Impacts Of Dams On The Riverine Ecosystem

Impacts of a dam on the riverine ecosystem

Most of the impact of the river engineering are extremely difficult and in the many cases impossible to predict with the certainty. Theories on the ecological dynamics of the river are mainly based on the short-term studies of the small temperate watershed so there is a limited understanding of the functioning of the large rivers in the temperate regions or of the rivers of any size in the tropics. Most of the major rivers in the Europe and the United States were diked straightened dredged and dammed long before their ecology or hydrology had been serious studies. In the tropics where research funds are few often the only scientific study of a river system has been done to find where best to dam it.

As every river is unique in the terms of its flow patterns the landscapes it flows through and the species it supports so the design and the operating pattern of every dam are unique as are the effects the dam has on the river and its associated ecosystems. While the greater majority of the world's largest dams and all of the major dams have been completed within the last six decades some of the environmental effects of a dam may not be realized for hundreds of years after the work of construction. A dam can be regarded as a huge long-term and largely irreversible environmental experiment without a control.

The two main categories of the environmental impacts of the dams are those which are inherent to dam construction and those which are due to the specific mode of the operation of each dam. The most significant of this myriad of the complex and the interconnected environmental disruptions is that they tend to fragment the riverine ecosystem, isolating populations of the population living up and downstream of the dam and cutting off the migrations and the other species movements. Because all of the other dams are reduce normal flooding, they also fragment ecosystems by isolating the river from its floodplain, turning what fish biologists term a floodplain river into a reservoir river. The elimination of the benefits provided by the natural flooding may be the single most ecologically damaging impact of a dam. This fragmentation of the river ecosystems has undoubtedly resulted in a massive reduction in the number of species in the world's watershed.

Some of the environmental effects of dams can benefit of some of the species. For example impounding a reservoir will create habitat for lake fish and warm water released from a reservoir can increase the abundance of the species of the fish which failed to thrive in the cool river. But because that the dams alter the conditions to which local ecosystem have adapted the overall impact of a dam will almost without exception to be reduced species diversity.

No one has yet managed to assess with any accuracy the global extent of the fragmentation of the river ecosystem by dams and water diversions. Two Swedish ecologists, however, have estimated the degree of damage to the river systems in the US , Canada, Europe and the former USSR . mats Dynesius and Christer Nilsson of the university of Umea found that fully 77 percent of the total water discharge of the 139 largest river systems in these countries is strongly or moderately affected by the fragmentation of the river system by the dams and by the water regulation resulting from the reservoir operation, interbasin diversion and the irrigation. As a result of the habitat destruction and obstruction to the organism dispersal Dynesius and the Nilsson conclude many riverine species may have become extinct over vast areas whereas populations of other have become fragmented and run the risk of the future extinction.

The permanent inundation of forests wetland and the wildlife is perhaps the most obvious ecological effect of a dam. The reservoir has flooded vast areas at least 400000 square kilometers have been lost worldwide. Yet it is not only the amount of the land lost which is important but also its quality river and the floodplain habitats are some of the world's most diverse ecosystems. Plants and the animals which are closely adapted to valley bottom habitats can often not survive along the edge of a reservoir. Dams also tend to be built in the remote areas which are the last refuge for species which have been displaced by the development in the other regions. No one has any idea how many species of the plants and the animals are now extinct because their last habitat was as destroying habitat reservoir can also cut off migratory routes across the valley and along the river. because it isolates populations this ecosystem fragmentation also leads to the risks of inbreeding from a smaller genetic pool.

The five dam Mahaweli Omega scheme in Sri Lanka the main purpose of which is to expand irrigation in the previously forested area has submerged and turned into the agricultural land the habitat of the seven endangered and two threatened animals species the purple-faced langur and the toque macaque both of which only occur on the island. One of the endangered species is the elephant 800 of which lived in the project area. An important migratory route for the animals has been cut off by the reservoirs and the canals and the animals have now become a dangerous pest for the farmers who have been brought into the area reducing the survival chances of the remaining animals.

It is often not just the forests within the reservoir area around the dam site and transmission lines and in the areas slated to be converted to the agriculture which is lost when a dam is built in a forested area . in many cases, farmers displaced by a reservoir have had to clear forests further up the sides of the valley to grow their crops and build new homes. The access to the previously remove areas allowed by new roads and the reservoirs can also accelerate deforestation every large dam which has been built in a forest area in the Thailand has attracted loggers as well as the developers who have built golf courses and resorts along the edges of the reservoirs.

The number of the fish species which thrive in the relatively uniform habitats created by the reservoirs is only the tiny fraction of the number which has evolved in the diverse niches provided by the rivers. Because pf the few area have economically valuable fish adapted to the still water of an artificial lake fishery departments across the worlds have introduced into the reservoirs a handful of species mainly types of the tilapia and the crap in the tropics and the trout bass and catfish in the temperate regions which can be reared in the hatcheries and can support reservoirs fisheries. This introduction which complete with those native species which persist in the reservoir and also spreads far upstream and the downstream of the dam have greatly magnified the effects of the dam and the diversion in harvesting the decline and extinction of the fish species sound the world.

As well as the flooding and the fragmentation of the world best wildlife habitats reservoirs have also updated some of the world most beautiful and the spectacular river scenery. Probably the greatest loss of the planet's scenic heritage to a reservoir was the inundation of the spectacular Sete Quedas waterfall at Guardian in the Brazilian-Paraguayan border now list a rock formation at the bottom of the Itaipu reservoir. At Guaira, the mighty Parana suddenly narrowed to a width of just 60 meters less than a tenth as wide as the horseshoe falls at Niagara and then thundered over 18 separate cataracts each more than 30 meters high. More water surged and boiled through the rocks and whirlpools of Sete Quedas than any other waterfall in the world about half as much again as over both falls at Niagara combined. A more imposing spectacle can Scarcely conceive a 19th-century French traveler wrote of Sete Quedas.

References:

Miller, Jr. G.T. Living in the Environment. Wadsworth Publication, 2003.

S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.