Reservoir
Artificial lake created by a dam to store fresh water.
A reservoir is a man-made lake, typically formed by building a dam across a river or valley, to store fresh water. Many reservoirs also generate hydroelectric power. They can be created in several ways: by damming a watercourse, diverting a river to flood a valley, excavating land, or constructing walls and levees to contain water.
There are several types. Dammed valley reservoirs are artificial lakes created by placing a dam across a valley, using the natural slopes as walls. These can be "on-stream," sitting directly on a riverbed and fed by streams and runoff, or "off-stream," filled by diverted water from other sources. Dams are usually built at the narrowest, strongest point of a valley to reduce cost. Building such reservoirs often requires relocating people, moving historical sites (like the temples of Abu Simbel), or preserving rare environments. During construction, the river is often diverted through a temporary tunnel. In hilly areas, existing lakes may be enlarged, sometimes requiring extra side dams if the water level rises above the watershed. Where a single large reservoir isn't practical, a chain of smaller ones may be built in a valley.
Coastal reservoirs store fresh water near a river mouth on the seacoast, capturing floodwater without flooding valuable land. They are common in Asia and Europe, with examples in South Korea, Singapore, China, and Hong Kong.
Bank-side reservoirs are built next to a river, often partly excavated and enclosed by an embankment up to 6 km around. The floor and walls must be waterproof, historically using puddled clay but now often rolled clay. Water can be stored for months, allowing natural processes to reduce contaminants and turbidity. Abstraction can also be paused during pollution or drought. London’s system, for instance, uses large bank-side reservoirs along the Thames and Lea.
Service reservoirs store treated drinking water near where it’s used. They can be elevated water towers, storage pools, tanks, or underground cisterns. Modern ones often use geomembrane liners to prevent seepage and floating covers to reduce evaporation. In the UK, Thames Water has many 1800s brick-lined underground reservoirs, like the Honor Oak Reservoir in London, once the world’s largest brick underground reservoir. It now sits beneath a golf course and is part of the Thames Water Ring Main. These reservoirs maintain water pressure, balance peak demand, and allow treatment plants to run efficiently. They can also be refilled during low-cost energy periods to reduce pumping costs.
Irrigation reservoirs store water for agriculture, filled by pumped groundwater, river water, or runoff, and are used mainly during dry seasons.
- type
- Artificial water storage
- common_uses
- Direct water supply, hydroelectric power, irrigation, flood control
- construction_methods
- Dammed valleys, coastal, bank-side, service, irrigation reservoirs
- notable_example
- Parakrama Samudra (Sri Lanka), Marib Dam (Yemen), Honor Oak Reservoir (London)
Lore & Background
Reservoirs are enlarged lakes created by controlling a watercourse, typically behind a dam, and are built primarily to store fresh water, often also serving hydroelectric power generation. They can be formed by damming a valley, interrupting a stream to create an embayment, excavating land, or constructing retaining walls and levees. Dammed valley reservoirs rely on natural topography, with dams placed at narrow points downstream of a basin for strength and cost efficiency; these may be on-stream, filled by rivers and runoff, or off-stream, receiving diverted water. Construction often requires river diversion via tunnels or channels, and may involve relocating communities, historical sites, or rare environments. In hilly regions, existing lakes are sometimes enlarged, and where a single large basin is unsuitable, chains of smaller reservoirs can be built. Coastal reservoirs store fresh water near river mouths, avoiding land submergence, while bank-side reservoirs are formed by excavation and encircling embankments, often with impermeable linings, allowing water to be stored for months to reduce contaminants and turbidity. Service reservoirs hold treated potable water near distribution points, ranging from elevated water towers to underground cisterns, and may use geomembrane liners to limit seepage and evaporation.
Reader's Guide
Reservoirs are significant for their role in storing fresh water for direct supply, irrigation, and hydroelectric power generation. They are built in various forms: dammed valleys, coastal reservoirs near river mouths, bank-side reservoirs that store water from variable-quality rivers, and service reservoirs that hold treated potable water near distribution points. The construction of dammed reservoirs often requires relocating people, historical artifacts, or rare environments, as seen with the temples of Abu Simbel moved before the Aswan Dam created Lake Nasser. Coastal reservoirs are preferred in some regions because they do not use scarce land area. Service reservoirs help ensure water pressure and manage peak demand, and can reduce pumping costs by refilling when energy costs are low. The article notes a debate over irrigation reservoirs: opponents view them as a monopolization of resources that may worsen droughts, while proponents see them as a solution for sustainable agriculture until a more durable model emerges.
Did You Know?
- The Kingdom of Kush invented the Hafir, a type of reservoir, during the Meroitic period.
- Coastal reservoirs store fresh water on the sea coast near a river mouth and do not use scarce land area.
Frequently Asked Questions
What is a Reservoir?
A reservoir is an artificial lake formed when a dam blocks a natural watercourse, trapping fresh water behind it. It functions as a managed storage body that can be drawn on for a variety of human needs.
What are the main purposes of a Reservoir?
Reservoirs serve four primary roles: supplying drinking water, generating hydroelectric power, providing irrigation for agriculture, and mitigating downstream flooding. Many installations combine several of these functions simultaneously.
How is a Reservoir constructed?
The most common method involves damming a valley to create a contained basin, but engineers also build coastal, bank-side, and service reservoirs using retaining walls, levees, or excavation. The key principle is interrupting or redirecting a natural drainage path so water pools in a controlled area.
What are some historically notable Reservoirs?
The Parakrama Samudra in Sri Lanka and the ancient Marib Dam in Yemen are often cited as landmark examples of large-scale water storage. In a modern urban context, London's Honor Oak Reservoir illustrates how such structures integrate into city water infrastructure.
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