23 CFR § 771.130 - Supplemental environmental impact statements.
(a) A draft EIS, final EIS, or supplemental EIS may be supplemented at any time. An EIS must be supplemented whenever the Administration determines that:
(1) Changes to the proposed action would result in significant environmental impacts that were not evaluated in the EIS; or
(2) New information or circumstances relevant to environmental concerns and bearing on the proposed action or its impacts would result in significant environmental impacts not evaluated in the EIS.
(b) However, a supplemental EIS will not be necessary where:
(1) The changes to the proposed action, new information, or new circumstances result in a lessening of adverse environmental impacts evaluated in the EIS without causing other environmental impacts that are significant and were not evaluated in the EIS; or
(2) The Administration decides to approve an alternative fully evaluated in an approved final EIS but not identified as the preferred alternative. In such a case, a revised ROD must be prepared and circulated in accordance with § 771.127(b).
(c) Where the Administration is uncertain of the significance of the new impacts, the applicant will develop appropriate environmental studies or, if the Administration deems appropriate, an EA to assess the impacts of the changes, new information, or new circumstances. If, based upon the studies, the Administration determines that a supplemental EIS is not necessary, the Administration must so indicate in the project file.
(d) A supplement is to be developed using the same process and format (i.e., draft EIS, final EIS, and ROD) as an original EIS, except that scoping is not required.
(e) In some cases, an EA or supplemental EIS may be required to address issues of limited scope, such as the extent of proposed mitigation or the evaluation of location or design variations for a limited portion of the overall project. Where this is the case, the preparation of a supplemental document must not necessarily:
(1) Prevent the granting of new approvals;
(2) Require the withdrawal of previous approvals; or
(3) Require the suspension of project activities, for any activity not directly affected by the supplement. If the changes in question are of such magnitude to require a reassessment of the entire action, or more than a limited portion of the overall action, the Administration must suspend any activities that would have an adverse environmental impact or limit the choice of reasonable alternatives, until the supplemental document is completed.