It should not be implicitly assumed that it will be meeting a real need. It may be that demand side management could reduce the need or that the demand has been overestimated.

For an investment as large as a dam, an accurate needs assessment is essential – otherwise the project may fail to satisfy its objectives and produce a suitable return.

These considerations are not purely academic. If a real demand has been established then any dam which is subsequently built is also likely to be a better investment.

On the other hand if a dam is built to satisfy unsubstantiated demand projections, it will be less secure as an investment.

Dam construction can have environmental benefits, although some of these may not be planned or foreseen.

• Reservoirs may become important for birdlife. Of 1200 wetlands of international importance under the Ramsar Convention, 100 are artificial with 78 of these being water storage areas.

• Dams often support commercial and/or recreational fisheries and the release of cold water from reservoirs encourage downstream populations of cold water fish, such as trout.

However, dams have caused much more environmental damage than they have brought benefits. For example:

• Dams can have a devastating effect on downstream fisheries as they obstruct the movement of migratory fish. Fish ladders have been used in some dams but not always effectively.

• Dams directly affect downstream wetlands as the incidence and severity of flooding decreases and water quality and sediment loads are altered.

• Biodiversity loss is caused by dam construction. Habitats are lost, both in the reservoir area and downstream. This is what is likely to happen should the Talo Dam In Djenne ( Mali ) go ahead. Some dam projects, like the Kárahnjúkar project in Iceland, threaten some of our last wilderness areas.

• Water quality can be degraded. Salinity increase can make the water unusable for drinking and for irrigation, as there is no longer enough water travelling downstream to flush the ecosystem. Decomposition of organic matter and the leaching of mercury from the soil may also introduce toxins.

• There is concern about the high levels of emissions of greenhouse gases such as carbon dioxide and methane, from some dams in the tropics with shallow reservoirs. In the initial stages, the emissions are mainly due to the rotting of flooded vegetation, but this process usually continues with the deposition of organic matter by in-flowing streams. More research into this matter is needed urgently.

• When large dams fail the results can be catastrophic, and can be as lethal as the floods that some dams are trying to contain. This can been seen with Vaiont dam in Italy.

• Dams can trigger earthquakes due to an effect known as ‘reservoir induced seismicity’. This is where the weight of water combined with water seeping deep into the pores of the underlying rock strata changes the area's response to existing stresses.