Dams and the Environment
A dam built across a river will obviously have a major effect on the river valley upstream of the dam which will be flooded as the new storage reservoir fills. Less obvious is that the river downstream of the dam will also be significantly affected. Large dam projects are highly individual in their design, geological setting and the construction materials used to build them. They are also individual in their impact on their environment. Some large dam projects in tropical Africa have created lakes hundreds of kilometres long in areas which had large local populations. The major impacts that these projects had on the plant, animal and human population of the area have been well documented, however it would be a mistake to assume that all dam projects necessarily have similar major impacts on the environment.
Some adverse effects of building a dam are easy to mitigate during the design of the dam as the following example shows. Fifty years ago a typical dam could release water only from the bottom of the storage reservoir. This water was very different from the water that would have flowed down the river before the building of the dam. Water from the bottom of a storage is usually cold and depleted in oxygen compared to normal river water and this had adverse effects on animal life in the river downstream of the dam. Since about the 1980s dam outlet works are usually specifically designed so that the adverse effects described above do not occur when water is released from the dam. Today's dams have an intake tower with withdrawal ports at different levels so that water can be released from the top layer of the reservoir regardless of the storage level at the time.
Provision of fish ladders is another example where dam design can remove or reduce an adverse effect of dam building. Today every reasonable effort is usually made to reduce the effect of the dam project on the environment eg borrow areas for clay, sand and gravel construction materials needed to build the dam are located, if possible, in the area which will be flooded by the reservoir so that the disturbed areas will not be visible after the dam is completed.
Not all adverse effects can be so easily removed. Building a dam changes forever the flow regime in the river: floods are much reduced in frequency and size and the natural pattern of short duration floods and long periods of low flows is changed to a less variable flow regime. In fact the reduction in flooding may be one of the reasons for building the dam in the first place. Flooding is damaging to humans and their property but may be necessary in the life cycles of some species of trees, fish and birds. It may be possible to at least partially mitigate these adverse effects on the natural environment by arranging water releases from the dam at specific times of the year to mimic the natural flooding that occurred before the dam was built.
"The Dam Site" tries to take an objective and scientific approach to the advantages and disadvantages of dams. Many large cities and developed, industrial societies could not exist without large dams but it cannot be denied that some large dams have caused major environmental problems as described on the following sites:
Critique of World Bank's Experience with Large Dams
Climate change dooms dams
Tailings dams associated with mining projects can pose additional environmental hazards eg Omai Tailings Dam in Guyana, South America. This gold tailings dam released cyanide slurries into the environment when it failed in 1995. Once mining has finished rehabilitation of the site may prevent future problems eg Mary Kathleen uranium mine in Australia.
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