When substantial new information is discovered or substantial changes with environmental ramifications are made to the proposal and alternatives, you should prepare a supplement to the EIS. Either a draft or a final EIS may be supplemented. Supplements may be prepared on previous final EISs if they still contain information than is 80 to 90 percent correct but require changes and updating on the remaining 10 to 20 percent. They may also be prepared on draft EISs where agency or public comments have raised substantial new issues that require full-blown analysis and many pages to address fully. Also, as stated above, if a decision-maker modifies an alternative after the EIS has been released as a final document, a supplement is required if the changes result in impacts not analyzed in the EIS.

You must circulate a supplement in the same manner as a draft or final EIS. If there is good reason to believe the interested and affected public will have a copy of the draft or final, you need only circulate the supplement. However, if the supplement has been prepared on an older EIS, or if there is reason to believe the public will not have the entire EIS, both the supplement and the EIS itself must be circulated. It should be made clear to the public whether the entire EIS and supplement are open to comment, or whether the supplement only is the subject of public scrutiny. If the EIS is older (2-3 years in most cases), the entire EIS and supplement should be available for public inspection and comment. You should consider a longer comment period for the supplement if the public needs to request the previously prepared EISs. If the EIS is seriously outdated, you should revise and update the entire document, and release it as a new draft, rather than attempting to supplement the EIS.

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