In the XXth century, dams were synonymous with development and economic progress. A lot of them were built and there are now about 50 000 dams over 15 metres and about 800 000 dams with a lesser height. We have now realised that these structures bring many disadvantages; they are features of a development model that is increasingly challenged. (1)
The largest dams are gargantuan constructions. The Tarbela Dam, in Pakistan, holds 106 million cubic metres of soil and stone. That is 40 times the volume of Egypt’s Great Pyramid. It took the equivalent of 380 Eiffel towers in iron and steel to build the Itaipu Dam, in Brazil. The Three Gorges Dam, on the Yangtze River, is the longest in the world: it is 2335 metres long.
Large dams can produce as much energy as several nuclear power stations put together. The Three Gorges Dam should produce almost 20 000 megawatts by itself which is equivalent to 20 nuclear reactors. As a result, most of the renewable energy in the world today is produced by dams (63% in 2005).
In Europe, almost 10 % of electricity is made hydraulically (15% in France). It should be noted that the installed capacity is potentially more than the available power as dams do not function continuously: they are used seasonally to produce more energy when demand peaks.
Most large dams in the world have been built exclusively, or mostly, for irrigation. Hence, 30 to 40 % of the 271 million hectares in the world that are irrigated are irrigated by dams. (2) A lot of the world’s agricultural production now depends on them.
Unlike oil or natural gas, hydroelectricity is considered clean and inexhaustible. It is clearly renewable.
For a long time, the greenhouse gas assessment of hydroelectric systems was distinctly positive, even if it takes several years for the CO2 emitted during their construction to be compensated by the electricity that is produced.
However, in tropical areas, rotting in the water impoundment releases large amounts of methane into the atmosphere. This greenhouse gas is almost 20 times stronger than CO2 : the carbon count for dams could therefore be negative in some cases.
Changing the watercourse
Dams often depopulate rivers of migratory species (eels, salmon,…), even if palliative systems like fish ladders are being installed more often. (3)
Building dams also stops sediments circulating. This can reduce the quality of farmland or cause delta imbalances (the Nile delta is thus receding because of the Assouan Dam). (4)
Lastly, water impoundments considerably increase water evaporation in hot areas.
Water impoundments often flood inhabited land which leads to the displacement of the populations living there. The Three Gorges Dam in China has thus led to the displacement of over a million people.
There are 400 000 km2 of land inundated by dams worldwide (more than Germany’s land surface) and between 40 and 80 million displaced persons. (5)
In many cases, the affected populations are not appropriately compensated. They are then moved to shanty towns or onto land that is not very productive. This has triggered many working-class movements. The largest is in the Narmada valley, in India. (6)
Some dams have burst or been damaged by accidents. When the Banqio dam in the South of China gave way, it killed 100 000 people. In 1963, a rock fall in the water impoundment of the Vajont Dam in Italy produced a giant wave which killed 2000 people in the six towns below. Some water impoundments can be damaged by earthquakes. This happened in Sichuan in China in 2008.
Some dams are coming to the end of their life and must be dismantled : this is a complex and costly operation. In countries in the North, they tend to be destroyed. Almost 400 dams of varying sizes have already been destroyed in the United States. In France, several smaller dams have been destroyed on watercourses inhabited by migrating fish, to restore their environmental quality. Amongst these are the Kernansquillec Dam on the Léguer (Côtes-d’Armor), the St- Etienne du Vigan Dam on the Allier river (Haute-Loire) and the Maisons-Rouges Dam on the Vienne river (Indre-et-Loire). (7)
Small hydroelectric installations
Even though large dams produce most of the hydroelectricity in the world, there are smaller, cheaper, more profitable structures that damage the environment less. About 50% of small hydroelectric capacity is installed in China.