Do you think that more dams should be built, or that we should start dismantling them?

Dams

Humans have been creating reservoirs of water for a long time, whether using the craters of extinct volcanoes to store water for irrigation as was done in Arabia 5,000 years ago, or creating mill ponds to drive waterwheels to grind corn, or today's hydroelectric dams which use the power of rivers to create electricity. Once created, these reservoirs may also be used for fishing or for leisure pursuits. In some cases, the main purpose is flood control or reclamation of low-lying land. Frequently a reservoir may have several purposes. There were no big dams (higher than 15m) before 1900; by 2000 there were 5,270. The first major (over 150m) modern dam was the Hoover dam in the USA, finished in 1935. By 1995 there were over 300 major dams. The Three Gorges Dam, China, will be the largest.

Of course, there are benefits to building dams - otherwise we would not build them in the first place:

• storing water (evening out the seasonal flow) for irrigation, for cities, or for industries such as mining
• providing electrical power
• flood control
• economic benefits (including recreation)
• dams can be symbols of great wealth and power
• once running, hydroelectric power stations do not add to global warming and other emissions like fossil fuel plants do
• hydroelectric power stations are a renewable source of energy

There are always consequences when dams are constructed.

• agricultural land, homes, habitats essential to endangered species and even priceless archaeology may be drowned (for example prehistoric rock art engravings by the Alqueva dam, Portugal, or Egyptian artifacts by the Aswan High Dam). This can cause great hardship for the people who lived or grew crops there
• the seasonal nature of the water flow may be even out, destroying a way of life for the people and animals that depended on it downriver
• if the water is not returned to the river, for instance if it is used for irrigation or to supply a town or city, then the people downriver may experience drought and water shortages. Rivers have even been known to run dry. In extreme circumstances this could lead to war.
• The life cycle of fish such as salmon which return upstream to spawn may be interrupted, as well as the food source of those animals that live on them and the fishing industry built around them.
• The gradient of the river upstream may be changed, making it flow more slowly, so that sediments settle out. This affects both vegetation and animals living in or beside the river. This can affect where pollution concentrates, too. This sediment will be lost to lands downstream. For example, the Nile Delta has not been receiving enough sediment to offset wave erosion since the Aswan Dam was built. The creation of large areas of still waters can also lead to increased risk from waterborne diseases, including mosquito-borne diseases such as malaria.
• If a dam collapses it can cause considerable devastation. For example, 62 dams broke in China in August 1975, killing more that 85 thousand people. The Teton Dam in the USA failed on June 5 1976. The Los Frailes tailings dam failure on April 25 1998 released 4-5 million cubic meters of toxic waste over agricultural land and into rivers, threatening the Doñana National Park, a UN World Heritage Area. There was massive flooding and several dam and reservoir failures the Saguenay River area of Quebec in July 1996.
• Changing from annual flooding to a river to irrigation increases the risk of salinization, which can eventually lead what was very fertile land to become unsuitable for agriculture.
• The process of construction of dams can cause considerable pollution and use up a lot of energy.
• Removal or drowning of trees may add to global warming
• Working out how to remove dams if they are no longer required can be a problem

Whether dams are worth building tends to be an emotive issue - those who stand to benefit the most end to be for them, and those who stand to lose the most - often those living where the dam is to go are against it. For those beside the river, it can be difficult to tell whether they will gain or lose more. In some parts of the world, the best hydroelectric sites have been dammed already, but there are still major plans for dams around the world.

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Do you think that more dams should be built, or that we should start dismantling them?

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