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Public-images-osm logo.svg water = reservoir
An artificial lake which may be used to store water. Usually formed by a dam over a natural water course. Show/edit corresponding data item.
Group: water management
Used on these elements
should not be used on nodesshould not be used on waysmay be used on areas (and multipolygon relations)should not be used on relations (except multipolygon relations)
Useful combination
See also
Status: approvedPage for proposal

A reservoir is, most commonly, an enlarged natural or artificial lake, pond, or impoundment, created using a dam to store water (reservoir on Wikipedia). While not as common, a reservoir need not be dammed by a dam. Some reservoirs are created from ordinary lakes by adding a drainage tunnel.

See also water=pond for very small artificial ponds and water=basin for an area of land artificially graded to hold water, and man_made=reservoir_covered for an underground or covered water reservoir.

How to map

Draw the reservoir area outline on the shoreline. For intermittent reservoirs (which are sometimes dry), draw an outline around the visible high water mark, and tag it with intermittent=yes.

Add natural=water and water=reservoir.

For the dam waterway=dam is used.

Tags used in combination

  • name=* - The primary name: in general, the most prominent signposted name or the most common name in the local language(s)
  • seasonal=* - Describes features that depend on seasonal conditions
  • intermittent=yes - Used to indicate whether or not a waterway or water body does not permanently contain water
  • swimming=* - Whether swimming is allowed at specific location

Types of reservoirs

Reservoirs include the following types:

  • Dammed valleys rely on the natural topography to provide most of the body of the reservoir. A dam is typically located at a narrow part of a valley downstream of a natural basin. Sometimes additional side dams are required to contain the reservoir. In hilly regions, reservoirs may be constructed by enlarging existing lakes.
  • Coastal reservoirs are fresh water storage reservoirs located on the sea coast near the river mouth to store the flood water of a river. Many coastal reservoirs were constructed in Asia and Europe.
  • Bank-side reservoirs, where water is pumped or siphoned from a river to store the water. Such reservoirs are usually formed partly by excavation and sometimes by building an encircling levee on Wikipedia or embankment: in OpenStreetMap these are tagged was water=basin or landuse=basin.
  • Service reservoirs are used for storage of potable drinking water close to the point of distribution. While these are called "reservoirs" in water systems jargon, these are usually not open bodies of water, but rather a storage tank, cistern, basin or a water tower, and thus should be tagged instead as man_made=storage_tank, water=basin / landuse=basin, man_made=reservoir_covered or man_made=water_tower as appropriate.


Reservoirs are used for multiple purposes:

  • Water supply. Many reservoirs are used to provide the raw water feed to a water treatment plant which delivers drinking water through water mains, or irrigation water used by agriculture.
  • Hydroelectricity. A reservoir generating hydroelectricity includes turbines 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.
  • Controlling watersources. Reservoirs can be used to control water flow in downstream waterways:
    • Water may be released from an upland reservoir so that it can be used downstream for drinking, irrigation, etc.
    • Flood control and flow balancing reservoirs collect water at times of high flow, then release it slowly during the following weeks or months.
    • Water may be released from a reservoir to supplement white water conditions for white-water sports, or to encourage natural migration behaviours in fish
  • Recreation. Many reservoirs often allow some recreational uses, such as fishing and boating.


Dammed valleys

  • relation Lake Mead, a dammed valley and the largest reservoir in the United States.
  • relation Lake Kariba, formed by damming the Zambezi Valley at the Kariba Gorge to generate power and to form the world's largest man-made lake and reservoir by volume.
  • way Llyn Clywedog in Wales, a dammed valley in which additional side dams are required to contain the reservoir.
  • relation Llwyn-on, relation Cantref and relation Beacons Reservoirs are examples of a chain of reservoirs in the  River Taff valley.

Coastal reservoirs

Bank-side reservoirs

Flood control reservoirs

Dam-less reservoirs

  • relation Ljosavatnet in Norway, is a reservoir without a dam. Regulation is achieved solely by a drainage tunnel having been mined into the bottom of what was once a lake. The riverbed of what used to be the river leading out of the lake now serves as a spillway.

Retired or disused reservoirs

Reservoirs may be retired or otherwise stop being in regular use. Below are some recommended strategies for retagging.

  1. The reservoir structures still exist, and the reservoir may be filled with water once or more per year: Add intermittent=yes, and map the area of the high-water mark.
  2. The reservoir structures are no longer intact, but the depression in the ground still results in a wetland: map the natural=wetland and add the appropriate value of wetland=*. The previous area of the reservoir can be tagged abandoned:water=reservoir.
  3. The reservoir structures are gone and there are no remaining features visible: Delete the reservoir. Or if there is inconsistency in the imagery sources that could lead to mapper confusion, leave area in place with abandoned:water=reservoir.

In all above cases, leaving the name=* is a matter for local judgement. In some cases the old name is no longer relevant, but in other cases locals still refer to the property by the original name, making it appropriate to tag.

Tagging elevation

Since reservoirs have water levels that vary significantly, it can be hard to interpret what is implied by an ele=* value.

In Norway, there is some precedence for using the following scheme to clarify the exact meaning of elevations:

  • ele:min=* for the lowest water level achievable by regulation.
  • ele:max=* for the highest water level achievable by regulation.
  • ele:natural=* for the elevation a potential lake would have without regulating structures.

Mapping partial intermittency

A common base practice for mapping a reservoir, is mapping the outline of the maximum water level. Since many reservoirs seldom achieve this level in practice, there is a desire to map part of the area of a reservoir as being intermittent. This usually applies to the area difference between the reservoir in its minimum water level state and its maximum water level state.

There is no clear, established practice for how to do this, but some examples exist:

  • relation Lake Mead is not tagged as intermittent, but is surrounded by another lake relation Lake Mead full-water area that is intermittent.
    • Advantages: Achieves desired result in terms of landscape rendering.
    • Disadvantages: Lake Mead is now two separate objects instead of a single one. Arguably, the name=* of relation Lake Mead really applies to the joint areas of the two relations.
  • relation Ljosavatnet is mapped with an outer way at the maximum water level. Inside the reservoir, there are additional small lakes, at layer=-1, that represent the minimum water level. The reservoir is tagged as intermittent.
    • Advantages: Achieves desired result in terms of landscape rendering. Also achieves correct size of name tag.
    • Disadvantages: Overlapping areas tagged natural=water. It could be considered misleading that the reservoir is tagged as intermittent, as it's only partially so.

Objects frequently associated with reservoirs

  • Dam - A barrier built across a river or stream to impound the water.
  • Intake tower - a vertical tubular structure with one or more openings used for capturing water from reservoirs. Tower with Tag:tower:type=intake.

Tagged objects distribution


Possible tagging mistakes

If you know places with this tag, verify if it could be tagged with another tag.
Automated edits are strongly discouraged unless you really know what you are doing!
If you know places with this tag, verify if it could be tagged with another tag.
Automated edits are strongly discouraged unless you really know what you are doing!

See also