Dam Dam - Any artificial barrier which impounds or diverts water. A dam is generally considered hydrologically significant if it is 1.One and one quarter feet (0.4 meters) or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet, or 2.Or has an impounding capacity of 50 acre-feet or more and is at least six feet (2 meters) above the natural bed of the stream.