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Reservoirs are engineered water bodies that store and regulate water for human and environmental needs. This explainer summarises how they are defined, built and managed, and how modern design seeks to balance supply with landscape and ecology.
A concise evidence-backed overview of reservoirs, covering definition, formation, uses, history, design principles and monitoring, drawn from authoritative sources on water infrastructure and landscape management.
Artificial lake definition
A reservoir is described as an artificial lake where water is stored. The definition is used in educational resources to distinguish managed water bodies from natural lakes. Most examples are created to hold water for later use, with the stored volume controlled by engineering works. The term is applied to a range of sizes from small service reservoirs to large valley impoundments.
Formation by damming rivers
Most reservoirs are formed by constructing dams across rivers. The dam creates a barrier that allows water to accumulate upstream, forming an enlarged body of water. This method is the common approach for water supply and hydroelectric schemes. The resulting impoundment follows the valley shape of the watercourse.
Natural lake conversion
A reservoir can also be formed from a natural lake whose outlet has been dammed to control the water level. In this case the original lake basin is retained and modified by engineering at the outlet. Water level management is then possible for supply, flood control or recreation while retaining natural basin characteristics.
Dam controls outflow
The dam controls the amount of water that flows out of the reservoir. Outflow regulation allows operators to release water for drinking supply, irrigation, industry or power generation as demand changes. Control structures such as spillways and outlet valves are integral to safe operation and water resource management.
Fresh water storage and power
A reservoir is an enlarged lake behind a dam, usually built to store fresh water, often doubling for hydroelectric power generation. The stored water provides a reliable source for municipal and agricultural use and can be used to drive turbines. The dual function is common in many large schemes worldwide.
Recreational reservoirs
Some reservoirs are made for recreational purposes rather than storing fresh water. Examples exist where the primary aim is leisure use, such as boating, fishing or landscape amenity. This shows the functional diversity of reservoirs beyond water supply and energy.
Artificial structure for fluids
A reservoir is an artificial structure designed to hold back fluids like water, gas or hydrocarbons. The definition covers bank reservoirs, dammed reservoirs and service reservoirs as examples of artificial water bodies. The structure is engineered to contain large quantities safely over long periods.
Supply for cities and farming
A reservoir is an artificial structure designed to hold back large quantities of fluids like water, ensuring a constant supply of water to cities, households and for farming. The stored water provides security during dry periods and supports food production and urban demand. Reliability is a core purpose of modern water resource planning.
Largest reservoirs by area and volume
Currently the largest reservoir by surface area is Volta Dam in Ghana, and the largest reservoir by volume is Kariba Dam between Zimbabwe and Zambia. These scale benchmarks illustrate the range of reservoir sizes built for regional water and power needs. Size comparisons help contextualise engineering and environmental impacts.
Ancient water storage in Arabia
In the year circa 300 BCE, farmers in Arabia used the craters of extinct volcanoes to store water that would be used for irrigation and watering their herds. This early practice shows long-standing human use of natural depressions for water conservation. It predates modern dam engineering by millennia.
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