Dams (synonyms: dikes, levees, weirs; example toponyms: The Beaverton Dam) are large barrier structures that stop the flow of water. They are often located on a river and may generate a large reservoir of water upstream. They may be hydroelectric dams. Dams are often used in narratives as locations of evil plots to destroy and flood a town, valley, or other location below. In post-apocalyptic worlds they may be in ruins.

When placing dams into fictional worlds, consider their location and the impact their construction may have had on both the upstream and downstream environment. Entire towns or regions may have been submerged as the new upstream reservoir filled a valley.

The earliest known dam dates to 3000 BC