A dam is an artificially constructed barrier which obstructs, redirects, or slows down the flow of water. The main purpose of dams is to retain water which can then by used by a populace. Although the engineering principles were developed on Earth in the 20th century, it wasn't until the 23rd century that the Klingons developed the engineering abilities necessary. By the 2260s, dams were only present on three Klingon worlds.
During the establishment of the Nisus science colony, a gigantic dam was placed near the colony so that the inhabitants would have fresh water, as well as aiding in the creation of electricity via the colony's power plant. (TOS novel: The IDIC Epidemic)