A large dam is defined as over 15 metres in height (wall height from the base up), or with a reservoir containing 3 million cubic metres of water or more. There is no firm definition of a 'mega-dam', but roughly ten times bigger than a large dam would seem to qualify, with a benchmark of around 150 metres in wall height. And then, on this website, references are made to the MONSTER dam, roughly 250 metres or more in wall height. Monster dams rank among the world's biggest (width or height or both).
In a class all of its own is the Three Gorges Dam, starting up on the mid-reaches of the Yangtse in 2009. The Three Gorges Dam has a wall-height of 180 metres, is over 2 kilometres wide, and has a generating capacity of 22.5GW (gigawatts) or 22,500 MW (megawatts). The 600-kilometre-long reservoir at the Three Gorges Dam reached full height in 2010, submerging 13 cities, 140 towns and 1,350 villages. This dam, which has displaced up to two million people, is the biggest dam in the Motherland, or the Mother of all Dams. Its environmental impact is uncertain.
China is building dams like there's no tomorrow—it is far and away the world's first builder of large dams. There are estimated to be over 50,000 large dams worldwide. Here's an astonishing statistic: almost half that number of large dams is situated within the borders of China—over 23,000 large dams are under operation in China today. Another astonishing statistic: there are at least 11,000 more large dams in South Asia—including over 4,500 large dams in India. That brings the total of large dams in China/South Asia to over 34,000, which means roughly 70 percent of the world's large dams are situated in China and South Asia. This does not bode well for the rivers of this region.
In the Himalayan region, although there are clear and urgent signs of imminent threat from climate change in the form of rapidly melting glaciers, both China and India are forging ahead with building of large dams. Both nations appear to be oblivious of the climate change threat—neither is adopting a more cautious approach to dam building in the Himalayan region. It does not make sense to build scores of large dams in a stark future where there may be no flow of water anyway—due to major rivers drying up.
China exports its mega-dam engineering expertise and financial backing to impoverished nations in Asia, Africa and elsewhere—initiating projects with very little impact assessment, if any. These projects are potentially devastating to indigenous people, who are rarely consulted about the projects. Mega-dams can potentially trigger ecological havoc on a huge scale. For more on Environmental Impact Assessments (or the lack of them) go to Secret Dam Construction.
World's Top Four Builders of Large Dams
|China||22,000 large dams|
Note: source is the World Commission on Dams, circa 2003, which means these figures are not current. Add a few thousand large dams in the case of China to bring the number up to date.
Xiluodu Dam on Yangtse
There are big dams, and then there are mega-dams. Sichuan and Yunnan provinces in southwest China are home to the MONSTER dam. Under construction are dams with wall heights approaching 300 metres—close to the height of a 95-storey building.
Xiluodu Dam, straddling the Yunnan-Sichuan border on the Jinsha River (upper Yangtse) is coming online. This concrete colossus stands around 280 metres tall, and has a generating capacity of 12.6GW by one estimate, and 13.8GW by another estimate. Xiluodu is the second-biggest dam in China after the Three Gorges Dam, and constructed by the same company, Three Gorges Project Corporation. Not far off, the third-biggest dam in the world, Xiangjiaba, is under way—projected for completion in 2014, with a generating capacity of 6.4GW. Together, Xiluodu and Xiangjiaba would generate 20GW (gigawatts) of energy—enough to power all the homes in England. Both dams have been constructed by the Three Gorges Project Corporation, which is involved in a cascade of dams on this river. These include two more monster dams, Beihaitan (capacity 12.6GW, wall-height 277m) and Wudongde (capacity 7.4GW, wall height 235m)—both projected for completion by 2020.
What does all this mean for the Jinsha (upper Yangtse) River? Death by dams, basically. Liu Jianqing, an environmental journalist and campaigner puts it this way:
“[The Jinsha] is big and beautiful. But if you have 25 dams and every 100 kilometres there is a dam, then you don't have a river. You will never have a river again. It means you won't have fish, you will lose a lot of land and many people have to lose their homes. We call that a dead river.”
People are incensed about losing homes and valuable farmland because of the dams. There have been outbreaks of unrest along the Jinsha. In 2011, riot police quelled a "mass disturbance" in Suijiang town, where 60,000 people are being forcibly relocated to make way for Xiangjiaba Dam.
Huge dams like this have the potential for creating environmental chaos—by disrupting fish migration and sediment flow, reducing biodiversity, and by degrading water quality. The huge dam itself poses a significant safety hazard as the structure ages. The resulting reservoir displaces entire communities, floods and fragments ecosystems, increases water-borne diseases, and could trigger earthquakes.
During the building of the Three Gorges Dam, over two million people had to be relocated because of the constant threat of landslides (a group of hydraulic engineers reported that over 4,700 landslides had taken place in the vicinity by March 2007). Construction of the Three Gorges, Xiluodu and Xiangjiaba dams on the Yangtse have been plagued by corruption, technical problems, human rights problems and profound environmental impact.