The environmental impact of reservoirs comes under ever-increasing scrutiny as the global demand for water and energy increases and the number and size of reservoirs increases. Dams and

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

s can be used to supply

drinking water Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often (but not always) supplied through taps, in which case it is also calle ...

, generate hydroelectric power, increase the water supply for

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

, provide recreational opportunities, and

flood control Flood management or flood control are methods used to reduce or prevent the detrimental effects of flood waters. Flooding can be caused by a mix of both natural processes, such as extreme weather upstream, and human changes to waterbodies and ru ...

. In 1960 the construction of

Llyn Celyn Llyn Celyn () is a reservoir constructed between 1960 and 1965 including the highly controversial Tryweryn flooding in the valley of the River Afon Tryweryn, Tryweryn in Gwynedd, Wales. This included the forcible removal of the Capel Celyn vil ...

and the flooding of

Capel Celyn Capel Celyn was a rural community to the northwest of Bala in Gwynedd, Wales, in the Afon Tryweryn valley. The village and other parts of the valley were flooded in the Tryweryn flooding of 1965 to create a reservoir, Llyn Celyn Llyn ...

provoked political uproar which continues to this day. More recently, the construction of

Three Gorges Dam The Three Gorges Dam (), officially known as Yangtze River Three Gorges Water Conservancy Project () is a hydroelectric gravity dam that spans the Yangtze River near Sandouping in Yiling District, Yichang, Hubei province, central China, downs ...

and other similar projects throughout

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

and

Latin America Latin America is the cultural region of the Americas where Romance languages are predominantly spoken, primarily Spanish language, Spanish and Portuguese language, Portuguese. Latin America is defined according to cultural identity, not geogr ...

have generated considerable environmental and political debate. Currently, 48 percent of rivers and their hydro-ecological systems are affected by reservoirs and dams.

Upstream impacts

Fragmentation of river ecosystems

A dam acts as a barrier between the upstream and downstream movement of migratory river animals, such as

salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...

and trout. Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge.

Reservoir sedimentation

Rivers carry sediment down their riverbeds, allowing for the formation of depositional features such as

river delta A river delta is a landform, archetypically triangular, created by the deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or with a body of stagnant water. The creat ...

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

braided river A braided river (also called braided channel or braided stream) consists of a network of river channel (geography), channels separated by small, often temporary, islands called ''braid bars'' or, in British English usage, ''aits'' or ''eyots''. ...

s, 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 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 "live storage" space for sediment.''Silenced Rivers: The Ecology and Politics of Large Dams'', by Patrick McCully, Zed Books, London, 1996. 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.Reservoir Sedimentation Handbook; Morris, Gregory & Fan, Jiahua; McGraw-Hill Publishers; 1998. The trapping of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

in reservoirs reduce sediment delivery downstream, which negatively impacts channel morphology, aquatic habitats and land elevation maintenance of

deltas A river delta is a landform, wikt:archetype#Noun, archetypically triangular, created by the deposition (geology), deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or ...

. Apart from dam removal, there are other strategies to mitigate reservoir sedimentation.

Flushing flow method

The flushing flow method involves partially or completely emptying the

behind a dam to erode the

stored on the bottom and

transport Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...

it downstream. Flushing flows aim to restore natural water and sediment fluxes in the river downstream of the dam, however the flushing flow method is less costly compared to removing dams or constructing bypass tunnels. Flushing flows have been implemented in the Ebro river twice a year in autumn and spring since 2003, except for two dry years in 2004 and 2005. The construction of multiple dams on the Ebro river disrupted the delivery of sediments downstream and as a result, the Ebro delta faces a

deficit. The river channel also narrowed and bank erosion increased. During experiments, it was found that suspended sediment concentration during flushing flows is double that of natural

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

s, although the total water discharge is lower. This means that flushing flows have a relatively high sediment transport capacity, which in turn suggests that flushing flows positively impact downstream river ecosystems, maximising sediment delivery to the lowest reaches of the river. A total of 340,000 t/year of sediment could be delivered to the Ebro delta, which could result in a net accretion rate of 1 mm per year.

Sediment bypasses

Sediment bypass tunnels can partially restore sediment dynamics in rivers downstream of dams, and are primarily used in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

and

Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

. Bypass tunnels divert part of the incoming

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and

s during

s into a tunnel around a

and dam. The water and sediment thus never enter the reservoir but join the river again below the dam. Bypass tunnels reduce riverbed

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

and increase morphological variability below the dam.

Impact below dam

River line and coastal erosion

As all dams result in reduced sediment load downstream, a dammed river is greatly demanding for sediment as it will not have enough sediment. This is 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 erodes 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 Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of Wind wave, waves, Ocean current, currents, tides, wind-driven water, waterborne ice, or other impacts ...

, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. Downstream channel erosion of dammed rivers is related to the morphology of the riverbed, which is different from directly studying the amounts of sedimentation because it is subject to specific long term conditions for each river system. For example, the eroded channel could create a lower water table level in the affected area, impacting bottomland crops such as

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...

corn Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...

, and resulting in a smaller supply.Sedimentation Engineering; American Society of Civil Engineers Committee; American Society of Civil Engineers Headquarters; 1975. In the case of the Three Gorges Dam in China the changes described above now appears to have arrived at a new balance of erosion and sedimentation over a 10-year period in the lower reaches of the river. The impacts on the tidal region have also been linked to the upstream effects of the dam. In addition to coastal erosion impacts, reduced river flow may also alter ocean currents and ecosystems.

Nutrients sequestration

Once a dam is put in place represents an obstacle to the flux of nutrients such as carbon (C), nitrogen (N), phosphorus (P), and silicon (Si) on downstream river, floodplains and delta. The increased residence time of these elements in the lentic system of a reservoir, compared to the lotic system of a river, promotes their sedimentation or elimination which can be up to 40%, 50%, and 60% for nitrogen, phosphorus and silica respectively and this ultimately changes nutrients

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

in the aquatic ecosystem downstream a dam. The stochiometric imbalance of nitrogen, phosphorus, and silicon of the outflow can have repercussion on downstream ecosystems by shifting the phytoplankton community at the base of the food web with consequences to the whole aquatic population. An example is the effect of the construction of the Aswan High dam in Egypt, where the drop in nutrient concentration to the Nile delta impeded the diatom blooms causing a substantial decrease the fish population of ''Sardinella aurita'' and ''Sardinella eba'', while the reduced load of mud and silt affected the micro-

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

fauna leading to the decline of shrimp population. The change in nutrients stoichiometry and silicon depletion at a

can also cause harmful algal and bacterial blooms to the detriment of

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

s' growth for whom silicon availability represents a milestone for shells' formation. Since dammed rivers store nutrients during their lifespan, it can be expected that when a dam is removed, these legacy nutrients are remobilized causing downstream ecosystems' eutrophication and probable loss of biodiversity, thereby achieving the opposite effect desired by the river restoration action at dam dismissal.

Water temperature

The water of a deep reservoir in temperate climates typically stratifies with a large volume of cold, oxygen poor water in the hypolimnion. Analysis of temperature profiles from 11 large dams in the Murray Darling Basin (Australia) indicated differences between surface water and bottom water temperatures up to 16.7 degrees Celsius. If this water is released to maintain river flow, it can cause adverse impacts on the downstream ecosystem including fish populations. Under worse case conditions (such as when the reservoir is full or near full), the stored water is strongly stratified and large volumes of water are being released to the downstream river channel via bottom level outlets, depressed temperatures can be detected downstream. The operators of Burrendong Dam on the Macquarie River (eastern Australia) are attempting to address thermal suppression by hanging a geotextile curtain around the existing outlet tower to force the selective release of surface water.

Natural ecosystems destroyed by agriculture

Many dams are built for irrigation and although there is an existing dry ecosystem downstream, it is deliberately destroyed in favor of irrigated farming. After the

Aswan Dam The Aswan Dam, or Aswan High Dam, is one of the world's largest embankment dams, which was built across the Nile in Aswan, Egypt, between 1960 and 1970. When it was completed, it was the tallest earthen dam in the world, surpassing the Chatuge D ...

was constructed in Egypt it protected Egypt from the droughts in 1972–1973 and 1983–1987 that devastated East and West Africa. The dam allowed Egypt to reclaim about 840,000 hectares in the

Nile Delta The Nile Delta (, or simply , ) is the River delta, delta formed in Lower Egypt where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's larger deltas—from Alexandria in the west to Port Said in the eas ...

and along the Nile Valley, increasing the country's irrigated area by a third. The increase was brought about both by irrigating what used to be desert and by bringing under cultivation 385,000 hectares that were natural flood retention basins. About half a million families were settled on these new lands. In 1983 the Franklin Dam project in Tasmania, Australia was cancelled following a campaign to protect surrounding forest from clearing and flooding.

Effects on flood-dependent ecology and agriculture

In many low lying developing countries the

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

and

forest A forest is an ecosystem characterized by a dense ecological community, community of trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, ...

ecology adjacent to

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

s and

s are irrigated by wet season annual floods. Farmers annually plant flood recession crops, where the land is cultivated after floods recede to take advantage of the moist soil. Dams generally discourage this cultivation and prevent annual flooding, creating a dryer downstream ecology while providing a constant water supply for irrigation. BaluchNomads

''Case studies'' *The Lake Manatali reservoir formed by the Manantali Dam in

Mali Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the List of African countries by area, eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east b ...

, West Africa intersects the migration routes of nomadic pastoralists and withholds water from the downstream savanna. The absence of the seasonal flood cycle causes depletion of grazing land, and is also drying the forests on the floodplain downstream of the dam. *After the construction of the Kainji Dam in

Nigeria Nigeria, officially the Federal Republic of Nigeria, is a country in West Africa. It is situated between the Sahel to the north and the Gulf of Guinea in the Atlantic Ocean to the south. It covers an area of . With Demographics of Nigeria, ...

, 50 to 70 percent of the downstream area of flood-recession cropping stopped.

Potential for disaster

Dams occasionally break causing catastrophic damage to communities downstream. Dams break due to engineering errors, attack or natural disaster. The greatest dam break disaster to date happened in China in 1975 killing 200,000 Chinese citizens. Other major failures during the 20th century were at Morbi, India (5,000 fatalities), at Vajont, Italy (2000 dead), while three other dam failures have each caused at least 1000 fatalities.

Flood control

The controversial

in China is able to store of floodwaters on the Yangtze River. The 1954 Yangtze River floods killed 33,000 people and displaced 18 million people from their homes. In 1998 a flood killed 4000 people and 180 million people were affected. The flooding of the reservoir caused over a million people to relocate, then a flood in August 2009 was completely captured by the new reservoir, protecting hundreds of millions of people downstream.

Mercury cycling and methylmercury production

The creation of reservoirs can alter the natural

biogeochemical cycle A biogeochemical cycle, or more generally a cycle of matter, is the movement and transformation of chemical elements and compounds between living organisms, the atmosphere, and the Earth's crust. Major biogeochemical cycles include the carbon cyc ...

of mercury. Studies conducted on the formation of an experimental reservoir by the flooding of a boreal wetland showed a 39-fold increase in the production of toxic

methylmercury Methylmercury is an organometallic cation with the formula . It is the simplest organomercury compound. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a bioaccumulative environment ...

(MeHg) following the flooding. The increase in MeHg production only lasted about 2–3 years before returning to near normal levels. However, MeHg concentration in lower food chain organisms remained high and showed no signs of returning to pre-flood levels. The fate of MeHg during this time period is important when considering its potential to bioaccumulate in predatory fish.

Effects beyond the reservoir

Effects on humans

Diseases
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 Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

) and

snails A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...

(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

built in

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. 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 The World Bank is an international financial institution that provides loans and Grant (money), grants to the governments of Least developed countries, low- and Developing country, middle-income countries for the purposes of economic development ...

and Dr. Thayer Scudder, a professor at the California Institute of Technology. As well, as resettlement of communities, care must also be taken not to irreparably damage sites of historical or cultural value. The Aswan Dam forced the movement of the Temple at Aswan to prevent its destruction by the flooding of the reservoir.

Greenhouse gases

Reservoirs may contribute to changes in the Earth's climate. Warm climate reservoirs generate

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

, a

greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...

when the reservoirs are stratified, in which the bottom layers are anoxic (i.e. they lack oxygen), leading to degradation of

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

through anaerobic processes.Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams. At a dam in Brazil, where the flooded basin is wide and the biomass volume is high the methane produced results in a pollution potential 3.5 times more than an oil-fired power plant would be.Graham-Rowe, Duncan (2005).
Hydroelectric Power's Dirty Secret Revealed
, ''NewScientist.com''. A theoretical study has indicated that globally hydroelectric reservoirs may emit 104 million metric tonnes of methane gas annually. Methane gas is a significant contributor to global climate change. This is not an isolated case, and it appears that especially hydroelectric dams constructed in lowland

rainforest Rainforests are forests characterized by a closed and continuous tree Canopy (biology), canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropi ...

areas (where inundation of a part of the forest is necessary) produce large amounts of methane. Bruce Forsberg and Alexandre Kemenes have demonstrated that the Balbina Dam for instance emits 39,000 tonnes of methane each year and three other dams in the Amazon produce at least 3 to 4× as much as an equivalent coal-fired power plant. Reasons for this being that lowland rainforests are extremely productive and thus stores far more carbon than other forests. Also, microbes that digest rotting material grow better in hot climates, thus producing more greenhouse gases. Despite this, as of 2020, another 150 hydroelectric dams are planned to be constructed in the Amazon basin. There is some indication that

greenhouse gas emissions Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (), from burning fossil fuels such as coal, petroleum, oil, and natural gas, is the main cause of climate chan ...

decline over the lifetime of the dam. "But even including

methane emissions Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...

, total GHG reen-House Gasper KWh generated from hydropower is still at least half that from the least polluting thermal alternatives.Thus, from the perspective of global warming mitigation, dams are the most attractive alternative to fossil fuel based energy sources." Research conducted at the Experimental Lakes Area indicates that creating reservoirs through the flooding of boreal wetlands, which are sinks for , converts the wetlands into sources of atmospheric carbon. In these ecosystems, variation in organic carbon content has been found to have little effect on the rates of greenhouse gas emission. This means that other factors such as the lability of carbon compounds and temperature of the flooded soil are important to consider. The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water.

References

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''
{{DEFAULTSORT:Environmental Impacts Of Reservoirs Reservoirs Reservoirs Reservoirs