Millions of people have been displaced by dams, while 60 per cent of the world’s rivers have been affected by dams and diversions, according to World Commission on Dams(WCD 2000).
WCD found that large dams have extensive impacts on rivers, watershed and aquatic ecosystems. These impacts are more negative than positive and, in many cases, have led to irreversible loss of species and ecosystems.
There has been limited success to counter the ecosystem impacts. The failure to account for impacts on downstream livelihoods, have lead to impoverishment and suffering of millions (WCD, 2000).
In the United States, whose 5,500 large dams, make it the second most dammed country in the world, building of large dams has been stopped and a huge amount is being spent on trying to fix the problems created by the existing dams.
The local groups who bear the social, environmental costs and risks of large dams, especially the poor, vulnerable and future generations are often not the same groups, which receive the water and electricity services or the social and economic benefits.
Studies lead to large inequities in the distribution of costs and benefits. The environmental and social costs of large dams are poorly accounted for, in economic terms. As such, the true profitability of these projects remains elusive (WCD, 2000).
Large dams provide opportunities for corruption, especially in the resettlement sector. Dams change landscapes and lead to irreversible impacts. Quite often, alternatives to large dams exit, but either these are not assessed fully or, the range of alternate options is narrowed by the ardent dam proponents.
A 1990 internal survey of the World Bank hydroelectric dam projects showed that 58 per cent were planned and built without any consideration of downstream impacts.
In 2000, around 3,800 cu km of fresh water was withdrawn from the world’s lakes, rivers and aquifers. This was twice the volume extracted over 50 years ago (WCD, 2000).
The increase in large dams in 20th century was rapid and, about 5,000 large dams were constructed by 1949. Later the speed picked up and by the year 2000, over 45,000 large dams were built in 140 countries. But, in the last two decades, there has been dramatic decline in the construction of large dams, especially in North America and Europe (WCD, 2000).
In view of large-scale problems and risks, associated with large dams, the current trend is towards the decommissioning of large dams.
According to the World Commission on Dams, momentum for river restoration is accelerating in many countries, especially in United States, where nearly 500 dams, mainly old small dams have been decommissioned. Since 1998, the decommissioning rate for large dams has overtaken the rate of construction in the United States (WCD, 2000).
An estimated 0.5-1 per cent of the total fresh water storage capacity of existing dams is lost each year to sedimentation in both large and small reservoirs worldwide. This means that 25 per cent of the world’s existing fresh water storage capacity may be lost in the next 25 to 50 years in the absence of measures to control sedimentation. This loss would mostly be in developing countries and regions, which have higher sedimentation rates (WCD, 2000).
China, United States, India, Spain and Japan are the top five dam building countries, which accounts for nearly 80 per cent of all large dams worldwide.
Environmental impacts: An important impact of large dams is the fragmentation of rivers. Dams are, by far, the main threats to the riverine ecosystems, fragmenting and transforming aquatic and terrestrial ecosystems, with a range of effects that vary in duration, scale and degree of reversibility (WCD, 2000).
In case of Sindh, construction of storage reservoirs and increased water withdrawal upstream, have adversely affected the ecology of riverine areas downsteam of Kotri barrage.
Sea intrusion has occurred to a large extent; mangroves population has been badly affected. There is increase in average groundwater salinity; soil erosion has occurred; lands have been rendered unfertile; fishermen have been rendered jobless; and people’s livelihood has been affected due to significant economic losses.
There appears to be a large-scale ecological disaster in the making.
The watersheds of the world are habitat of 40 per cent of the world’s fish species and, provide many ecosystem functions, ranging from nutrient recycling and water purification to soil replenishment and flood control. At least 20 per cent of the world’s more than 9,000 fresh water fish species have become extinct, threatened or endangered in recent years (WCD, 2000).
Dams retain sediments on their upstream side, especially the heavy gravel and cobbles. The river, which looses, its sediment load, seeks to recapture it by eroding the downstream banks, undermining bridges and other river structures.
Riverbeds are typically eroded by several meters within a decade of first closing a dam. The damage can extend for tens or even hundred of kilometres below a dam.
Within nine years of closing Hoover Dam in the US, the riverbed below the dam had lowered by more than four meters. Riverbed deepening will also lower the groundwater table along a river, threatening vegetation and local wells in the floodplain and requiring crop irrigation in places where there was previously no need.
The depletion of riverbed gravels reduces habitat for many fish that spawn in the gravelly bottom and, for the invertebrates such as insects, mollusks and crustaceans. Changes in the physical habitat and hydrology of rivers are implicated in 93 per cent of freshwater fauna declines in North America.
According to the US Environmental Protection Agency (EPA), building dams can generate and release a multitude of non-point source pollutants, both upstream and downstream from the dam. Impacts from these pollutants can cause a number of problems, including changes in water temperature, dissolved oxygen values, salinity, turbidity, habitat and living resources.
The silting of dams can lead to the loss of habitat resulting from the inundation of wetlands, riparian areas and, farmland in upstream areas of the impounded waterway, or erosion of these resources in downstream areas (USEPA, 1995).
Dams and river diversions have wiped out vital habitat, decimating fish populations and have pushed numerous aquatic species to the brink of extinction (Postel (1999), Pillar of Sand, WWI).
In Colorado River Delta, flow diversion caused severe environmental consequences. Today, the delta is a desiccated, place of mud-cracked earth, salt flats and murky pools. Virtually the entire Colorado’s flow is captured and siphoned off upstream to fill swimming pools in Los Angeles, generate electricity in Las Vegas and irrigate crops in the deserts of Arizona,California and Mexicali Valley. Most of the wildlife is gone, and the fishermen are at risk of extinction (Postel (1992), Last Oasis, WWI).
In the Nile River basin, the Aswan High Dam has greatly altered the river system. Out of 47 commercial fish species in the river prior to - the dam’s construction, only 17 were still harvested a decade after its completion. The annual sardine harvest in the eastern Mediterranean dropped by 83 per cent, likely a side-effect of the reduction in nutrient-rich silt entering that part of the sea (Postal (1996), Dividing the Waters, WWI).
A significant impact of large dams is the displacement of large population atones, who thrived on downstream riverine resources. The loss of access to natural resources of riverine communities and, uprooting of their heritage has profound economic, social and cultural impacts on them.
The construction of large dams has led to the displacement of an estimated 40-80 million people worldwide. Many of them have not been resettled or received adequate compensation. Between 1986 and 1993, an estimated four million people were displaced annually by an average of 300 large dams starting construction each year (WCD, 2000).
Dams have been responsible for the production of greenhouse gases. Their environmental impacts are felt at on a wider scale, far downstream from the place where they are constructed. Change in ecosystem functions, due to flow regulation, alters the biological diversity and the carrying capacity of the downstream areas. This, in turn, affects the communities’ abilities to make an endured living.
Inundation of reservoir area destroys terrestrial plants and forests. They displace wildlife and animals. Native species are wiped out and, at times, exotic and invasive species are introduced, resulting in ecological consequences.
An impact, which has received relatively less attention, is the affect of dams on human health. Dams are notorious in increasing the population of vector-borne diseases. In case of Aswan High Dam, dramatic increases in snail’s population lead to the propagation of schistosomiasis. Health problems associated with large dams include diseases like yellow fever, sleeping sickness, Chagas’s disease, and leishmaniasis.
These arise from the proximity of human habitations to undeveloped jungle, bush or desert, in which the animal carriers of the diseases are found. In addition, large dams radically change the ecological regimes and, with it, a new pattern of the following infectious diseases is established: schistosomiasis, onchocerciasis (river blindness) transmitted by blackfly Simulium; malaria transmitted by Anopheles mosquitoes; arboviral infections caused by numerous viruses; and filariasis caused by Culex and Aedes mosquitoes (Cairncross and Feachem, 1993).
Water resources development in the Senegal River Basin has resulted in epidemics of bilharzias (schistosomiasis) and rift valley fever in areas that had previously been unaffected by the depilating diseases. Malaria cases have proliferated as mosquito vectors found many new breeding sites (Abranaovitz (1996), Imperilled Waters, Impoverished Future, WWI).
Eutrophication problem is associated with dams. Excessive nutrients cause algal blooms (toxic cyanobacteria) and weed growth. In China, a high degree of primary liver cancer has been linked to the presence of cyanobactenal toxins in drinking-water.
Bacteria feeding on the rotting biomass in reservoirs transforms the naturally-present, harmless mercury to methyl-mercury, which is harmful for the central nervous system (WCD, 2000).
Although, it has now become very difficult to build destructive river projects in the US and many other dammed countries, dam proponents and financial institutions continue to export this obsolete technology, much in the same way, the chemical industry continued to export pesticides long after they had been banned in the county of origin.
At dam conferences, the talk these days always centres around finding fresh markets to exploit and new ways to sell dams to skeptical public (IRN, 2001).
Examples: The down-to-earth case history of some dams is reviewed here, so as to fully comprehend the environmental impacts of large dams.
Prior to the construction of Aswan High Dam, the Nile River carried about 124 million tons of sediment to the sea each year, depositing nearly 10 million tons on the floodplain and delta.
Today, 98 per cent of that sediment remains behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt’s floodplain agriculture. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river (IRN, 2001).
Along the mouth of the Volta River in Ghana, Akosombo Dam has cut off the supply of sediment to the Volta Estuary, affecting also neighbouring Togo and Benin. Their coasts are now being eaten away at a rate of 10-15 meters per year. A project to strengthen the Togo coast has cost $3.5 million for each kilometre protected. The story is the same on coastline after coastline, where dams have stopped a river’s sediments (IRN, 2001).
Unhindered development of the irrigated agriculture took place, in the last over three decades resulting in a rapid decline in the annual flow of waters from the Amu-Darya and Syr-Darya to the sea. In some years, the water of Amu-Darya did not even reach the seashore.
As a result, the Aral Sea began to shrink. The water level of the sea fell by 14 meters. Its area shrunk by more than 40 per cent and its volume decreased by more than 60 per cent in 30 years. In 1989, the sea receded into two separate portions.
The average salinity of the water increased from nine per cent in 1957 to 30 per cent in 1989.
All these actions lead to a trail of adverse environmental impacts. Most of the 20 species of fish, which the sea originally had, died due to declining food, increase in water salinity and drying-up of shallow spawning areas.
The exposed seabed of the Ara Sea has become a source of large dust storms, blowing up to 75,000 tons of dust each year from the saline “solonchak” soils. Deposition of this salty material on the irrigated cropland of the delta, has badly affected the crop and soil yields.