Lh. Seitinthang
[dropcap]G[/dropcap]overnment institutions campaign mightily for construction of new dams as they have no other reason for existing and their survival depends on building and maintaining dams and projects. Politicians often want to make certain of their constituents wealthy by encouraging ‘growth’ and join in pressuring for new dams to be built. But why do we need more of them?Once a dam is built and formed its reservoirs, the region is served with developed, filled with cities,roads,parking lots, and houses. This unfortunately, lowers the water table and river even further. Eventually, the new human populace will run out of water-but they will still want to grow demands yet another dam. Building of dams leads to the building of more dams until there is no water left to take. Whether reservoir projects are ultimately beneficial or detrimental to either the environment or surrounding human populations has been debated since the 1960s. The construction of Llyn Celyn and the flooding of Capel Calyn provoked political uproar which continues to this day. More recently, the construction of Three Georges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate. Lake Nasser behind the Aswan Dam, Egypt, 5250 km2, displaced 60,000 people. With these conflicts and pressures in mind, the World Commission on Dams began its work in May, 1998 with first points of agreement was that dams are only a means to an end. What is the end? The WCD concluded that the ‘end’ that any projects achieves must be the sustainable improvement of human welfare i.e., significant advance of human development on a basis that is economically viable, socially equitable and environmentally sustainable. If dams are the best way to achieve this goal, it deserves support.The World Commission on Dam, 2000 reported, large dams emerged as one of the most significant and visible tools for the management of water resources. More than 45,000 large dams around the world have played an important role in helping communities and economies harness water resources for food production, energy generation, flood control and domestic use, estimated 30-40 % of irrigated land worldwide now relies on dams and which generated 19% of world electricity and trend peaked in average of eight or nine large dams were commissioned each day somewhere in the world with total investment in large dams worldwide of estimated at more than $7 trillion.
Positive Impacts
From the 1930s to the 1970s, the construction of large dams became in the eyes of many synonymous with development and economic progress. Viewed as symbols of modernization and humanity’s ability to harness nature, dam construction accelerated dramatically. The benefits were regarded as self-evident, while the construction and operational costs tended to be limited to economic and financial considerations that justified dams as highly competitive options. Government officials noted that the dam will relieve the danger of flooding and reduce emissions of sulphur dioxide and carbon dioxide. It contributed to 12-16% of world irrigated areas; generating 19% of the world’s total electricity supply in 150 countries and 24 countries depend on dams for 90% of their power supply. Construction of dam is one of the main results of Industrialization for the development of the government which increased the agricultural productions through more irrigation and extend cropland. It facilitates the water way means of transportation for the surrounding areas and stabilized the amount of water flow. In 1990, over a billion people had access to less than the minimum required of 50 litres per person per day, but now due to these dams it increases triple to that situation. Fishing can be improved especially in lakes but also in rivers with introduced species, leisure i.e., boating on lakes, extended rafting season and it increased riparian vegetation if dam discharges are steady.
Negative Impacts
The first effect of a dam is to alter the pattern of disturbances that the plants and animals of a river have evolved for. Many aquatic animals coordinate their reproductive cycles with annual flood seasons. Every flood is valuable in that it takes nutrients from the land and deposits them in the river, providing flood for the stream’s residents. Floods also provide shallow backwater areas on vegetated and shaded riversides; the young of many animals depends on these backwaters to protect them from large predators. Rivers possess a delicate ecology that depends on a regular cycle of disturbance within certain tolerances. The plant and animal communities that inhabit the river and river margins have evolved to adapt to their rivers own peculiar pattern of flood and drought, slow and fast current. Dams disrupt this ecology. In many developing countries, the savannah and forest ecology of the floodplains depend on seasonal flooding from rivers. Also flood recession cropping is practiced extensively whereby the land is cultivated taking advantage of the residual soil moisture after floods recede. Dams attenuate flood which may affect the ecology and agriculture seriously. If the dam is allowed to release water from its reservoir, it will often do so only once in a while, rather than in frequent, small floods as are seen in nature. This leads to scouring and armouring of the riverbed. The higher energy of the sudden floods picks up and removes smaller sediments like silt, sand, and gravel, as well as aquatic plants and animals, leafy debris, and large wood debris. Complex sets of habitats are erased. The riverbed below the dam becomes like a pavement of cobbles and loses its value as habitat for plants, macro invertebrates, and fish.Fish passage is a concern with dams. Many fishes must move upstream and downstream to complete their lifecycles. Dams are often built without fish ladders. When fish ladders are provided, they seldom work as needed. If enough adult fishes do manage to climb above a dam, there remains the issue of their young: how will they get back downstream? Many are killed by predators while they wander, lost, in the reservoir above the dam. Many are killed in their fall downward through the dam to the river below. They aren’t killed by the fall itself, but by the high levels of nitrogen gas at the base of the dam. In other words, like divers who go too deep, they get the “bends.”
Are many fishes that cannot climb dam ladders or leap over low dams? Some of these fishes swim upstream every year to breed, then let the water carry them back downstream. The eggs of pelagic spawners float downstream, too, which is why the adults must swim far upriver to breed. Otherwise, the baby fish would soon end up out to sea. Perhaps deadliest of all to salmon and steelhead species is the typical hydropower practice of releasing large amounts of water in powerful surges during the day in order to provide electricity when demand and prices are highest and cutting down flow during the night in order to replenish reservoirs for the next day. The cyclic floods caused by this popular practice contribute to the extinction of salmon by flushing away their spawning gravels during the day and leaving them high and dry at night. Riverbeds become scoured, stripped of their organic materials, sediment, vegetation, and macro invertebrates. The damming of a river creates a reservoir upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam’s construction. The newly created reservoir has more surface area than the river would have had, and therefore more evaporation occurs than previously. This can lead to a loss of up to 2.1 meters in depth per year in some climates. Reservoirs can also contribute to greenhouse gas emissions.
The initial filling of a reservoir floods the existing plant material, leading to the death and decomposition of the carbon-rich plants and trees. The rotting organic matter releases large amounts of carbon into the atmosphere. The decaying plant matter itself settles to the non-oxygenated bottom of the reservoir, and the decomposition—unmitigated by a flow pattern that would oxygenate the water—produces and eventually releases dissolved methane.
Temperature is another problem. Rivers tend to be fairly homogenous in temperature. Reservoirs, on the other hand, are layered. They are warm at the top and cold at the bottom. If water is released downstream, it is usually released from the bottom of the dam, which means the water in the river is now colder than it should be. Many macro invertebrates depend on a regular cycle of temperatures throughout the year. When we change that, we compromise their survival. For instance, a certain stonefly may feel the cold temperatures and delay its metamorphosis. This may mean that at a certain life stage it will be living in the depth of winter rather than in autumn as it should have been. Whilst reservoirs are helpful to humans, they can also be harmful as well. One negative effect is that the reservoirs can become breeding grounds for disease vectors. This holds true especially in tropical areas where mosquitoes (which are vectors for malaria) and snails (which are vectors for Schistosomiasis) can take advantage of this slow flowing water.Reservoirs may contribute to changes in the Earth’s climate. Warm climate reservoirs generate methane, a greenhouse gas when the reservoirs are stratified, in which the bottom layers are anoxic (i.e. they lack oxygen), leading to degradation of biomass through anaerobic processes.In some cases, where flooded basins are wide and biomass volumes are high the amount of biomass converted to methane results in pollution potential 3.5 times more than an oil-fired power plant would for the same generation capacity.
Impact below dam
As all dams result in reduced sediment load downstream, a dammed river is said to be “hungry” for sediment. Because the rate of deposition of sediment is greatly reduced since there is less to deposit but the rate of erosion remains nearly constant, the water flow eats away at the river shores and riverbed, threatening shoreline ecosystems, deepening the riverbed, and narrowing the river over time. This leads to a compromised water table, reduced water levels, homogenization of the river flow and thus reduced ecosystem variability, reduced support for wildlife, and reduced amount of sediment reaching coastal plains and deltas. This prompts coastal erosion, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. Channel erosion of rivers has its own set of consequences. The eroded channel could create a lower water table level in the affected area, impacting bottomland crops such as rice or wheat, and resulting in a smaller supply.
Dams and the creation of reservoirs require relocation of potentially large human populations if they are constructed close to residential areas. It affects society in three ways; an economic disaster; human trauma and social catastrophe (Michael Cernea and Thayer Scudder). Most of the major hydro-projects submerged some of the most famous historical scenery and tourist sites. The weight of millions of tons of power behind dam can increase the chances of an earthquake. Natural fertilizers are not replenished by annual floods, and the use of artificial fertilizers increases toxic minerals and salinity. These enter the food chain in potentially toxic levels. Waste of human and non-human origin is concentrated by lack of drainage, in places leading to increase disease threats. Downstream, where such contaminated water enters an ocean or sea, even the large bodies of water are affected.
