Illustration of U.S. Army Corps of Engineers dams and levee systems. A federally regulated dam is any artificial barrier between six and 25-feet high, storing at least 50 acre-feet of water. Structures below six feet are not considered dams if there is no life safety risk. Any structure 25 feet and over storing at least 15 acre-feet of water is a dam. There are 90,000 dams in the U.S. that meet this definition.