18 CFR 1318.103: Supplements.

(a) After completion of an environmental document, if TVA makes substantial changes in the proposed action that are relevant to environmental concerns or there is substantial new circumstances or information about the significance of the adverse effects that bear on the proposed action or its impact, and important components of the proposed action remain to be implemented, TVA will determine how the environmental document should be supplemented or modified. TVA may supplement an environmental document by issuing a new document that addresses changes in the proposed action or new circumstances or information or may modify the existing document so that revisions and edits to the original document are apparent. A supplemented or modified environmental document will be published.

(b) After completion of an environmental document, if TVA determines that changes to the proposed action that are relevant to environmental concerns are not substantial or new circumstances or information are not significant or relevant to environmental concerns, and therefore supplemental analysis is not warranted, TVA will document this determination consistent with § 1318.102.