Tag:waterway=dam
 waterway = dam
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A barrier built across a river or stream to block and regulate the flow of the river  
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| Rendering in openstreetmap-carto |
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| Group: Waterways |
| Used on these elements |
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| Useful combination |
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| See also |
Status: approved
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| Wikidata |
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A barrier built across a river or stream to impound the water. A dam normally does not have water flowing over the top of it. The water behind a dam is usually water=reservoir. A dam may also be used to hold back a man_made=tailings_pond.
Sometimes there is a waterway=fish_pass around it.
How to Map
- Narrow/small dams can be drawn as a way
and tagged with waterway=dam. - Wide/big dams can be drawn as an area
and tagged with waterway=dam + name=*.
If drawn as way waterway=dam it will be probably directly adjacent with the outline of the reservoir although this is not a requirement. If drawn as area it may share ways with the outline of the reservoir, partly overlap with the water area or be adjacent without touching the "area" of the dam.
Validators and QA tools generally want that the waterway=river have shared node(s) with the waterway=dam if and where they cross it (flow through it). If the river runs in a tunnel well underneath the dam it should be mapped as waterway=river + tunnel=yes + layer=-1 without any shared nodes with the dam.
The artificial lake created by the water impounded behind the dam wall (commonly called a 'dam' in Australia) should be tagged as natural=water + water=reservoir or water=pond as appropriate.
Example
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small dam as a node 
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small dam as a way
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big dam as an area
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(High-)ways over a dam
 | The following section is disputed. You can join the discussion. |
This subject was not part of the proposal but the most straightforward solution is that the highway=* and waterway=dam share a way if a highway crosses over the dam and the dam is mapped as a line.
Validators and QA tools may complain about this combination because many have an overly generic rule saying that highways and waterways can not share a single way - this is one of the valid exceptions and tools should be fixed to accept it.
Alternative mapping methods are to map the dam as an area, or the map the dam and the highway as two separate ways which might share some of the same nodes.
Example of ways over dam:
See also
- waterway=weir - Smaller barrier where the water can still flow over the top.
- natural=water + water=reservoir - Water area impounded behind.
- man_made=dyke - Embankment for holding back flood water
- power=generator + generator:source=hydro - A device which converts one form of energy to another, for example, an electrical generator. + Turbine of a hydroelectric power station
- waterway=fish_pass - as natural=water + water=fish_pass - A fish pass, also known as a fish ladder, a fishway or fish steps, is a structure on or around artificial barriers (such as dams and locks) to facilitate diadromous fishes natural migration.