan Dam on the Nile River has not lived up to its advance expectations (it was built with aid from the Soviets after the US declined to help out). In addition to silt, its upstream reservoir is filling with dead water-hyacinths and other aquatic vegetation that have increased explosively in the warm, calm, nutrient-laden reservoir. The Nile's downstream waters, tamed into slow movement through canals and irrigation ditches in Egypt's rural areas, have begun to ruin the fertile floodplain soils through salt deposits in the fields that previously had been washed out of the farmlands by the annual floods which no longer occur. Expensive fertilizers must now be used to replace the soil nutrients formerly deposited on the fields by the floods, and a debilitating human disease, schistosomiasis, flourishes in rural areas because stagnant-water snail vectors can survive in the canals. And the Nile's discharge to the Mediterranean is so low that saltwater intrusion has affected the groundwater in wells near the coast.
Big dams can always be built, but that doesn't mean they will work. Why doesn't China listen to its excellent environmental and geological scientists and chart a better course for its mighty Yangtze River?
