The National Environmental Policy Act (NEPA) of 1969 requires United States (U.S.) federal agencies to consider the potential impacts on the human, natural, or cultural environment before making a decision on a proposed action. An Environmental Impact Statement is a detailed public document that provides an assessment of the potential effects that a major federal action might have on the environment. The U.S. Department of the Navy’s analysis under NEPA extends through the United States’ territorial sea (from shore to 12 nautical miles). An Overseas Environmental Impact Statement is prepared to comply with Executive Order 12114, Environmental Effects Abroad of Major Federal Actions, of 1979, which requires federal agencies to consider the potential effects of their actions on the physical environment outside the United States, its territories, and possessions.
A supplement to an environmental impact analysis is prepared when there are new circumstances, new information, or when the agency determines the purposes of NEPA will be furthered by doing so (Title 40 U.S. Code § 1502.9).