A dam is a barrier that prevents the flow of water. It could be either manmade or  made by animals such as beavers.

Positive: Dreaming of a dam may symbolize one’s potential. It could represent a reserve or source of great power. It could also represent a way over an obstacle (as in crossing a stream by walking across a beaver dam).

Negative: A dam in a dream may also represent a block, restriction, or hindrance in one’s life.

(Joshua 3:16).