Today, to make a case for a major hydropower project, a thorough environmental assessment will usually be necessary and in most cases it will be mandatory. The effects of the project, including any necessary resettlement, effects on bio- diversity, the potential for seismic activity, and the impact on areas downstream of the project, will all have to be evaluated. Such a study should include pro- posals for the mitigation of any negative effects of the development. In many cases, particularly where international lending agencies are involved, a project will not be permitted to proceed unless the environmental assessment is favor- able. This is equally true of public sector and private sector projects.


The most divisive effect of any large hydropower project is likely to be the need to resettle people whose homes or communities will be destroyed. In building the Three Gorges Dam, the Chinese government moved 1.3 million people and more people might need to move if the reservoir banks become unstable. This represents one of the largest resettlement programs for a hydropower project, but even with much smaller numbers the result will be extremely disruptive for those involved.

Resettlement numbers can be large as in the case of the Three Gorges Dam, but how is one to judge if they are too large? One way of at least comparing projects is to determine the number displaced for each megawatt of generating capacity installed. For the Three Gorges Dam this ratio was 71. The Kedung Ombo Dam in Indonesia, a 29 MW project that led to the displacement of 29,000 people, had a ratio of 1000 people/MW. In contrast, the Grand Coulee Dam built in the 1930s in the United States had a ratio of 2.

If people are to be displaced then a rule of thumb for modern developments is that they should be better off economically afterwards than before. More than that, people being moved should have a large say in where they are moved to and a stake in the project. If the project can provide wide community benefits then people will support it. If not, then development should be questioned.

The situation becomes more difficult when whole communities and their cultural and religious sites are likely to be destroyed. It is sometimes possible to move such sites; the most high-profile example of this is the rebuilding of an Egyptian temple before inundation of the Nile behind the High Aswan Dam in Egypt. However, cultural and religious beliefs may make such a solution unacceptable. In many cases, particularly in remote parts of the world, such considerations are all too easily ignored.


Even if a dam and reservoir does not displace many people it can still have an enormous impact if it affects a large area. The relative impact in this case can be crudely assessed by calculating the area inundated for each megawatt of generating capacity. This ratio for the Three Gorges Dam was 317 ha/MW while for the Grand Coulee Dam it was 5 ha/MW. Meanwhile, the Kompienga Dam in Burkina Faso achieved the score of 1426 ha/MW, the highest of any recent project. Again this is only a broad indication of the effect since it will also depend on the type of terrain that is being submerged. However, large, shallow reservoirs will always have more impact by this measure than deep, narrow ones.

The greatest danger to biodiversity is that a project will destroy the home of an endangered species. Since hydropower projects take a long time to develop, it is possible to create a new habitat to replace the one that is threatened while work continues on dam construction. This can be relatively straightforward for plant species but can be much more difficult for animal species. However, it is feasible. Indeed, some older projects are now introducing managed habitats that were not considered when the plant was initially built.

The effect of dam and reservoir construction on aquatic species is less obvious but can be equally dramatic. In France several rivers no longer support salmon as a consequence of dams, and in China the Yangtze dolphin was declared extinct in 2006, partly as a consequence of hydropower developments along the river that prevented its movement.

The water in deep reservoirs can become deoxygenated, affecting aquatic life that might otherwise live there. On the other hand, the creation of a reservoir can provide new opportunities for fish species and large reservoirs can allow fish farms to develop, creating a new industry that did not previously exist.

Geological Effects

There is a growing body of evidence that the construction of dams and inundation of reservoirs can generate seismic activity in the underlying strata as a result of the pressures generated at the surface. Such effects are normally only found with large dams, over 100 m high. The activity is generally short-lived but in some cases it can persist. And earthquake in Sichuan province in China in 2008 has been linked to a dam. This earthquake caused the loss of 80,000 lives.

Another danger is of landslips in the region around the reservoir. A case of this type in Italy in 1983 caused a reservoir to overtop, leading to the loss of 2600 lives. Such landslips are not only potentially fatal, as in this case, but they also reduce the volume of the reservoir and therefore its utility.

Sedimentation and Downstream Effects

All rivers carry a load of small particles that are borne downstream with the water. When a river is dammed, this load of sediment will often simply settle in the bottom of the reservoir and slowly fill it up. The reservoir for the Sanmen Gorge hydropower plant on the Yellow River in China lost 40% of its volume to sediment in four years. While this is a dramatic case, most reservoirs have to deal with this problem on some scale.

Normally sediment deposition will reach a steady state with enough sediment being carried past the dam to balance that being deposited each year. It is sometimes possible to wash sediment away periodically by opening the sluice gates of the dam. However, sediment is made of abrasive particles and its passage through the plant’s turbines will cause wear that may eventually lead to the need for repair or replacement.

Many of the most important environmental effects of sediment deposition are felt downstream of the dam. One immediate effect of loss of sediment is to increase erosion immediately below the dam site. More dramatic is likely to be the effect of loss of sediment on downstream habitats that rely on it. When the Aswan Dam was built on the Nile River it prevented vital sediment reaching the Nile delta. This sediment was the source of the delta’s fertility and its loss led both to delta erosion and to a rapid increase in the use of artificial fertilizers. Problems in the Black Sea with algae have also been linked to loss of sedimentary material as a result of dams on rivers such as the Danube.

Against this, a dam on a major river such as the Danube or the Yangtze can help control flooding, making downstream regions much safer. It can also make the river navigable upstream, which can be a benefit for local communities.

Greenhouse Gases

Hydropower projects are generally classified among the power generation schemes with the lowest greenhouse gas emissions. Typical greenhouse gas emissions are 10–13 kg/MWh, similar to that of wind power plants. Not all hydropower schemes are low emitters, however. Some can generate significant quantities of methane, a potent greenhouse gas.

Methane is produced when organic material collects in the bottom of a reservoir where the water is deoxygenated. Under these conditions anaerobic digestion takes place releasing methane gas. To prevent this, project developers should try to remove as much organic material as possible from the region to be inundated before submersion takes place by felling trees and clearing undergrowth where possible. Even so it will be impossible to remove everything.

The Canadian utility Hydro Quebec, which has studied this effect, has found that methane production from reservoirs normally follows a predictable cycle. Production peaks between 3 and 5 years after the reservoir is filled. After 10 years emissions are no greater than for natural lakes. However, there have been cases where much higher levels of methane emissions have been detected.

Interregional Effects

Dam construction can lead to political disputes when rivers cross national boundaries. For example, the damming of the Euphrates River in Turkey has reduced water flow through Syria and Iraq and this has led to frequent disputes. Reduced water flow when water is taken upstream for irrigation or drinking is one problem. Others may relate to sedimentation issues discussed earlier. In all cases, however, friction is likely unless great care is taken with such developments.

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