Reservoir (water)

For other uses of the term, see reservoir (disambiguation).

A reservoir is, most broadly, anything used for storage of large amounts of a liquid, which can be put into and drawn from the reservoir. It is in essence a buffer.

Most often, a reservoir refers to an artificial lake, used to store water for various uses. They are created first by building a sturdy dam, usually out of cement, earth, rock, or a mixture of all three. Once the dam is completed, a stream is allowed to flow behind it and eventually fill it to capacity.

Reservoir uses

Primary uses

Reservoirs are most usually made for one of the following reasons:

to hold water for domestic, agricultural, and industrial use, particularly as drinking water
to hold water to power machinery, such as a water mill, or to generate electricity or to store electricity
To supply water to a canal or irrigation system.
To hold water that would flood a community, where it can slowly and safely be released.

Secondary uses

Reservoirs may have a secondary use other than a primary use. These may include:

holding fish
leisure uses, such as sailing, fishing, or water skiing

Criticisms of reservoirs

dispossession of population and destruction of property via eminent domain
destruction of habitats under reservoir
suggestions that large water bodies can act a host for mosquitoes and associated disease
increased erosion downstream because of the increased energy of water flow, that results due to the deposition of sediment that results when the water loses energy when it collects in the reservoir
deposition of sediments behind the dam results in reduced reservoir size
prevention of downstream flooding can have negative impacts on farming as fertile silt is not deposited
reduced water supply downstream, less water is available for use by those downstream
increased humidity in desert areas
interruption of migration and spawning for aquatic species

Construction of reservoirs and dams

To build dams and reservoirs, surveyors have to find river valleys which are deep and narrow; the valley sides can then act as natural walls. The best place for building a dam has to be determined. If necessary, humans have to be re-housed and/or historic sites have to be moved, e.g. the temples of Abu Simbel before the construction of the Aswan Dam, creating Lake Nasser in the Nile River, Egypt.

First, the river must be redirected slightly so as to not disturb construction of the dam. Once that is done, building of the dam may commence. This may take anywhere from a few months to a few years, depending on its size and complexity. After the dam is complete, the river is allowed to begin filling the large area behind the dam.

In the UK, many smaller reservoirs (for instance, those used to supply a village) are underground, either completely invisible from the surface or covered by a roof and grassed over. Thames Water has a large brick-built underground reservoir beneath London, Honor Oak, which was completed in 1909 and is the largest of its type on Europe. The roof is used as a golf course.

Dam and reservoir operation

Standard reservoir

A normal, standard reservoir simply holds water until it is needed. Water is withdrawn from the reservoir to be cleaned (for drinking water). In the event that major rain occurs, water can be released, decreasing the reservoir's water level.

Hydroelectric reservoir

Inside of a hydroelectric dam, there are large turbines that, when water flows through the pipes and turbine, spins to generate electricity. As with a normal reservoir, water could be withdrawn for use as drinking or irrigation water, or could be released if the reservoir is too full.

Irrigation reservoir

Water in an irrigation reservoir is released into networks of canals mainly for use in farmlands or secondary water systems. Water in an irrigation reservoir is generally not used for drinking water, but in some cases is. As with all reservoirs, water can be released if the reservoir is too full.

Flood control reservoir

Used to prevent flooding to lower lying lands, flood control reservoirs collect water mainly in spring, then release it slowly over the course of the summer.

Recreational reservoir

Very rarely is a reservoir built solely for a recreational purpose. Most reservoirs are built to a civic purpose, but still allow fishing, boating, and other activities. At most reservoirs, special rules apply for the safety of the public.

Largest reservoirs

Lake Volta in western Africa covers around 8,500 km². Lokka is Northern Europe's largest artificial lake, 417 km² in size. The IJsselmeer is Western Europe's largest artificial lake, 1250 km² in size

Surface area

The following are the world's ten largest reservoirs by surface area:

Lake Volta (8,482 km²; Ghana)
Samara Reservoir (6,450 km²; Russia)
Smallwood Reservoir (5,698 km²; Canada)
Lake Kariba (5,580 km²; Zimbabwe, Zambia)
Bukhtarma Reservoir (5,490 km²; Kazakhstan)
Bratsk Reservoir (5,426 km²; Russia)
Lake Nasser (5,248 km²; Egypt, Sudan)
Rybinsk Reservoir (4,580 km²; Russia)
Caniapiscau Reservoir (4,318 km²; Canada)
Lake Guri (4,250 km²; Venezuela)

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