Most dams are built to store water to compensate for fluctuations in river flow, thereby providing a measure of human control over water resources, or to raise the level of water upstream to either increase hydraulic head or enable diversion of water into a canal. The creation of storage and head allows dams to generate electricity; to supply water for agriculture, industries, and municipalities; to mitigate flooding; and to assist river navigation. However, the environmental costs far outweigh the benefits.
Environmental problems caused by dams include:
- Soil Erosion
Dams hold back the sediment load that is normally found in a river flow, thus depriving the downstream of fertile soil. In order to make up for the sediments, the downstream water erodes its channels and banks. This lowering of the riverbed threatens vegetation and wildlife.
- Species Extinction
Many fish species rely on the ability to move about freely in a river to seek food or to return to the place they were born. The vast majority of large dams do not include proper bypass systems for these animals, interfering with their life cycles and sometimes driving the species to extinction. Migratory species are the worst affected in this scenario. The reproduction of migrating fishes is also hindered by the floods that harm the egg beds.
- Spread of Disease
Dam reservoirs in tropical areas become breeding grounds for mosquitoes, snails and flies due to their slow movement. These vectors are responsible for spreading malaria, Schistosomiasis and river blindness. Dams may also cause increases in water sourced illnesses like typhus, typhoid and cholera.
- Changes to Earth’s Rotation
Large dams cause changes to the earth’s rotation because of the shift of water weight from oceans to reservoirs. Due to the number of dams which have been built, the Earth’s daily rotation has apparently sped up eight-millionths of a second since the 1950s.
- Sedimentation
Sedimentation is the process by which larger sediments in water entering a reservoir are deposited at its upper end forming a delta and steadily raising the level of the upper reaches of the reservoir. This results in flooding.
- Salinisation
The arid and semi-arid areas are incapable of handling large amounts of water brought by irrigation. Irrigation water has more saline content and adds more salt to the system leading to the increase of salinization. Changes in the salt regime can affect the entire ecosystem and disrupts breeding of fishes.
- Changes in Water Quality
The temperature of water, salt and oxygen distribution may change vertically as a consequence of reservoir formation. The water quality will also be changed seriously as a result of drainage water returning from irrigation which contains fertilisers. This results in algal blooms which suck the oxygen out of the water, making the water acidic and potentially harmful to people and animals.
- Increase in Earthquakes
Some increase in earthquakes may be there because of the filling of big dam reservoirs.
- Changes in Climate
As reservoirs fill, upstream forests are flooded, eliminating their function as carbon sinks. As the drowned vegetation decomposes, decaying plants in manmade reservoirs release methane, which is a powerful greenhouse gas.
- Increased Evaporation
Since more surface area of the water gets exposed to the sun, reservoirs result in much more evaporation than the natural flow of the river before that dam existed.
Although dams do represent a renewable source of energy, these manmade structures are far from green. Solar, wind and geothermal energies should be explored as alternatives to hydroelectricity which is a bane not only for the environment but also for the people living downstream the dams.
