Environmental Impacts Of Large Dams Environmental Sciences Essay
About 48000 large dams have been built as a response to meet energy or water need. Nearly half of the worlds rivers have at least one large dam. One-third of the countries in the world rely on hydropower for more than half their electricity supply, and large dams generate 19% of electricity overall. Half the worlds large dams were built exclusively or primarily for irrigation, and some 30-40% of the 271 million hectares irrigated worldwide rely on dams.
There are dams on nearly half of the rivers of the world (Table 2.6). Six percent of the energy consumed in the world is produced from hydraulic power. Additionally, hydraulic power is in the second rank within the renewable energy sources and every year it increases 4 percent in the world. Dams whose height is more than 15 meters are referred to as big dams. Clearly, dams can play an important role in meeting people’s needs (Table 2.7).
Table 2.6. Distribution of Dams on the World
Number of Dams
Percentage of the total
Source: World Commission on Dams, Dams and Development, 2000.
There are always two opinions about dams – the supporters talk about the economic benefits of irrigation, electricity generation, flood control and water supply, the opponents highlight the adverse impacts of displacement and impoverishment of people, destruction of ecosystems and fishery resources, and possibility of disaster if the dam breaks.
Environmental Impacts of Large Dams
Land and water are ecologically linked in a natural system called a watershed. From the smallest droplet to the mightiest river, water works to shape the land, taking with it sediment and dissolved materials that drain to watercourses and, in most cases, eventually to the sea. The river is a product of the land it flows through – the type of rock and soil, the shape of the land, and the amount of vegetation are some of the factors that determine the river’s shape, size and flow.
When a large dam is constructed, these ties between the land and the river are broken and the consequences are felt throughout the watershed, as well as by the web of life it supports. Some 40,000 large dams, most of which were built in the past 50 years, now obstruct the world’s rivers. More than 400,000 square kilometers – an area larger than Zimbabwe, have been inundated by reservoirs worldwide. The world’s largest impoundment, the 8,500 sq km Volta Reservoir behind Ghana’s Akasombo Dam, flooded 4% of that nation’s land area. An internal survey of hydroelectric dam projects by World Bank has shown that 58% of the dams were planned and built without any consideration of downstream impacts.
Table 2.7. Countries having the Biggest Dams according to Size and Function
Countries having Biggest Dams
Ranking with respect to Dam’s Function
Water For Drinking and Daily Use
Source: World Commission on Dams, Dams and Development, 2000.
The following are a few serious environmental impacts of dams:
(i) Effects on River Systems
Reducing the flow of water from a river changes the landscape it flows through, which in turn can affect the ecosystem’s flora and fauna. A dam holds back sediments, especially the heavy gravel and cobbles. The river, deprived of its sediment load, seeks to recapture it by eroding the downstream channel and banks, undermining bridges and other riverbank structures. Riverbeds are typically eroded by several meters within a decade of first closing a dam; the damage can extend for tens or hundreds of kilometers below a dam. Riverbed deepening lowers 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 river bottom, and for invertebrates such as insects, molluscs and crustaceans. Changes in the physical habitat and hydrology of rivers are implicated in 93% of freshwater fauna declines in North America.
Before construction of the Aswan Dam in Egypt, 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% of that sediment remains behind the dam. The result has been a drop in soil productivity. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river.
Another example of this problem is along the mouth of the Volta River in Ghana. Akosombo Dam has cut off the supply of sediment to the Volta Estuary, affecting also neighboring Togo and Benin, whose coasts are now being eaten away at a rate of 10-15 meters per year.
(ii) Hydrological Effects
Dams change the pattern of the flow of a river, both reducing its overall volume and changing its seasonal variations. The nature of the impacts depends on the design, purpose and operation of the dam. All parts of a river’s ecology can be impacted by changes to its flow.
A river’s estuary, where fresh water meets the sea, is a particularly rich ecosystem. Some 80% of the world’s fish catch comes from these habitats, which depend on the volume and timing of nutrients and fresh water. The alteration of the flows reaching estuaries because of dams and diversions is a major cause of the precipitous decline of sea fisheries in the Gulf of Mexico, the Black and Caspian Seas, California’s San Francisco Bay, the Eastern Mediterranean and others.
(iii) Changes to Flooding
The storage of water in dams delays and reduces floods downstream. River and floodplain ecosystems are closely adapted to a river’s flooding cycle. The native plants and animals depend on its variations for reproduction, hatching, migration and other important lifecycle stages. Annual floods deposit nutrients on the land, flush out backwater channels, and replenish wetlands. It is generally recognized by biologists that dams are the most destructive of the many abuses causing the rapid disappearance of riverine species. About 20% of the world’s recognized 8,000 freshwater species are threatened with extinction.
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 food for the stream’s residents. Floods also provide shallow backwater areas on vegetated and shaded riversides; the young of many animals depend on these backwaters to protect them from large predators.
As an example, a fish on a certain river may only reproduce during April of every year so that its offspring will have abundant food and places to hide. If the flood never comes because a dam holds the river back (because people want the water for themselves), the offspring may be produced during a time when they cannot possibly survive. If the fish can wait until the next flood, which may be in July, its young will be born during the wrong time of year, and will have to contend with the absence of their normal food supply and temperatures for which they are not prepared.
Vegetation, too, depends upon these regular cycles of flood. Quite often, people will decide that they can spare no water at all and no flooding will occur. Or they may have built the dams specifically to stop flooding, so they can build houses in the floodplains. When this happens, riparian vegetation, the vegetation bordering the river, changes forever.
(iv) Removing Sediment
Another reason that riverbeds become scoured and armored is that dams remove all the sediment from the river. It is natural that the river, which is accustomed to carrying sediment and now has none, will pick up the sediment from the streambed below the dam. It is almost as though the river has been “starved” of its sediment. As in everything else in nature, balance will be achieved one way or the other, often at the expense of one or more species.
The sediment in a dammed river reaches the slow-moving reservoir above the dam and drops out, settling behind the dam. Each dam is engineered to withstand the force of a particular amount of water (this may be very very large). The dam is not engineered to withstand the additional force of tons of wet sediment pressing on the backside. The muddier the river, the faster this heap of sediment will build up. When it builds up very high, either the dam bursts, killing people and destroying settlements downstream, or the reservoir’s water pours over the top of the dam.
The river downstream of the dam will be like a dead river. It will not have a living river ecosystem filled with fish and birds. The water will be starved of nutrients and provide little or no habitat for animals. In addition, animals that once used the “muddiness” of the river’s water to conceal them from predators are now overly vulnerable to predation, and may quickly go extinct. A river with dams eventually becomes little more than a dead channel of water.
(v) Starving the River
Dams hold back not only sediment, but also debris. The life of organisms (including fish) downstream depends on the constant feeding of the river with debris. This debris includes leaves, twigs, branches, and whole trees, as well as the organic remains of dead animals. Debris not only provides food, it provides hiding places for all sizes of animals and surfaces for phytoplankton and microorganisms to grow. Without flooding and without a healthy riparian zone, this debris will be scarce. Adding to the problem, although debris might come from the river above the dam, it is instead trapped in the reservoir, and never appears downstream. The bottom level of the food web is removed. All in all, the loss of sediment and debris means the loss of both nutrients and habitat for most animals.
(vi) Changing Temperature
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 this is changed, their survival is threatened.
The typical practices in a hydroelectric station are to release large amounts of water in powerful surges during the day in order to provide electricity when demand and prices are highest, and to cut 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 many species like the 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.
(viii) Stopping Fish Migration
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? Predators kill many 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.
There are many fishes that cannot climb dam ladders or leap over low dams. Some of these fishes swim upstream every year to breed, and 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 in sea.
(ix) Social Impacts of Dams: Conflict with people
The most important social impact of a dam is displacement of people. The forced removal of people from their homes and the land by which they make a living has been the tragic consequence of dams. Although the people are offered resettlement, the situation leads to disintegration of self-identity and place-connection for both individuals and communities. The social connections are lost and the people find it hard to recover a sense of belonging to both community and the physical environment that supports their existence. The major issues related to displacement are summarized below:
Flooding of reservoir
Water diversion and hydrological changes
Soil compaction and paving
Mountain top removal and stream filling
Reduction/depletion of minerals and species
Creation of barriers to species migration
Eviction and resettlement
Loss of resource due to construction and/or flooding
Landslide, flood, and earthquakes from dams
Water quality decline
Loss (or gain) of fish and wildlife populations
Ecosystem changes leading to pest problems or disease
Aquifer disruption causing problems downstream
Loss of fish species leads to loss of migratory bird species
Loss of access to resources and property
Unemployment with project completion
Creation of new identities
Urbanization as labour camps become permanent
Unsustainable agriculture in resettlement area leading to soil erosion
Ethnic conflict due to resettlement
The inundation of the river valley has significant adverse social impacts by blocking access to natural and social resources of the river valley. This puts pressure on the ecosystem, as fewer resources are available to serve the needs of the population located within the region. Increased competition for commonly held resources, such as wild fruits and vegetables, timber, fodder and firewood, disrupts the subsistence routines of riverine populations. This has the impact of forcing people to forge in new areas that may be further from their homes in and/or in locations that are used by other groups for either similar or conflicting purposes.
The dams also seriously affect human health through the spread of disease. For instance, schistosomiasis and malaria proliferate in areas around the still water of dam reservoirs. Beyond creating habitat for disease vectors, dams have been linked with the spread of non-communicable diseases like mercury poisoning. The increase in disease is also attributable to the influx of migratory workers during the time of dam construction. The overall social impact of a surge in disease in communities is to increase social malaise and to circumscribe livelihood opportunities.
The process of displacement has affected most the weaker sections of the people in India. The scheduled tribes and castes and backward caste people constitute a large chunk of the displaced people. Women and children suffer the maximum in any displacement. The conditions of people displaced by the various dams in the Narmada have attracted the attention of the Supreme Court of India, and it is observed that the measures at resettlement and in making provision for civic amenities in the new settlements are far below expectation.
The total number of people displaced by dams is estimated to be 40 – 80 million throughout the world – a substantial number of these people are Indians (the estimates vary, but the figure is likely to be around 4.4 million people). The people get compensation, but the process is often long-drawn, and sometimes, the actual sufferers are not getting true value for the losses incurred. These are inadequate when compared to the permanent loss of livelihood, and the social and mental cost of displacement.
Large Dams in India
At independence, in 1947, there were not more than 300 large dams in India. By 2000, the number grew to over 4000, more than half of them built between 1971 and 1989. India ranks third in the world in dam building after US and China. While some of these dams were built primarily for flood control, water supply, and hydroelectric power generation, the primary purpose of most Indian dams (96 percent) remains irrigation. In fact, large dam construction has been the main form of investment in irrigation undertaken by the Indian government. However, much controversy has evoked since 1980s with the Sardar Sarovar Project on the Narmada when the people started asking questions on the social, environmental, and economic costs of dams and their benefits.
Most irrigation dams in India are embankment dams. They consist of a wall built across a river to impound water forming a reservoir upstream and a system of spillways and gates to bypass the wall to maintain normal flow and the impounded water flows to canals feeding agriculture fields downstream. People living in the upstream catchment area, lose property and livelihood and gain very little, while people living in the command area (downstream) gain the most from irrigation.
Between 1951 and 2000, India’s production of food grains increased fourfold, from 51 million tonnes to about 200 million tonnes resulting in considerable foreign exchange savings in food grains import, and making India a food grain surplus or at least self-sufficient country. About two thirds of this increase has been attributed to rise in irrigated areas, 35 % of which are irrigated by dams.
The world is building more dams every year. New dams promise more electricity and at the same time, they are devastating to others. A few case studies are briefly described below:
The Enawene Nawe, Brazil
The Enawene Nawe, a small Amazonian tribe (over 420) who live by fishing in Mato Grosso state, Brazil, is a relatively isolated people. They grow manioc and corn in gardens and gather forest products, like honey but fishing is their main livelihood and fish are a vital part of their diet, as they are one of the few tribes who eat no red meat. During the fishing season, the men build large dams across rivers and spend several months camped in the forest, catching and smoking the fish which is then transported by canoe to their village.
For decades, the Enawene Nawe has faced invasion of their lands by rubber tappers, diamond prospectors, cattle ranchers and more recently soya planters – Maggi, the largest soya company in Brazil, illegally built a road on their land in 1997 (this was subsequently closed by a federal prosecutor). Although their territory was officially recognized and ratified by the government in 1996, a key area known as the Rio Preto was left out. This area is tremendously important to the Enawene Nawe both economically and spiritually – this is where they build their fishing camps and dams, and where many important spirits live.
Now, up to 11 dams are planned along the Juruena River, which flows through the Indians’ territory. The dams will be funded by a consortium of businesses, many of whom are involved in the soya industry. The Enawene Nawe is opposing the dams, and has launched an appeal for support to halt the construction.
The Penan, Malaysia
In 2008, a leaked map exposed the huge dam plans of the national electricity company in Sarawak, Malaysia. The local Penan people, who are familiar with destructive interference from outsiders, now face a new challenge to their land and livelihoods. Blueprints were accidentally posted on the internet for dams that will submerge homes and villages. To make matters worse, these dams are projected to produce far more electricity than Sarawak uses.
The Borneo forests
The island of Borneo, a fragile treasure house of rainforests, rare animals and plants, is under threat from plans for Chinese engineers to build 12 dams that will cut through virgin land and displace thousands of native Dayak people. The government of the Malaysian state of Sarawak says the dams are the first stage of a “corridor of renewable energy” that will create 1.5 million jobs through industries powered by safe, clean hydro-electricity. Campaigners are furious but appear powerless in the face of a project they fear will compound the devastation wrecked on Borneo’s peoples and land by previous dam projects and the felling of its forests.
They point to the ruin caused by the levelling of millions of acres of trees for oil palm plantations to meet the world’s demand for biofuels. The dams would slice across a vast sweep of Sarawak, a place where wisps of cloud cling to remote, tree-clad peaks, huge butterflies flit through the foliage and orang-utans, sun bears and leopards roam.
The Bakun dam, a separate project due to be completed by 2011, has already displaced an estimated 10,000 indigenous people, leading to bitter legal battles and a chorus of dismay from economists about cost overruns. For all that, it may be too late to save the natural bounty of Borneo itself. Orphaned orang-utans, piteously holding the outstretched hands of their human saviours, are the most conspicuous symbols of its fragility.
Divided between Malaysia and Indonesia, with Brunei occupying a tiny enclave in the north, Borneo’s riches have ensured its plunder. One reason is the voracious world demand for timber. The other is the biofuels made from palm oil. Almost half of Borneo’s rainforests have been cut down. Two million acres have vanished every year as trees are felled, the wood sold and the land turned over to oil palms. Enormous fires cast a perpetual pall of toxic haze, making Indonesia the world’s third largest greenhouse gas polluter after China and the United States. “Green gold”, or palm oil, poses an even more insidious threat because it promises prosperity and development to the numerous poor of Borneo – along with immense rewards for the elites. The vegetable oil comes from crushed palm husks. Long used for cooking, cosmetics and soap, it has now become a principal source of biodiesel fuel.
Malaysia and Indonesia produce about 85% of the world’s supply of palm oil – most of it from Borneo. The price of this apparently environment-friendly fuel is high as the damages far outweigh the benefits. All over Sarawak, tribal people have lost their ancestral lands to similar gambits.
The situation in India
This dam is situated in the town “Bhakra” in Punjab and is Asia’s biggest dam. The dam is built on the “Sutlej River”. It is 225.55 m high above sea level. The dam is 518.25 m long and 304.84 m broad. Its huge reservoir known as the “Gobindh Sarovar”, stores up to 9621 million cu m of water, enough to drain the whole of Chandigarh, parts of Haryana, Punjab and Delhi.
It has 4 floodgates, which are fully functional during the period of floods, and also has 2 power- stations situated on either side of the dam. Each of the power plants comprise of 5 generators, and a power station. The total electricity produced in both the generators is 1325 MW.
The Narmada River Dams
The Narmada River originates from the Maikal ranges at Amarkantak, 1057 m above the sea-level, now in Shahdol district of Madhya Pradesh. The river flows for 1312 km through the three states of Madhya Pradesh (MP), Maharashtra and Gujarat before falling into the Arabian Sea. The valley has been the seat of an uninterrupted flow of human civilization from pre-historic times. The river has supported a variety of people and diverse socio-cultural practices ranging from the relatively autonomous adivasi (tribal) settlements in the forests to non-tribal rural population.
The Narmada basin extends over an area of 98,796 km2 and lies between 720 32′ E to 810 45′ E and 21o 20′ N to 23o 45′ N. The basin covers large areas in the states of Madhya Pradesh (86%), Gujarat (14%) and a comparatively smaller area (2%) in Maharashtra. In the river course of 1,312 km, there are 41 tributaries, out of which 22 are from the Satpuda range and the rest on the right bank are from the Vindhya range.
The valley experiences extremes of hydrometeorological and climatic conditions with the upper catchment having an annual precipitation in the range of 1000 mm to 1850 mm and with half or even less than half in its lower regions (650 mm-750 mm); the diversity of vegetation from lush green in the upper region to dry deciduous teak forest vegetation in the lower region is testimony to this feature.
The Narmada basin is drought affected and a large part of North Gujarat, Saurashtra and Kutch constitute semi-arid or arid regions on account of extreme unreliability of rainfall, rendering them ‘chronically’ drought prone and subject to serious drinking water problems. Teak and India’s best hardwood forests are found in the Narmada River basin and they are much older than the ones in the Himalayas. The lower Narmada River Valley and the surrounding uplands, covering an area of 169,900 km2 consists of dry deciduous forests. The natural vegetation of the region is a three-tiered forest. Tectona grandis is the dominant canopy tree, in association with Diospyros melanoxylon, Dhaora (Anogeissus latifolia), and Boswellia serrata. Riperian areas along the regions’ rivers and streams, which receive year-round water, are home to moist evergreen forests. The ecoregion is home to 76 species of mammals and to 276 bird species none of which are endemic. According to the World Wildlife Fund (WWF), about 30% of the ecoregion is covered in relatively intact vegetation. The ecoregion includes some large blocks of habitat in the Vindhya and Satpura ranges. About 5% of the ecoregion lies within protected areas, including Bandhavgarh, Panna, and Sanjay National Parks.
The valley has some of the important national parks and wild life sanctuaries. Kanha national park located in the upper reaches of Narmada, about 18 km from Mandla, boasts of several wild animals including the Tiger. Two tributaries of Narmada, namley, Hallon and Banjar, flow through this park. It is one of the best National Parks of Asia, which has been described vividly by Rudyard Kipling in his famous creation “Jungle Book”. Satpura National Park, set up in 1981, is located in Hoshangabad district of Madhya Pradesh and covers an area of 524 km2. Along with the adjoining Bori and Panchmarhi Sanctuaries, it constitutes an area of 1,427 km2 of unique Central Indian Highland ecosystem. Satpura National Park, being part of a unique ecosystem, is very rich in biodiversity. The fauna comprises tiger, leopard, sambar, chital, bhedki, nilgai, four-horned antelope, chinkora, bison (gour), wild boar, wild dog, bear, black bear, black buck, fox, porcupine, flying, mouse deer, Indian joint squirrel etc. There are a variety of birds. Hornbills and peafowl are the common birds. The flora of the national park consists of mainly sal, teak, tendu, aonla, mohua, bel, bambo, and a variety of grasses and medicinal plants.
Madla plant Fossil National park, Dindori National fossils park Ghughuya is situated in Din dori district of Madhya Pradesh in India. This national park has plants in fossil form that existed in India anywhere between 40 million and 150 million years ago spread over seven villages of Mandla District (Ghuguwa, Umaria, Deorakhurd, Barbaspur, Chanti-hills, Chargaon and Deori Kohani). The Mandla Plant Fossils National Park is an area that spreads over 274,100 m2.
The Pachmarhi Biosphere Reserve covers part of three civil districts viz., Hoshangabad, Betul and Chhindwara of Madhya Pradesh. The total area is 4926.28 km2. It envelops three wildlife conservation units viz., Bori Sanctuary (518.00 km²), Satpura National Park (524.37 km2), and Pachmarhi Sanctuary (461.37 km2). Satpura National Park comprises the core zone and the remaining area of 4,501.91 km2, surrounding the core zone serves as buffer zone. The area comprises 511 villages. The area exhibits variety of geological rock and soil formations. There is a wide spectrum of floral and faunal features that occupy the Satpura conservation area. It is one of the oldest forest reserves, which has an established tradition of scientific management of forests. It constitutes a large contiguous forest block that harbours a community of plant and animal species typical of the central highland region.
Of the 30 big dams proposed along the Narmada, Sardar Sarovar Project (SSP) and Narmada Sagar Project (NSP) are the mega dams. The Maheshwar and Omkareshwar dams along with SSP and NSP, are to form a complex which would ultimately cater to the needs of SSP. The struggle of the people of the Narmada valley against large dams began when the people to be displaced by SSP began organizing in 1985-86. Since then the struggle has spread to encompass other major dams in various stages of planning and construction chiefly Maheshwar, Narmada Sagar, Maan, Goi and Jobat. Tawa and Bargi Dams were completed in 1973 and 1989 respectively.
Sardar Sarovar project
The Sardar Sarovar Project (SSP) faced stiff opposition from the people right from the planning stage. The Narmada Bachao Andolan (Save the Narmada movement) has been at the forfront of this opposition and at one time the NBA was successful in stopping World Bank funding for the project. A number of cases were filed against the project. However, the Supreme Court of India in 2000 issued a final ruling allowing construction to proceed. It is estimated that nearly 200,000 people would be displaced to construct the reservoir and a large number of people will lose land or livelihood due to project activities. A majority of the displaced people are tribal people.
Medha Patkar (born 1 December 1954 in Bombay) is the founder of the Narmada Bachao Andolan and has vowed to work for the displaced people. She is one of India’s most important environmental activists. Her uncompromising insistence on the right to life and livelihood has brought to the fore the basic questions of natural resources, human rights, environment, and developm