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A reservoir (; from French ''réservoir'' ) is an enlarged
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
behind a
dam 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 ...
. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an
embayment A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a na ...
within it, through excavation, or building any number of
retaining wall Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to ...
s or levees. In other contexts, "reservoirs" may refer to storage spaces for various fluids; they may hold liquids or gasses, including
hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...
. ''Tank reservoirs'' store these in ground-level, elevated, or buried
tanks A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engi ...
. Tank reservoirs for water are also called cisterns. Most underground reservoirs are used to store liquids, principally either water or
petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
.


Types


Dammed valleys

Dammed reservoirs are
artificial lake A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...
s created and controlled by a
dam 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 ...
constructed across a
valley A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ove ...
, and rely on the natural
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...
to provide most of the basin of the reservoir. These reservoirs can either be ''on-stream reservoirs'', which are located on the original streambed of the downstream
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...
and are filled by creeks, rivers or
rainwater Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water fo ...
that runs off the surrounding forested catchments; or ''
off-stream reservoir {{Unreferenced, date=December 2009 An off-stream reservoir is a reservoir that is not located on a streambed, and is supplied by a pipeline, aqueduct or an adjacent stream. San Luis Reservoir is the largest off-stream reservoir in the United Sta ...
s'', which receive diverted water from a nearby stream or aqueduct/ pipelined water from other on-stream reservoirs. Dams are typically located at a narrow part of a downstream of a natural basin. The valley sides act as natural walls, with the dam located at the narrowest practical point to provide strength and the lowest cost of construction. In many reservoir construction projects, people have to be moved and re-housed, historical artifacts moved or rare environments relocated by Roman peoples Examples include the temples of
Abu Simbel Abu Simbel is a historic site comprising two massive rock-cut temples in the village of Abu Simbel ( ar, أبو سمبل), Aswan Governorate, Upper Egypt, near the border with Sudan. It is situated on the western bank of Lake Nasser, about ...
(which were moved before the construction of the
Aswan Dam The Aswan Dam, or more specifically since the 1960s, the 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. Its significance largely eclipsed the previous Aswan L ...
to create Lake Nasser from the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest ...
in
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
), the relocation of the village 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 1965 to create a reservoir, Llyn Celyn, in order to supply Liverpool and Wirra ...
during the construction of
Llyn Celyn Llyn Celyn () is a reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in Gwynedd, Wales. It measures roughly long by wide, and has a maximum depth of . It has the capacity to hold of water. It was originally ...
, and the relocation of Borgo San Pietro of Petrella Salto during the construction of Lake Salto. Construction of a dammed reservoir will usually need the river to be diverted during part of the build, often through a temporary tunnel or by-pass channel. In hilly regions, reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs, the new top water level exceeds the watershed height on one or more of the feeder streams such as at Llyn Clywedog in Mid Wales. In such cases additional side dams are required to contain the reservoir. Where the topography is poorly suited to a single large reservoir, a number of smaller reservoirs may be constructed in a chain, as in the
River Taff The River Taff ( cy, Afon Taf) is a river in Wales. It rises as two rivers in the Brecon Beacons; the Taf Fechan (''little Taff'') and the Taf Fawr (''great Taff'') before becoming one just north of Merthyr Tydfil. Its confluence with th ...
valley where the Llwyn-on, Cantref and Beacons Reservoirs form a chain up the valley.


Coastal

Coastal reservoirs are fresh water storage reservoirs located on the sea
coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in ...
near the river mouth to store the flood water of a river. As the land-based reservoir construction is fraught with substantial land submergence,
coastal reservoir Coastal reservoir is a type of reservoir to store fresh water on sea coast area near a river delta. Saemanguem in South Korea, Marina Barrage in Singapore, Qingcaosha and Plover Cove in China, Delta Works in Netherlands, Thanneermukkom Bund in In ...
is preferred economically and technically since it does not use scarce land area. Many coastal reservoirs were constructed in Asia and Europe. Saemanguem in South Korea,
Marina Barrage Marina Barrage is a dam in southern Singapore built at the confluence of five rivers, across the Marina Channel between Marina East and Marina South. First conceptualised in 1987 by then prime minister Lee Kuan Yew to help achieve greater sel ...
in Singapore, Qingcaosha in China, and
Plover Cove Plovers ( , ) are a widely distributed group of wading birds belonging to the subfamily Charadriinae. Description There are about 66 species in the subfamily, most of them called "plover" or "dotterel". The closely related lapwing subf ...
in Hong Kong, are a few existing coastal reservoirs.


Bank-side

Where water is pumped or siphoned from a river of variable quality or size, bank-side reservoirs may be built to store the water. Such reservoirs are usually formed partly by excavation and partly by building a complete encircling bund or
embankment Embankment may refer to: Geology and geography * A levee, an artificial bank raised above the immediately surrounding land to redirect or prevent flooding by a river, lake or sea * Embankment (earthworks), a raised bank to carry a road, railwa ...
, which may exceed 6 km (4 miles) in circumference. Both the floor of the reservoir and the bund must have an impermeable lining or core: initially these were often made of puddled clay, but this has generally been superseded by the modern use of
rolled Rolling is a Motion (physics)#Types of motion, type of motion that combines rotation (commonly, of an Axial symmetry, axially symmetric object) and Translation (geometry), translation of that object with respect to a surface (either one or the ot ...
clay. The water stored in such reservoirs may stay there for several months, during which time normal biological processes may substantially reduce many contaminants and almost eliminate any
turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...
. The use of bank-side reservoirs also allows water abstraction to be stopped for some time, when the river is unacceptably polluted or when flow conditions are very low due to
drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
. The London water supply system is one example of the use of bank-side storage: the water is taken from the
River Thames The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the second-longest in the United Kingdom, after the R ...
and
River Lee The River Lee (Irish: ''An Laoi'') is a river in Ireland. It rises in the Shehy Mountains on the western border of County Cork and flows eastwards through Cork, where it splits in two for a short distance, creating an island on which Cork's ...
; several large Thames-side reservoirs such as
Queen Mary Reservoir The Queen Mary Reservoir is one of the largest of London's reservoirs supplying fresh water to London and parts of surrounding counties, and is located in the Borough of Spelthorne in Surrey. The reservoir covers and is above the surrounding are ...
can be seen along the approach to
London Heathrow Airport Heathrow Airport (), called ''London Airport'' until 1966 and now known as London Heathrow , is a major international airport in London, England. It is the largest of the six international airports in the London airport system (the others be ...
.


Service

Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed as
water tower A water tower is an elevated structure supporting a water tank constructed at a height sufficient to pressurize a distribution system for potable water, and to provide emergency storage for fire protection. Water towers often operate in conju ...
s, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs can be storage pools,
water tank A water tank is a container for storing water. Water tanks are used to provide storage of water for use in many applications, drinking water, irrigation agriculture, fire suppression, agricultural farming, both for plants and livestock, chemi ...
s or sometimes entirely underground cisterns, especially in more hilly or mountainous country. In the United Kingdom,
Thames Water Thames Water Utilities Ltd, known as Thames Water, is a large private utility company responsible for the public water supply and waste water treatment in most of Greater London, Luton, the Thames Valley, Surrey, Gloucestershire, north Wiltsh ...
has many underground reservoirs built in the 1800s, most of which are lined with brick. A good example is the
Honor Oak Honor Oak is an inner suburban area principally of the London Borough of Lewisham, with part in the London Borough of Southwark. It is named after the oak tree on One Tree Hill that Elizabeth I is reputed to have picnicked under. Overview One Tr ...
Reservoir in London, constructed between 1901 and 1909. When it was completed it was said to be the largest brick built underground reservoir in the world and it is still one of the largest in Europe. This reservoir now forms part of the southern extension of the
Thames Water Ring Main The Thames Water Ring Main (TWRM, formerly the London Water Ring Main) is a system of approximately of concrete tunnels which transfer drinking water from water treatment works in the Thames and River Lea catchments for distribution within c ...
. The top of the reservoir has been grassed over and is now used by the Aquarius Golf Club. Service reservoirs perform several functions, including ensuring sufficient head of water in the
water distribution system A water distribution system is a part of water supply network with components that carry potable water from a centralized treatment plant or wells to consumers to satisfy residential, commercial, industrial and fire fighting requirements. Defini ...
and providing water capacity to even out peak demand from consumers, enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to reduce the cost of pumping, by refilling the reservoir at times of day when energy costs are low.


History

Circa 3000 BC, the craters of extinct
volcano A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Ear ...
es in
Arabia The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plat ...
were used as reservoirs by farmers for their
irrigation Irrigation (also referred to as watering) 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 been devel ...
water. Dry climate and
water scarcity in India Water scarcity in India is an ongoing water crisis that affects nearly hundreds of million of people each year. In addition to affecting the huge rural and urban population, the water scarcity in India also extensively affects the ecosystem and a ...
led to early development of
stepwell Stepwells (also known as vavs or baori) are wells or ponds with a long corridor of steps that descend to the water level. Stepwells played a significant role in defining subterranean architecture in western India from 7th to 19th century. So ...
s and
water resource management Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the Earth is salt water and only three percent is fresh water; slightl ...
techniques, including the building of a reservoir at
Girnar Girnar is an ancient hill in Junagadh, Gujarat, India. Geology Mount Girnar is a major igneous plutonic complex which intruded into the basalts towards the close of the Deccan Trap period. The rock types identified in this complex are ga ...
in 3000 BC. Artificial lakes dating to the 5th century BC have been found in ancient Greece. The artificial Bhojsagar lake in present-day
Madhya Pradesh Madhya Pradesh (, ; meaning 'central province') is a state in central India. Its capital is Bhopal, and the largest city is Indore, with Jabalpur, Ujjain, Gwalior, Sagar, and Rewa being the other major cities. Madhya Pradesh is the seco ...
state of India, constructed in the 11th century, covered .
Kush Kush or Cush may refer to: Bible * Cush (Bible), two people and one or more places in the Hebrew Bible Places * Kush (mountain), a mountain near Kalat, Pakistan Balochistan * Kush (satrapy), a satrapy of the Achaemenid Empire * Hindu Kush, a ...
invented the Hafir, which is a type of reservoir, during the Meroitic period. 800 ancient and modern hafirs have been registered in the Meroitic town of Butana.Fritz Hintze, Kush XI; pp.222-224. The Hafirs catch the water during raining season in order to ensure water is available for several months during the dry season to supply drinking water, irrigate fields and water cattle. The Great Reservoir near the Lion Temple in
Musawwarat es-Sufra Musawwarat es-Sufra (Arabic: , Meroitic: Aborepi, Old Egyptian: jbrp, jpbr-ˁnḫ), also known as Al-Musawarat Al-Sufra, is a large Meroitic temple complex in modern Sudan, dating back to the early Meroitic period of the 3rd century BC. It is l ...
is a notable hafir in Kush. In Sri Lanka, large reservoirs were created by ancient
Sinhalese Sinhala may refer to: * Something of or related to the Sinhalese people of Sri Lanka * Sinhalese people * Sinhala language, one of the three official languages used in Sri Lanka * Sinhala script, a writing system for the Sinhala language ** Sinha ...
kings in order to save the water for irrigation. The famous Sri Lankan king Parākramabāhu I of Sri Lanka said "Do not let a drop of water seep into the ocean without benefiting mankind". He created the reservoir named
Parakrama Samudra Parakrama Samudra (or King Parakrama's sea or the Sea of King Parakrama) is a shallow reservoir (wewa), consisting of five separate wewa (reservoirs) (thopa, dumbutulu, erabadu, bhu, kalahagala tanks) connected by narrow channels in Polonnaruwa, S ...
(sea of King Parakrama). Vast artificial reservoirs were also built by various ancient kingdoms in Bengal, Assam and Cambodia.


Uses


Direct water supply

Many dammed river reservoirs and most bank-side reservoirs are used to provide the raw water feed to a
water treatment Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...
plant which delivers
drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
through water mains. The reservoir does not merely hold water until it is needed: it can also be the first part of the
water treatment Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...
process. The time the water is held before it is released is known as the ''retention time''. This is a design feature that allows particles and
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
s to settle out, as well as time for natural biological treatment using algae,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
that naturally live in the water. However natural
limnological Limnology ( ; from Greek λίμνη, ''limne'', "lake" and λόγος, ''logos'', "knowledge") is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characterist ...
processes in temperate climate lakes produce temperature
stratification Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...
in the water, which tends to partition some elements such as
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
and
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
into deep, cold anoxic water during the summer months. In the autumn and winter the lake becomes fully mixed again. During drought conditions, it is sometimes necessary to draw down the cold bottom water, and the elevated levels of manganese in particular can cause problems in water treatment plants.


Hydroelectricity

In 2005, about 25% of the world's 33,105 large dams (over 15 metres in height) were used for hydroelectricity. The U.S. produces 3% of its electricity from 80,000 dams of all sizes. An initiative is underway to retrofit more dams as a good use of existing infrastructure to provide many smaller communities with a reliable source of energy. A reservoir generating
hydroelectricity Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined an ...
includes
turbines A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful Work (physics), work. The work produced by a turbine can be used ...
connected to the retained water body by large-diameter pipes. These generating sets may be at the base of the dam or some distance away. In a flat river valley a reservoir needs to be deep enough to create a head of water at the turbines; and if there are periods of drought the reservoir needs to hold enough water to average out the river's flow throughout the year(s). Run-of-the-river hydro in a steep valley with constant flow needs no reservoir. Some reservoirs generating hydroelectricity use pumped recharge: a high-level reservoir is filled with water using high-performance electric pumps at times when electricity demand is low, and then uses this stored water to generate electricity by releasing the stored water into a low-level reservoir when electricity demand is high. Such systems are called pump-storage schemes.


Controlling watersources

Reservoirs can be used in a number of ways to control how water flows through downstream waterways: :Downstream water supply – water may be released from an upland reservoir so that it can be abstracted for drinking water lower down the system, sometimes hundreds of miles further downstream. :Irrigation – water in an
irrigation Irrigation (also referred to as watering) 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 been devel ...
reservoir may be released into networks of
canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface flo ...
s for use in
farm A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is used ...
lands or secondary water systems. Irrigation may also be supported by reservoirs which maintain river flows, allowing water to be abstracted for irrigation lower down the river. :Flood control – also known as an "attenuation" or "balancing" reservoirs, flood control reservoirs collect water at times of very high rainfall, then release it slowly during the following weeks or months. Some of these reservoirs are constructed across the river line, with the onward flow controlled by an
orifice plate An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a '). Description An orifice plate is a thin plate with a hole in it, which is usually placed in ...
. When river flow exceeds the capacity of the orifice plate, water builds up behind the dam; but as soon as the flow rate reduces, the water behind the dam is slowly released until the reservoir is empty again. In some cases, such reservoirs only function a few times in a decade, and the land behind the reservoir may be developed as community or recreational land. A new generation of balancing dams are being developed to combat the possible
consequences of climate change The effects of climate change impact the physical environment, ecosystems and human societies. The environmental effects of climate change are broad and far-reaching. They affect the water cycle, oceans, sea and land ice ( glaciers), sea le ...
. They are called "Flood Detention Reservoirs". Because these reservoirs will remain dry for long periods, there may be a risk of the clay core drying out, reducing its structural stability. Recent developments include the use of composite core fill made from recycled materials as an alternative to clay. :Canals – Where a natural watercourse's water is not available to be diverted into a
canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface flo ...
, a reservoir may be built to guarantee the water level in the canal: for example, where a canal climbs through
lock Lock(s) may refer to: Common meanings *Lock and key, a mechanical device used to secure items of importance *Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainment * ''Lock ...
s to cross a range of hills. Another use is to reduce costs or construction time when the canal must be dug through rock, as used on the Rideau Canal with
The Narrows __NOTOC__ The Narrows is the tidal strait separating the boroughs of Staten Island and Brooklyn in New York City, United States. It connects the Upper New York Bay and Lower New York Bay and forms the principal channel by which the Hudson Riv ...
locks dividing the two Rideau's and essentially turning the upper Rideau into an enlarged reservoir, albeit only by two or three feet. :Recreation – water may be released from a reservoir to create or supplement
white water Whitewater forms in a rapid context, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and w ...
conditions for kayaking and other white-water sports. On
salmon Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus ''Salmo'') and North Pacific (genus '' Oncorhy ...
id rivers special releases (in Britain called ''
freshet The term ''freshet'' is most commonly used to describe a spring thaw resulting from snow and ice melt in rivers located in upper North America. A spring freshet can sometimes last several weeks on large river systems, resulting in significant in ...
s'') are made to encourage natural migration behaviours in fish and to provide a variety of fishing conditions for anglers.


Flow balancing

Reservoirs can be used to balance the flow in highly managed systems, taking in water during high flows and releasing it again during low flows. In order for this to work without pumping requires careful control of water levels using
spillway A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure th ...
s. When a major storm approaches, the dam operators calculate the volume of water that the storm will add to the reservoir. If forecast storm water will overfill the reservoir, water is slowly let out of the reservoir prior to, and during, the storm. If done with sufficient lead time, the major storm will not fill the reservoir and areas downstream will not experience damaging flows. Accurate weather forecasts are essential so that dam operators can correctly plan drawdowns prior to a high rainfall event. Dam operators blamed a faulty weather forecast on the
2010–2011 Queensland floods A series of floods hit Queensland, Australia, beginning in November 2010. The floods forced the evacuation of thousands of people from towns and cities. At least 90 towns and over 200,000 people were affected. Damage initially was estimated a ...
. Examples of highly managed reservoirs are
Burrendong Dam Burrendong Dam is a rock-fill embankment major gated dam with a clay core across the Macquarie River upstream of Wellington in the central west region of New South Wales, Australia. The dam's purpose includes flood mitigation, irrigation, ...
in Australia and
Bala Lake Bala Lake ( cy, Llyn Tegid ) is a large freshwater glacial lake in Gwynedd, Wales. The River Dee, which has its source on the slopes of Dduallt in the mountains of Snowdonia, feeds the long by wide lake. It was the largest natural body of ...
(''Llyn Tegid'') in
North Wales North Wales ( cy, Gogledd Cymru) is a region of Wales, encompassing its northernmost areas. It borders Mid Wales to the south, England to the east, and the Irish Sea to the north and west. The area is highly mountainous and rural, with Snowdonia N ...
. Bala Lake is a natural lake whose level was raised by a low dam and into which the River Dee flows or discharges depending upon flow conditions, as part of the River Dee regulation system. This mode of operation is a form of hydraulic
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...
in the river system.


Recreation

Many reservoirs often allow some recreational uses, such as
fishing Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment, but may also be caught from stocked bodies of water such as ponds, canals, park wetlands and reservoirs. Fishing techniques inclu ...
and boating. Special rules may apply for the safety of the public and to protect the quality of the water and the ecology of the surrounding area. Many reservoirs now support and encourage less formal and less structured recreation such as natural history,
bird watching Birdwatching, or birding, is the observing of birds, either as a recreational activity or as a form of citizen science. A birdwatcher may observe by using their naked eye, by using a visual enhancement device like binoculars or a telescope, by ...
,
landscape painting Landscape painting, also known as landscape art, is the depiction of natural scenery such as mountains, valleys, trees, rivers, and forests, especially where the main subject is a wide view—with its elements arranged into a coherent compo ...
, walking and
hiking Hiking is a long, vigorous walk, usually on trails or footpaths in the countryside. Walking for pleasure developed in Europe during the eighteenth century.AMATO, JOSEPH A. "Mind over Foot: Romantic Walking and Rambling." In ''On Foot: A Histor ...
, and often provide information boards and interpretation material to encourage responsible use.


Operation

Water falling as rain upstream of the reservoir, together with any
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...
emerging as springs, is stored in the reservoir. Any excess water can be spilled via a specifically designed spillway. Stored water may be piped by
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
for use as
drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
, to generate
hydro-electricity Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined an ...
or to maintain river flows to support downstream uses. Occasionally reservoirs can be managed to retain water during high rainfall events to prevent or reduce downstream flooding. Some reservoirs support several uses, and the operating rules may be complex. Most modern reservoirs have a specially designed
draw-off tower An intake tower or outlet tower is a vertical tubular structure with one or more openings used for capturing water from reservoirs and conveying it further to a hydroelectric or water-treatment plant. Unlike spillways, intake towers are intended ...
that can discharge water from the reservoir at different levels, both to access water as the water level falls, and to allow water of a specific quality to be discharged into the downstream river as "compensation water": the operators of many upland or in-river reservoirs have obligations to release water into the downstream river to maintain river quality, support fisheries, to maintain downstream industrial and recreational uses or for a range of other purposes. Such releases are known as ''compensation water''.


Terminology

The units used for measuring reservoir areas and volumes vary from country to country. In most of the world, reservoir areas are expressed in square kilometres; in the United States, acres are commonly used. For volume, either cubic metres or cubic kilometres are widely used, with acre-feet used in the US. The capacity, volume, or storage of a reservoir is usually divided into distinguishable areas. ''Dead'' or ''inactive'' storage refers to water in a reservoir that cannot be drained by gravity through a dam's
outlet works A gatehouse, gate house, outlet works or valve house for a dam is a structure housing sluice gates, valves, or pumps (in which case it is more accurately called a pumping station). Many gatehouses are strictly utilitarian, but especially in the n ...
, spillway, or power plant intake and can only be pumped out. Dead storage allows sediments to settle, which improves water quality and also creates an area for fish during low levels. ''Active'' or ''live'' storage is the portion of the reservoir that can be used for flood control, power production,
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
, and downstream releases. In addition, a reservoir's "flood control capacity" is the amount of water it can regulate during flooding. The "surcharge capacity" is the capacity of the reservoir above the spillway crest that cannot be regulated. In the United States, the water below the normal maximum level of a reservoir is called the "conservation pool". In the United Kingdom, "top water level" describes the reservoir full state, while "fully drawn down" describes the minimum retained volume.


Modelling reservoir management

There is a wide variety of software for modelling reservoirs, from the specialist Dam Safety Program Management Tools (DSPMT) to the relatively simple WAFLEX, to integrated models like the Water Evaluation And Planning system (WEAP) that place reservoir operations in the context of system-wide demands and supplies.


Safety

In many countries large reservoirs are closely regulated to try to prevent or minimise failures of containment. While much of the effort is directed at the dam and its associated structures as the weakest part of the overall structure, the aim of such controls is to prevent an uncontrolled release of water from the reservoir. Reservoir failures can generate huge increases in flow down a river valley, with the potential to wash away towns and villages and cause considerable loss of life, such as the devastation following the failure of containment at
Llyn Eigiau Llyn Eigiau is a lake on the edge of the Carneddau range of mountains in Snowdonia, Conwy, Wales. The name ''Eigiau'' is thought to refer to the shoals of fish which once lived here. Early maps refer to it as ''Llynyga''. It is thought that ...
which killed 17 people.(see also
List of dam failures A dam failure or dam burst is a catastrophic type of structural failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of such an uncontrolled release. Between the years 2000 and 2009 more than ...
) A notable case of reservoirs being used as an instrument of war involved the British
Royal Air Force The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) an ...
Dambusters raid on Germany in World War II (codenamed "
Operation Chastise Operation Chastise or commonly known as the Dambusters Raid was an attack on Nazi Germany, German dams carried out on the night of 16/17 May 1943 by No. 617 Squadron RAF, 617 Squadron RAF Bomber Command, later called the Dam Busters, using sp ...
"), in which three German reservoir dams were selected to be breached in order to damage German infrastructure and manufacturing and power capabilities deriving from the Ruhr and Eder rivers. The economic and social impact was derived from the enormous volumes of previously stored water that swept down the valleys, wreaking destruction. This raid later became the basis for several films.


Environmental impact


Whole life environmental impact

All reservoirs will have a monetary cost/benefit assessment made before construction to see if the project is worth proceeding with. However, such analysis can often omit the environmental impacts of dams and the reservoirs that they contain. Some impacts, such as the greenhouse gas production associated with concrete manufacture, are relatively easy to estimate. Other impact on the natural environment and social and cultural effects can be more difficult to assess and to weigh in the balance but identification and quantification of these issues are now commonly required in major construction projects in the developed world


Climate change


Reservoir greenhouse gas emissions

Naturally occurring lakes receive organic sediments which decay in an
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
environment releasing
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 relative abundance of methane on Ea ...
and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
. The methane released is approximately 8 times more potent as a greenhouse gas than carbon dioxide. As a man-made reservoir fills, existing plants are submerged and during the years it takes for this matter to decay, will give off considerably more greenhouse gases than lakes do. A reservoir in a narrow valley or canyon may cover relatively little vegetation, while one situated on a plain may flood a great deal of vegetation. The site may be cleared of vegetation first or simply flooded. Tropical flooding can produce far more greenhouse gases than in temperate regions. The following table indicates reservoir emissions in milligrams per square meter per day for different bodies of water.


Hydroelectricity and climate change

Depending upon the area flooded versus power produced, a reservoir built for hydro-electricity generation can either reduce or increase the net production of greenhouse gases when compared to other sources of power. A study for the National Institute for Research in the Amazon found that hydroelectric reservoirs release a large pulse of carbon dioxide from decay of trees left standing in the reservoirs, especially during the first decade after flooding. This elevates the global warming impact of the dams to levels much higher than would occur by generating the same power from fossil fuels. According to the
World Commission on Dams The World Commission on Dams (WCD) existed between April 1997 and 2001, to research the environmental, social and economic impacts of the development of large dams globally. The self-styled WCD consisted of members of civil society, academia, the ...
report (Dams And Development), when the reservoir is relatively large and no prior clearing of forest in the flooded area was undertaken,
greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and ...
from the reservoir could be higher than those of a conventional oil-fired thermal generation plant. For instance, In 1990, the impoundment behind the Balbina Dam in Brazil (inaugurated in 1987) had over 20 times the impact on global warming than would generating the same power from fossil fuels, due to the large area flooded per unit of electricity generated. Another study published in the Global Biogeochemical Cycles also found that newly-flooded reservoirs released more carbon dioxide and methane than the pre-flooded landscape, noting that forest lands, wetlands, and preexisting water features all released differing amounts of carbon dioxide and methane both pre- and post-flooding. The
Tucuruí Dam The Tucuruí Dam (Tucuruí means "grasshopper's water", translated from Tupí language; pt, Tucuruí) is a concrete gravity dam on the Tocantins River located on the Tucuruí County in the State of Pará, Brazil. The main purpose of the dam is ...
in Brazil (completed in 1984) had only 0.4 times the impact on global warming than would generating the same power from fossil fuels. A two-year study of carbon dioxide and methane releases in Canada concluded that while the hydroelectric reservoirs there do emit greenhouse gases, it is on a much smaller scale than thermal power plants of similar capacity. Hydropower typically emits 35 to 70 times less greenhouse gases per TWh of electricity than thermal power plants. A decrease in air pollution occurs when a dam is used in place of
thermal power A thermal power station is a type of power station in which heat energy In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary ...
generation, since electricity produced from hydroelectric generation does not give rise to any flue gas emissions from fossil fuel combustion (including sulfur dioxide, nitric oxide and
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
from
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
).


Biology

Dams can produce a block for migrating fish, trapping them in one area, producing food and a habitat for various water-birds. They can also flood various
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s on land and may cause extinctions. Creating reservoirs can alter the natural biogeochemical cycle of mercury. After a reservoir's initial formation, there is a large increase in the production of toxic methylmercury (MeHg) via microbial methylation in flooded soils and peat. MeHg levels have also been found to increase in
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
and in fish.


Human impact

Dams can severely reduce the amount of water reaching countries downstream of them, causing water stress between the countries, e.g. the Sudan and
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
, which damages farming businesses in the downstream countries, and reduces drinking water. Farms and villages, e.g.
Ashopton Ashopton was a small village in Derbyshire, England, in the valley of the River Ashop. The village population was less than 100. Details are included in the civil parish of Aston, Derbyshire. In the early 1940s, the village (along with neighbou ...
can be flooded by the creation of reservoirs, ruining many livelihoods. For this very reason, worldwide 80 million people (figure is as of 2009, from the Edexcel GCSE Geography textbook) have had to be forcibly relocated due to dam construction.


Limnology

The
limnology Limnology ( ; from Greek λίμνη, ''limne'', "lake" and λόγος, ''logos'', "knowledge") is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characteris ...
of reservoirs has many similarities to that of lakes of equivalent size. There are however significant differences. Many reservoirs experience considerable variations in level producing significant areas that are intermittently underwater or dried out. This greatly limits the productivity or the water margins and also limits the number of species able to survive in these conditions. Upland reservoirs tend to have a much shorter residence time than natural lakes and this can lead to more rapid cycling of nutrients through the water body so that they are more quickly lost to the system. This may be seen as a mismatch between water chemistry and water biology with a tendency for the biological component to be more
oligotroph An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...
ic than the chemistry would suggest. Conversely, lowland reservoirs drawing water from nutrient rich rivers, may show exaggerated
eutrophic Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplan ...
characteristics because the residence time in the reservoir is much greater than in the river and the biological systems have a much greater opportunity to utilise the available nutrients. Deep reservoirs with multiple level draw off towers can discharge deep cold water into the downstream river greatly reducing the size of any
hypolimnion The hypolimnion or under lake is the dense, bottom layer of water in a thermally- stratified lake. The word hypolimnion is derived from the Greek "limnos" meaning "lake". It is the layer that lies below the thermocline. Typically the hypolimni ...
. This in turn can reduce the concentrations of phosphorus released during any annual mixing event and may therefore reduce
productivity Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...
. The dams in front of reservoirs act as
knickpoint In geomorphology, a knickpoint or nickpoint is part of a river or channel where there is a sharp change in channel slope, such as a waterfall or lake. Knickpoints reflect different conditions and processes on the river, often caused by previous ...
s-the
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
of the water falling from them reduces and deposition is a result below the dams.


Seismicity

The filling (impounding) of reservoirs has often been attributed to reservoir-triggered seismicity (RTS) as seismic events have occurred near large dams or within their reservoirs in the past. These events may have been triggered by the filling or operation of the reservoir and are on a small scale when compared to the amount of reservoirs worldwide. Of over 100 recorded events, some early examples include the tall Marathon Dam in Greece (1929), the tall Hoover Dam in the U.S. (1935). Most events involve large dams and small amounts of seismicity. The only four recorded events above a 6.0- magnitude (Mw) are the tall
Koyna Dam The Koyna Dam is one of the largest dams in Maharashtra, India. It is a rubble-concrete dam constructed on Koyna River which rises in Mahabaleshwar, a hillstation in Sahyadri ranges. It is located in Koyna Nagar, Satara district, in the Wes ...
in India and the
Kremasta Dam The Kremasta Dam is an earth-fill embankment dam on the Achelous River in Aetolia-Acarnania, Greece. It is located just downstream of where the Agrafiotis, Tavropos and Trikeriotis rivers meet to form the Achelous. The dam was constructed betw ...
in Greece which both registered 6.3-Mw, the high Kariba Dam in
Zambia Zambia (), officially the Republic of Zambia, is a landlocked country at the crossroads of Central, Southern and East Africa, although it is typically referred to as being in Southern Africa at its most central point. Its neighbours are t ...
at 6.25-Mw and the Xinfengjiang Dam in China at 6.1-Mw. Disputes have occurred regarding when RTS has occurred due to a lack of hydrogeological knowledge at the time of the event. It is accepted, though, that the infiltration of water into pores and the weight of the reservoir do contribute to RTS patterns. For RTS to occur, there must be a seismic structure near the dam or its reservoir and the seismic structure must be close to failure. Additionally, water must be able to infiltrate the deep rock stratum as the weight of a deep reservoir will have little impact when compared the deadweight of rock on a crustal stress field, which may be located at a depth of or more.


Microclimate

Reservoirs may change the local micro-climate increasing humidity and reducing extremes of temperature, especially in dry areas. Such effects are claimed also by some
South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
n
wineries A winery is a building or property that produces wine, or a business involved in the production of wine, such as a wine company. Some wine companies own many wineries. Besides wine making equipment, larger wineries may also feature warehouses, b ...
as increasing the quality of the wine production.


List of reservoirs

In 2005 there were 33,105 large dams (≥15 m height) listed by the International Commission on Large Dams (ICOLD).


List of reservoirs by area


List of reservoirs by volume


See also

*
Ab Anbar An ''ab anbar'' ( fa, آب‌انبار, literally "Cistern") is a traditional reservoir or cistern of drinking water in Greater Iran in antiquity. Structure To withstand the pressure the water exerts on the containers of the storage tank, ...
*
Coastal sediment supply Coastal sediment supply is the transport of sediment to the beach environment by both fluvial and aeolian transport. While aeolian transport plays a role in the overall sedimentary budget for the coastal environment, it is paled in comparison to ...
*
Colourful lakelets Colourful lakelets (Polish "Kolorowe Jeziorka") is the name of three (sometimes four) artificial ponds formed in place of former mines at the slope of Wielka Kopa mountain (871 m) in Rudawy Janowickie, range in Sudetes Mountains, Poland. The bigg ...
(in Poland) *
Dam failure A dam failure or dam burst is a catastrophic type of structural failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of such an uncontrolled release. Between the years 2000 and 2009 more than ...
*
Drainage basin A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
* Forebay *
Head of the reservoir In hydrology, the head is the point on a watercourse up to which it has been artificially broadened and/or raised by an impoundment. Above the head of the reservoir natural conditions prevail; below it the water level above the riverbed has be ...
*
Mill pond A mill pond (or millpond) is a body of water used as a reservoir for a water-powered mill. Description Mill ponds were often created through the construction of a mill dam or weir (and mill stream) across a waterway. In many places, the c ...
*
Quarry lake A quarry lake is a lake that is formed after a quarry has been dug through a mining operation. Formation During the mining process, water must be emptied. But after the mining operation has been abandoned, groundwater is allowed to seep in, ...
* Reservoir safety *
Shade balls Shade balls are small plastic spheres floated on top of a reservoir for environmental reasons, including to slow evaporation and prevent sunlight from causing reactions among chemical compounds present in the water. Also known as bird balls, the ...


References


External links

* * {{authority control Lakes by type Bodies of water