Environmental impacts of dams
Encyclopedia
The environmental impact of reservoirs is coming under ever increasing scrutiny as the world demand for water and energy increases and the number and size of reservoirs increases.
Dam
s and the reservoir
s can be used to supply drinking water
, generate hydroelectric power
, increasing the water supply for irrigation
, provide recreational opportunities and to improve certain aspects of the environment. However, adverse environmental and sociological impacts have also been identified during and after many reservoir constructions. Whether reservoir projects are ultimately beneficial or detrimental—to both the environment and surrounding human populations— has been debated since the 1960s and probably long before that. In 1960 the construction of Llyn Celyn
and the flooding of Capel Celyn
provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam
and other similar projects throughout Asia
, Africa
and Latin America
have generated considerable environmental and political debate.
upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam’s construction. To date, over 400,000 km2 of the earth have been flooded due to damming. 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 climate
s. 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.
and trout
. Dams block their migration upstream to spawning
areas, threatening to decrease reproduction numbers and reduce the species population. In light of this consequence, efforts have been made to allow the fish a passage upstream, and newer dams often include artificial "fishways" or "fish ladders".
Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge. Fish sometimes have difficulty migrating downstream through a dam, meaning that downstream populations are often reduced, unless the fish are able to swim safely through the dams’ spillways. Permanent inundation caused by reservoir flooding also alters the wetlands, forests and other habitats surrounding the river. Further ecosystem disruption occurs along the banks of the river and downstream. The areas surrounding riverbanks are of a particularly rich bio-diversity, supported by the natural flooding of a dam-free river. Dammed rivers reduce flood rates, and this has negative consequences on the floodplains downstream that depend on seasonal waters for survival. The comparatively invariable ecosystem created by a reservoir-river supports a far-reduced range of wildlife. Dams hold back sediments that would replenish down stream ecosystems naturally. Endemic species may or may not survive the environmental change, and new species are likely to adopt the altered habitat as a home. However, since dams change a key ecosystem to which all surrounding ecosystems have adapted, dam construction nearly always reduces wildlife diversity, for better or for worse.
It also results in the loss of habitat of many living organisms.
or corn, and resulting in a smaller supply.
on the River Towy has been shown to be related to the reduced water temperature caused by the bottom water discharge from Llyn Brianne
, a dam created in the 1960s. Recent shifts in the populations of fish in the Snake River and Klamath region have, for example, prompted the renewal of research and restoration projects aimed at relieving the stress of temperature change.
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.
Resettlement
Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas. The record for the largest population relocated belongs to the Three Gorges dam
built in China
. Its reservoir submerged a large area of land, forcing over a million people to relocate. "Dam related relocation affects society in three ways: an economic disaster, human trauma, and social catastrophe", states Dr. Michael Cernea of the World Bank
and Dr. Thayer Scudder, a professor at the California Institute of Technology
.
and forest
ecology of the floodplain
s 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 floods which may affect the ecology and agriculture seriously.
Case studies
, 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.
Dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
s and the reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
s can be used to supply drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
, generate hydroelectric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
, increasing the water supply for irrigation
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
, provide recreational opportunities and to improve certain aspects of the environment. However, adverse environmental and sociological impacts have also been identified during and after many reservoir constructions. Whether reservoir projects are ultimately beneficial or detrimental—to both the environment and surrounding human populations— has been debated since the 1960s and probably long before that. In 1960 the construction of Llyn Celyn
Llyn Celyn
Llyn Celyn is a large reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in Gwynedd, North Wales. It measures roughly 2½ miles long by a mile wide, and has a maximum depth of...
and the flooding of Capel Celyn
Capel Celyn
Capel Celyn was a rural community to the north west of Bala in Gwynedd, north Wales, in the Afon Tryweryn valley. The village and other parts of the valley were flooded to create a reservoir, Llyn Celyn, in order to supply Liverpool and The Wirral with water for industry...
provoked political uproar which continues to this day. More recently, the construction of Three Gorges Dam
Three Gorges Dam
The Three Gorges Dam is a hydroelectric dam that spans the Yangtze River by the town of Sandouping, located in the Yiling District of Yichang, in Hubei province, China...
and other similar projects throughout Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and Latin America
Latin America
Latin America is a region of the Americas where Romance languages – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...
have generated considerable environmental and political debate.
Upstream impacts
Creation of a reservoir
The damming of a river creates a reservoirReservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
upstream from the dam. The reservoir waters spill out into the surrounding environments, flooding the natural habitats that existed before the dam’s construction. To date, over 400,000 km2 of the earth have been flooded due to damming. 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 climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
s. 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.
Fragmentation of river ecosystems
A dam also acts as a barrier between the upstream and downstream movement of migratory river animals, such as salmonSalmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
and trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
. Dams block their migration upstream to spawning
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
areas, threatening to decrease reproduction numbers and reduce the species population. In light of this consequence, efforts have been made to allow the fish a passage upstream, and newer dams often include artificial "fishways" or "fish ladders".
Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge. Fish sometimes have difficulty migrating downstream through a dam, meaning that downstream populations are often reduced, unless the fish are able to swim safely through the dams’ spillways. Permanent inundation caused by reservoir flooding also alters the wetlands, forests and other habitats surrounding the river. Further ecosystem disruption occurs along the banks of the river and downstream. The areas surrounding riverbanks are of a particularly rich bio-diversity, supported by the natural flooding of a dam-free river. Dammed rivers reduce flood rates, and this has negative consequences on the floodplains downstream that depend on seasonal waters for survival. The comparatively invariable ecosystem created by a reservoir-river supports a far-reduced range of wildlife. Dams hold back sediments that would replenish down stream ecosystems naturally. Endemic species may or may not survive the environmental change, and new species are likely to adopt the altered habitat as a home. However, since dams change a key ecosystem to which all surrounding ecosystems have adapted, dam construction nearly always reduces wildlife diversity, for better or for worse.
It also results in the loss of habitat of many living organisms.
Reservoir sedimentation
Rivers carry four different types of sediment down their riverbeds, allowing for the formation of riverbanks, river deltas, alluvial fans, braided rivers, oxbow lakes, levees and coastal shores. The construction of a dam blocks the flow of sediment downstream, leading to downstream erosion of these Sedimentary depositional environment, depositional environments, and increased sediment build-up in the reservoir. While the rate of sedimentation varies for each dam and each river, eventually all reservoirs develop a reduced water-storage capacity due to the exchange of storage space for sediment. Diminished storage capacity results in decreased ability to produce hydroelectric power, reduced availability of water for irrigation, and if left unaddressed, may ultimately result in the expiration of the dam and river.Riverline and coastal erosion
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 alfalfaAlfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
or corn, and resulting in a smaller supply.
Water temperature
The water of a reservoir is usually warmer in the winter and cooler in the summer than it would be without a dam. As this water flows into its river, the altered temperature also affects the temperature of the river. This impacts the plant and animal life present in both the reservoir and the river, often creating environments that are unnatural to the endemic species. A substantial decrease in fishing success for Atlantic salmon and SewinBrown trout
The brown trout and the sea trout are fish of the same species....
on the River Towy has been shown to be related to the reduced water temperature caused by the bottom water discharge from Llyn Brianne
Llyn Brianne
Llyn Brianne is a man-made lake or reservoir in the headwaters of the River Tywi in central Wales.-Construction:The reservoir was constructed by Wimpey Construction in the late 1960s and early 1970s in order to regulate the flow in the River Tywi to support large potable water abstraction at...
, a dam created in the 1960s. Recent shifts in the populations of fish in the Snake River and Klamath region have, for example, prompted the renewal of research and restoration projects aimed at relieving the stress of temperature change.
Effects on humans
DiseasesWhilst 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
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
) and snails (which are vectors for Schistosomiasis
Schistosomiasis
Schistosomiasis is a parasitic disease caused by several species of trematodes , a parasitic worm of the genus Schistosoma. Snails often act as an intermediary agent for the infectious diseases until a new human host is found...
) can take advantage of this slow flowing water.
Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas. The record for the largest population relocated belongs to the Three Gorges dam
Three Gorges Dam
The Three Gorges Dam is a hydroelectric dam that spans the Yangtze River by the town of Sandouping, located in the Yiling District of Yichang, in Hubei province, China...
built in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. Its reservoir submerged a large area of land, forcing over a million people to relocate. "Dam related relocation affects society in three ways: an economic disaster, human trauma, and social catastrophe", states Dr. Michael Cernea of the World Bank
World Bank
The World Bank is an international financial institution that provides loans to developing countries for capital programmes.The World Bank's official goal is the reduction of poverty...
and Dr. Thayer Scudder, a professor at the California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
.
Effects on flood-dependent ecology/agriculture
In many developing countries the savannaSavanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
and forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
ecology of the floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
s 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 floods which may affect the ecology and agriculture seriously.
- The Manatali reservoirLake ManantaliLake Manantali is a large artificial lake, formed by the 1989 construction of the Manantali Dam, on the Bafing River in Mali. Its northern point is located 90 km to the south-east of the city of Bafoulabé.-Size:...
formed by the Manantali damManantali DamThe Manantali Dam is a multi-purpose dam on the Bafing river in the Senegal River basin, 90 km to the south-east of Bafoulabé, in Mali's Kayes Region.-History:...
in MaliMaliMali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
intersects the migration routes of nomadic pastoralistsPastoralismPastoralism or pastoral farming is the branch of agriculture concerned with the raising of livestock. It is animal husbandry: the care, tending and use of animals such as camels, goats, cattle, yaks, llamas, and sheep. It may have a mobile aspect, moving the herds in search of fresh pasture and...
and destroyed 43000 ha of savannah, probably leading to overgrazingOvergrazingOvergrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, or by overpopulations of native or non-native wild animals.Overgrazing reduces the...
and soil erosion elsewhere. Further, the reservoir destroyed 120 km² of forest. The depletion of groundwater aquifers, which is caused by the suppression of the seasonal flood cycle, is damaging the forests downstream of the dam. - In the case of the Lower Omo River, Ethiopia, hundreds of thousands of nomads will be affected because the flood recession agriculture on which their livelihoods are based would disappear once the Gibe III dam will be completed.
- After the closure of the KainjiKainjiKainji may refer to several locations in Nigeria:*Kainji Lake*Kainji Dam*Kainji National ParkIt is also used to refer to a group of languages spoken around and to the east of the lake*Kainji languages...
dam, NigeriaNigeriaNigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
, 50 to 70 per cent of the downstream area of flood-recession cropping was lost.
Effects on the Earth itself
Reservoirs may contribute to changes in the Earth's climate. Warm climate reservoirs generate methaneMethane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
when the reservoirs are stratified, in which the bottom layers are anoxic (i.e. they lack oxygen), leading to degradation of biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
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.
External links
- Rivers No More: The Environmental Effects of Large Dams at International Rivers (an excerpt for Rivers No More: The Environmental Effects of Large Dams)
- World Commission on Dams