The production potential of hydroelectricity is considerable. It amounts, theoretically, to 36 000 TWh, while the resources that can be harnessed in practice are estimated at 14000TWh, more than the present world electric energy production of 12000TWh. In 1990 hydroelectricity produced was only 2200 TWh. From these numbers, it would appear that hydroelectricity could be the main alternative to the use of fossil fuels for electricity produc­tion. However, several factors will limit this possibility:

• Most of the potential is located in Asia (27%), South America (24%) and the former USSR (24%). For the industrially developed countries of Europe, North America and Japan, the unused potential is small.

• The local environmental impact of large hydroelectricity is, usually, quite significant. Not only are the local climate and ecosystem disturbed, but large populations have to be displaced. This is, certainly, the main limiting factor to the establishment of large hydroelectric dams.

• The risks of catastrophic dam rupture. In the recent past, dam ruptures led to some of the most catastrophic technological events, certainly compar­able with Chernobyl. Some of the most dreadful events were:

Morvi (India 1979) 30000 dead Vaiont (Italy, 1963) 2118 dead L’Oros (Brazil, 1960) 1000 dead St Francis (USA, 1928) 700 dead Gleno (Italy, 1923) 600 dead Logan (USA, 1972) 450 dead Malpasset (France, 1959) 423 dead.

Each year, although with less catastrophic outcomes, several dam ruptures are observed in the world.

These limitations led the World Energy Council [27] to anticipate a maximum share of 7% for hydroelectricity in the fulfilment of world energy needs.

Hydroelectricity, after the large initial investment is repaid over a period of 15 to 30 years, is very cheap. In many cases the kWh cost is less than 20.

Finally, note that if hydroelectricity is easily modulated according to need, it is dependent upon the pluviometric regime of the region where the dams are implanted. Long droughts may significantly affect its availability, as has been experienced recently in California.