Hydropower is often described as a renewable source, but it is so different in many respects from the other renewable sources that it is best treated by itself. It is a well-tried source that now accounts for about 3% of world energy production at prices comparable to those of the major energy sources. Renewables are expensive because the available energy, though very abundant, is spread over huge areas and has to be collected. In the case of hydropower the collecting is done for us by the river valleys. To ensure a steady flow of water and to increase the depth of fall, the rivers are dammed to form lakes. The water flows through the turbines to produce electricity. Hydroelectric power provides much of the energy in mountainous countries like Norway, Switzerland, Iceland and Austria.

Hydropower differs from the other renewables in being generally reliable and relatively rather dangerous due to the possibility of dam bursts. It cannot be significantly expanded because most of the suitable rivers in the developed countries have already been used. There are several possible sites in the developing countries, but these are usually far from centres of population and so are vulnerable to guerrilla attacks. Saboteurs have often blown up the pylons carrying electricity from the Calboro Bassa dam on the Zambesi river. Hydropower occupies large areas of land and ultimately the lakes silt up and are no longer useful. Severe problems have been experienced in several countries heavily dependent on hydropower (Norway, New Zealand, Brazil and the USA) during periods of prolonged drought.

Hydropower inevitably inundates large areas of land, often including villages and fertile agricultural land. It can also disrupt the ecosystems of the surrounding areas. The dam prevents the upstream migration of fish such as salmon, though this may be overcome by building fish ladders. The areas below the dam are deprived of silt that is valuable for the soil; this was an important disadvantage of the Aswan dam on the Nile.

In a few places there are areas of land below sea level that could be used to generate electricity hydroelectrically from sea water flowing into them. It has been estimated that 4000 MW could be generated in this way using the Quattara depression in North Africa.

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