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 reservoirs can be used to supply drinking water, generate hydroelectric power, increase the water supply for irrigation, provide recreational opportunities, and flood control. 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. Currently, 48 percent of rivers and their hydro-ecological systems are affected by reservoirs and dams. [1]
A dam acts as a barrier between the upstream and downstream movement of migratory river animals, such as salmon and trout. [3]
Some communities have also begun the practice of transporting migratory fish upstream to spawn via a barge. [3]
Rivers carry sediment down their riverbeds, allowing for the formation of depositional features such as 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 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. [4] 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. [5]
The trapping of sediment in reservoirs reduce sediment delivery downstream, which negatively impacts channel morphology, aquatic habitats and land elevation maintenance of deltas. [6] Apart from dam removal, there are other strategies to mitigate reservoir sedimentation.
The flushing flow method involves partially or completely emptying the reservoir behind a dam to erode the sediment stored on the bottom and transport it downstream. [7] [6] 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. [8] [9] The construction of multiple dams on the Ebro river disrupted the delivery of sediments downstream and as a result, the Ebro delta faces a sediment deficit. The river channel also narrowed and bank erosion increased. [7] During experiments, it was found that suspended sediment concentration during flushing flows is double that of natural floods, although the total water discharge is lower. This means that flushing flows have a relatively high sediment transport capacity, [8] which in turn suggests that flushing flows positively impact downstream river ecosystems, maximising sediment delivery to the lowest reaches of the river. [10] 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. [7]
Sediment bypass tunnels can partially restore sediment dynamics in rivers downstream of dams, and are primarily used in Japan and Switzerland. [11] Bypass tunnels divert part of the incoming water and sediments during floods into a tunnel around a reservoir and dam. The water and sediment thus never enter the reservoir but join the river again below the dam. [12] Bypass tunnels reduce riverbed erosion and increase morphological variability below the dam. [13]
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. [5] This prompts coastal erosion, as beaches are unable to replenish what waves erode without the sediment deposition of supporting river systems. [14] 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 or corn, and resulting in a smaller supply. [15] 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. [16]
In addition to coastal erosion impacts, reduced river flow may also alter ocean currents and ecosystems. [17]
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 [18] which can be up to 40%, 50%, and 60% for nitrogen, phosphorus and silica respectively [19] and this ultimately changes nutrients stoichiometry 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. [20] [21] [22] 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 fauna leading to the decline of shrimp population. [23] The change in nutrients stoichiometry and silicon depletion at a river delta can also cause harmful algal and bacterial blooms to the detriment of diatoms' 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.
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. [24] If this water is released to maintain river flow, it can cause adverse impacts on the downstream ecosystem including fish populations. [25] 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 250–350 kilometres (160–220 mi) downstream. [24] 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. [26]
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 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 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. [27]
In many[ quantify ] low lying developing countries[ example needed ] the savanna and forest ecology adjacent to floodplains and river deltas 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.
Case studies
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.
The controversial Three Gorges Dam in China is able to store 22 cubic kilometres (5.3 cu mi) 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.
The creation of reservoirs can alter the natural biogeochemical cycle 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 (MeHg) following the flooding. [31] 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. [32]
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) and snails (which are vectors for schistosomiasis) can take advantage of this slow flowing water. [33]
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. [2] 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.
Reservoirs may contribute to changes in the Earth's climate. Warm climate reservoirs generate methane, 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. [34] [ page needed ] 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. [35] A theoretical study has indicated that globally hydroelectric reservoirs may emit 104 million metric tonnes of methane gas annually. [36] 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 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 [37] and three other dams in the Amazon produce at least 3 to 4× as much CO2 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. [38] There is some indication that greenhouse gas emissions decline over the lifetime of the dam. "But even including methane emissions, total GHG [Green-House Gas] per 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." [34]
Research conducted at the Experimental Lakes Area indicates that creating reservoirs through the flooding of boreal wetlands, which are sinks for CO2, converts the wetlands into sources of atmospheric carbon. [31] 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. [39]
The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water. [40]
Location | Carbon Dioxide | Methane |
---|---|---|
Lakes | 700 | 9 |
Temperate reservoirs | 1500 | 20 |
Tropical reservoirs | 3000 | 100 |
Hydropower, also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the gravitational potential or kinetic energy of a water source to produce power. Hydropower is a method of sustainable energy production. Hydropower is now used principally for hydroelectric power generation, and is also applied as one half of an energy storage system known as pumped-storage hydroelectricity.
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aquaculture, and navigability. Hydropower is often used in conjunction with dams to generate electricity. A dam can also be used to collect or store water which can be evenly distributed between locations. 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.
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. The soils usually consist of clays, silts, sands, and gravels deposited during floods.
A river delta is a landform shaped like a triangle, created by the deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs at a river mouth, when it enters an ocean, sea, estuary, lake, reservoir, or another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the uppercase Greek letter delta, Δ. The size and shape of a delta are controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution.
Hydroelectricity, or hydroelectric power, is electricity generated from hydropower. Hydropower supplies one sixth of the world's electricity, almost 4,500 TWh in 2020, which is more than all other renewable sources combined and also more than nuclear power. Hydropower can provide large amounts of low-carbon electricity on demand, making it a key element for creating secure and clean electricity supply systems. A hydroelectric power station that has a dam and reservoir is a flexible source, since the amount of electricity produced can be increased or decreased in seconds or minutes in response to varying electricity demand. Once a hydroelectric complex is constructed, it produces no direct waste, and almost always emits considerably less greenhouse gas than fossil fuel-powered energy plants. However, when constructed in lowland rainforest areas, where part of the forest is inundated, substantial amounts of greenhouse gases may be emitted.
Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the final result of the settling process.
The ca. 1000 km long Tana River is the longest river in Kenya, it is also called Sagana River in the Mt Kenya region and gives its name to the Tana River County. Its catchment covers ca. 100,000 km2 and can be divided into the headwaters and the lower Tana consisting of the section downstream of Kora where the river flows for ca. 700 km through semi-arid plains. Its tributaries include some major rivers in the Central Region like Thika, Ragati River, Nyamindi, Thiba ,Mathioya, Chania, Thuci and Mutonga. The river rises from Mt Kenya in Nyeri. It initially runs southwest before turning south around the massif of Mount Kenya and meanders all the way up to the Indian Ocean. A series of hydroelectric dams has been constructed along the river. These include the Masinga Dam, the Kamburu Dam, the Gitaru Dam, the Kindaruma Dam and the Kiambere Dam. en The Masinga Reservoir and the Kiambere Reservoir, created by the Masinga and Kiambere dams respectively, serve a dual purpose: hydro-electric power (HEP) generation and agricultural irrigation. The other three are used exclusively for HEP generation. A 2003 study reported that two-thirds of Kenya's electrical needs were supplied by the series of dams along the Tana River. Many people believe this river has groundwater underneath it, but it doesn't. The electricity is then supplied to the national grid system and distributed countrywide through a series of substations, transformers and cables.
A reservoir is an enlarged lake behind a dam, usually built to store fresh water, often doubling for hydroelectric power generation.
The Elwha Dam was a 108-ft high dam located in the United States, in the state of Washington, on the Elwha River approximately 4.9 miles (7.9 km) upstream from the mouth of the river on the Strait of Juan de Fuca.
Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces are constructed during land development. During rain, storms, and other precipitation events, these surfaces, along with rooftops, carry polluted stormwater to storm drains, instead of allowing the water to percolate through soil. This causes lowering of the water table and flooding since the amount of water that remains on the surface is greater. Most municipal storm sewer systems discharge untreated stormwater to streams, rivers, and bays. This excess water can also make its way into people's properties through basement backups and seepage through building wall and floors.
A river is a natural freshwater stream that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it runs out of water, or only flow during certain seasons. Rivers are regulated by the water cycle, the processes by which water moves around the Earth. Water first enters rivers through precipitation, whether from the runoff of water down a slope, the melting of glaciers or snow, or seepage from aquifers beneath the surface of the Earth.
Dam removal is the process of demolishing a dam, returning water flow to the river. Arguments for dam removal consider whether their negative effects outweigh their benefits. The benefits of dams include hydropower production, flood control, irrigation, and navigation. Negative effects of dams include environmental degradation, such as reduced primary productivity, loss of biodiversity, and declines in native species; some negative effects worsen as dams age, like structural weakness, reduced safety, sediment accumulation, and high maintenance expense. The rate of dam removals in the United States has increased over time, in part driven by dam age. As of 1996, 5,000 large dams around the world were more than 50 years old. In 2020, 85% percent of dams in the United States are more than 50 years old. In the United States roughly 900 dams were removed between 1990 and 2015, and by 2015, the rate was 50 to 60 per year. France and Canada have also completed significant removal projects. Japan's first removal, of the Arase Dam on the Kuma River, began in 2012 and was completed in 2017. A number of major dam removal projects have been motivated by environmental goals, particularly restoration of river habitat, native fish, and unique geomorphological features. For example, fish restoration motivated the Elwha Ecosystem Restoration and the dam removal on the river Allier, while recovery of both native fish and of travertine deposition motivated the restoration of Fossil Creek.
The environmental impact of irrigation relate to the changes in quantity and quality of soil and water as a result of irrigation and the subsequent effects on natural and social conditions in river basins and downstream of an irrigation scheme. The effects stem from the altered hydrological conditions caused by the installation and operation of the irrigation scheme.
The Ebro is a river of the north and northeast of the Iberian Peninsula, in Spain. It rises in Cantabria and flows 930 kilometres (580 mi), almost entirely in an east-southeast direction. It flows into the Mediterranean Sea, forming a delta in the Terres de l'Ebre region, in southern Catalonia. In the Iberian peninsula, it ranks second in length after the Tagus and second in discharge volume, and drainage basin, after the Douro. It is the longest river entirely within Spain; the other two mentioned flow into Portugal. It is also the second-longest river in the Mediterranean basin, after the Nile.
A mouth bar is an element of a deltaic system, which refers to the typically mid-channel deposition of the sediment transported by the river channel at the river mouth.
An alluvial river is one in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. For this reason, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology; and the amount, size, and type of sediment that they carry.
Shihmen Dam is a major rock fill dam across the Dahan River in northern Taoyuan City. It forms the Shihmen Reservoir (石門水庫), Taiwan's third largest reservoir or artificial lake. It provides irrigation in Taoyuan, flood control for the Taipei Basin, and hydroelectricity and domestic water supply for more than three million people in northern Taiwan.
The estimated hydropower potential of Mekong River Basin about 58,930 Megawatts (MW). As of February 2024, there are an estimated 167 Hydropower Plants (HPPs) in the Mekong, with a combined installed capacity of some 36,376.3 MW. An additional 20 HPPs are currently under construction and at various stages of completion. These have a combined installed capacity of an additional 4,535.5 MW.
Legacy sediment (LS) is depositional bodies of sediment inherited from the increase of human activities since the Neolithic. These include a broad range of land use and land cover changes, such as agricultural clearance, lumbering and clearance of native vegetation, mining, road building, urbanization, as well as alterations brought to river systems in the form of dams and other engineering structures meant to control and regulate natural fluvial processes (erosion, deposition, lateral migration, meandering). The concept of LS is used in geomorphology, ecology, as well as in water quality and toxicological studies.
Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of sea-level rise. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term sustainability of deltas.