The construction of large dams completely change the relationship of water and land that destroy the existing ecosystem balance whereas in many cases, has taken thousands of years to create. Currently there are around 40,000 large dams which obstruct the world's rivers, completing changing their circulation systems and this is not going to occur without dire environmental impacts. About past few years, the negative impacts of dams have become so well known that most countries have stopped building them altogether and are now forced to invest their money into fixing the problems created by existing dams.
Construction of the dam does not necessarily bring benefits only. But on the other hand there are adverse effects of dam construction that existed before and after construction must be considered. It will not only affect the construction environment, but can be up to the river mouth. Among the effects of dam construction is:-
ECOLOGICAL SYSTEM PROBLEMS
No doubt that problem will arise in our ecosystem. It usually starts from the beginning of construction until the affected downstream natural river. Many cases studies and much information can not be used as reference material before extinct. Indirectly, the genetic diversity of biological resources and destroyed. Many endangered species of aquatic animals, land, forest issues.
Construction of dams required extensive land clearing without any control. As in Bakun and at Lake Chini has a significant impact and can be seen clearly. The most obvious example of the Bakun dam has destroyed the natural habitat of 70.000 hectares of Singapore. Another example is the dam of Lake Chini, Pahang is also clear that there are 60 hectares of forest. Vegetative damaged ecosystems and hence have an impact on wildlife. Disturbed the natural habitat and cause the organism to find a new habitat or the continued extinction of the area.
Extinction of flora and fauna, and disruption
For example, the reproduction of freshwater turtles is threatened by this situation. It also prevents the passage of freshwater turtles and fish and thus interferes with their breeding habits and movements.
In Temenggor Dam have been identified this problem with the number of fish in the Sungai Perak terubuk decreasing. Another problem is the transfer of animals from the forest to the occupation. This causes problems for local residents who live nearby. An example is the construction of dam ordinary Bakun. Flora extinction without knowing the depth study. This problem will increase if no action is taken and control.
Others, a reservoir fishery, sometime more productive than the previous fishery alongside the river, however is created. In rivers with biologically productive estuaries, both marine and estuarine fish and shellfish suffer from changes in water flow and quality. Changes in freshwater flows and thus the salinity balance in an estuary will alter species distribution and breeding pattern of fish. Changes in nutrient levels and decrease in the quality of the river water can also have profound impacts on the productivity of an estuary. These changes can also have major effects on marine species which feed or spend part of their life cycle in the estuary, or are influenced by water quality changes in the coastal areas. The greatest impact on wildlife will come from loss of habitat resulting from reservoir filling and land use changes in the catchment area. Migratory patterns of wildlife may be disrupted by the reservoir and associated developments. Aquatic fauna, including waterfowl, amphibians and reptiles can increase because of the reservoir.
Water pollution / Water quality
Usually, the river will be tough on soil erosion and silting occurs during construction and disposal of water. From here, the water becomes brown growth of aquatic plants caused a problem occurs. But on the other hand, the river will be a shortage of minerals due to system problems slowed the flow of water caused water reservoir. Rate of mineral and oxygen was decreasing.
Further problems arise when people cultivating their crops. Use of pesticides can not be drained and recycled in a river. Steel consumption rates also affect water quality when water is released from the dam.
Damming in the river and create a lake-like environment profoundly changes the hydrology and limnology of the river system. Dramatic changes occur in the timing of flow, quality, quantity and use of water, aquatic biota, and sedimentation in the river basin. The area of influence of a dam construction projects extend from the upper limits of the catchment of the reservoir to as far downstream as the estuary, coast and offshore zone. While there are direct environmental impacts associated with the construction of the dam for examples, dust, erosion, borrow and the disposal problems. The greatest impacts result from the impoundment of water, flooding of land to form the reservoir and alteration of water flow downstream. These effects have direct impacts on soils, vegetation, wildlife and wild lands, fisheries, climate and especially the human populations in that area.
Increased pressure on upland areas above the dam is a common phenomenon caused by the resettlement of people from the inundated areas by the uncontrolled influx of newcomers into the basin catchment. On site environmental deterioration as well as a decrease in water quality and increase in sedimentation rates in the reservoir result from clearing of forest land for agriculture, grazing pressures, use of agricultural chemicals, and tree cutting for timber or fuel wood.
Construction of the dam has an adverse impact on river hydro ecosystem. Other disadvantages are affecting fish population in the river if the construction of the dam occurred at the upper river. That situation took place on the Temenggor Dam, Bersia Kenering. Deep study was done in Lake Chenderoh. Fishery catches in the dam is low and only 22 species were captured, and 15 of them consisted of fish caught by family Cyprinid. Most using drift nets, and this small shows lack large fish in that dam. Results show catch per unit effort will be between (2.7 - 12.8 kg. per fisherman-day with a value between RM4.94-32.43-per-day fishing.