Dams are any barrier constructed across the river, streams or other water bodies etc to regulate the flow of water. Dam construction brings both economic and social development of the region. It boosts the economy of the region by providing employment to the local people during its constructions and providing a source of recreation after its completion. Apart from becoming a major tourist spot, the reservoir and the adjoining areas of dams can also be used for navigation, fishing, irrigation etc.
Based on its functions dams are usually divided into the following types:
Storage dams
Storage dams stores water in its reservoirs for various purposes like providing drinking water, irrigational purposes during lean periods etc.
Check dams
Check dam are usually small in size. It controls the soil erosion and also checks flood by regulating the amount of water flowing downstream.
Diversion dams
Diversion dams divert the water from its natural course to different channels, storage ponds etc.
Overflow dams
Overflow dams increase the water level of inland water ways for navigation of boats, barges etc. It is also used for measuring the flow of water
A multipurpose dam serves many purposes including the generation of electricity using the energy of the water that falls down.
Adverse Impacts
Though the dam serves many purposes, the environmentalists have often expressed great concerns about the adverse impacts it has on the environment, particularly the construction of large dams. According to the International Committee On Large Dams (ICOLD), large dams have been defined as those dams that are more than 15m in height or has more than 1 Mm3 storage capacity. Some of the major impacts of large dams are:
Submergence of lands
Construction of dam causes submergence of low lying areas around the reservoirs. The submerged lands include both residential areas and agricultural lands. It was reported that the construction of Narmada dam had affected 162 villages of which 82 were submerged completely. When submergence of land occur resettlements of affected human population is often a big issue as generally it would mean restarting their life in a totally new environment. If the nature and the productivity of the land are quite different from their previous land, it could be quite risky for these new settlers particularly for those who were practicing agriculture, animals rearing or fishery.
Deforestation
Often forests have to be cleared at and around the site for structural constructions and also for making the temporary residence for the workers. Consequently, the natural habitats of many animals and plants would be destroyed. This could threaten the existence of the population of wildlife including many of endangered species. For instance, the population of Irrawaddy dolphin (Orcaella brevirostris) is threatened by the construction of Irrawady Mali dam.
Riparian vegetations
Riparian agriculture is the agriculture practice near the river banks. Here the productivity of the crops is dependent on the fertile silt brought down by the river water. When dams are constructed, the downstream region receives less silt and sediment, affecting the nature of vegetation growing. Mention may be made of the change that took place at the river Colorado River basin in Southwest United States which was once rich in vegetations like cottonwood and marsh now consist of dry, barren areas of tamarisk and grass, after the construction of Hoover Dam, Glen Canyon Dam, Davis Dam, Palo Verde Dams etc
Sedimentation
While the downstream river receives less sediment, the reservoir is also affected by the deposited sediments. The deposited sediments go on piling up in the reservoir and reduce its storage capacity gradually. At a same time, the pressure created by its weight and the large volume of stored water may cause the reservoir induced Seismicity (or RIS which means the earthquake caused by the reservoir). The pressure created by the water in the reservoir is so much that sometimes it could also lead to bursting of the dams such as the incident of Doñana Disaster that took place in South Spain on 25 April 1998
Deterioration of Water Quality
As compared to the flowing river water less oxygenation take place in the stagnant water of the reservoir. As a result, the dead plants and animals which are submerged under water at the reservoir start decomposing under anaerobic conditions. Besides releasing gases like carbon-dioxide and methane, it can change the nature of the water. Reservoir can also change the chemical components of the water. For instance, mercury is naturally present in the soil in harmless inorganic form. But the bacteria that anaerobically decompose the dead organism convert the inorganic mercury to organic methyl mercury which affects the nervous system in living organism. Water pollution also increases when they are stopped from flowing downward to the open sea. To cite an example, the Yangte river of China has become more polluted with the construction of Three Gorges Dam
Scouring the Riverbed
When water is released from the dam, it will flow downstream under great pressure and energy. As a result, it tends to pick up greater materials like sediments, silts, debris, large woody materials etc on the way transforming the river bed into a cobbled like thing. Often, the woody debris serves as the habitat for smaller fish and aquatic organisms while leaves, phyto-planktons, zooplankton serves as the food materials for other higher aquatic organisms. So, when the riverbeds are scoured, the populations of many aquatic organisms are affected by the destruction of the habitats and the decrease in the populations.
Change in the water temperature
River water has almost same temperature throughout but in the reservoir as the sunlight cannot penetrate through the deeper layers, the top layer is quite warmer while the bottom layer is comparatively cold. Normally when the water is released from the dams, it is released from the lower portion which contains only cold water. Such difference in the water temperature disrupts the lifecycle of those species down- stream which are sensitive to the change in temperature such as the stonefly or the salmon fish.
Affects fish population
Dam construction affect the fish population in different ways. As already said, dam destroys habitat of many smaller organism by scouring the riverbed. Another important issue is the disruption in the lifecycle of different species of fish which breeds in the river water but migrates in the sea or ocean for maturation e.g. salmon, trout etc. They again move upstream during breeding season. With the construction of dams, the fish are unable to complete their lifecycle in a suitable environment.
Water-Related Diseases
The stagnant water of the reservoirs becomes good breeding place for some vectors of diseases like mosquitoes and snails, thus spreading diseases like malaria and schistosomiasis. Such stagnant water if contaminated are also likely to spread water borne diseases like dysentery, cholera etc.