The National Environmental Policy Act's (NEPA's) requirement that environmental impact statements (EISs) be prepared for "major Federal actions significantly affecting the quality of the human environment"¹ has been well defined by the courts and is now a fact of life with which federal agencies routinely contend. Less well defined or accepted, however, is the requirement that impact statements be supplemented when important new information comes to light or when a project has been modified such that the original EIS is rendered inadequate. To this relatively immature area of the case law has recently been added a major ruling by the Fifth Circuit.

In Environmental Defense Fund, Inc. v. Marsh,² the court ordered that construction on major portions of the controversial Tennessee-Tombigbee Waterway project (Tenn-Tom or TTW) be enjoined pending completion of a supplemental environmental impact statement (SEIS). The court, although reluctant to impede the completion of the mammoth project, was unable to find a reasonable basis for the Corps of Engineers' failure to prepare an SEIS given the sweeping revisions that had been made in the design of the project since the initial EIS was prepared.