PUBLISHED: 26 February 2015

What are Dams and Reservoirs?

Dams are structures that are built on a river in order to retain water for one or more specific purposes (e.g. hydroelectricity generation). Reservoirs are formed behind a dam. 

A reservoir is a body of water that has been “formed or modified by human activity for specific purposes, in order to provide a reliable and controllable [water] resource” [1]. It can be confusing to talk about reservoirs and dams, as sometimes these terms are used interchangeably. However, dams and reservoirs are two entirely different things. An easy distinction to remember is that a dam is a physical structure that retains water; a reservoir is the water body that is created by a dam. 

Glenmore Reservoir 
Glenmore Reservoir” by Dave Ketchup is licenced under CC BY 2.0

Why build dams and reservoirs?

Dams and reservoirs can serve a variety of purposes. The purpose of a reservoir and dam can be singular or multiple and can also change over time.  

Dams and reservoirs can serve the following purposes:

  • Water storage: Water can be stored for municipal use or for irrigation purposes.
  • Hydroelectric generation: Dams can be used to generate hydroelectricity. According to the British Dam Society, over “80% of the world’s reservoir storage is used for hydropower” [2].
  • Recreation: Reservoirs may allow for recreational activities such as boating and fishing.
  • Flood protection: Some dams are designed specifically for flood protection. These specific dams are usually designed to “reduce flood peaks by 30-50%  and allow for downstream areas to be evacuated” [3]. In some cases, a dam may have a secondary function that can be used for flood protection. If appropriate planning is undertaken, the level of a reservoir can be lowered to hold back some floodwater. 
  • Waste containment: Reservoirs can be built to contain mine tailings.


[1] J. Thornton, A. Steel & W. Rast Chapman, Deborah (ed). (1992, 1996). Chapter 8 – Reservoirs. Retrieved from

[2] The British Dam Society. “Dams and Reservoirs.” Retrieved from: 

[3] Ibid.