(1) The agency shall prepare supplements to either draft or final environmental impact statements whenever:
(a) the agency or the applicant makes a substantial change in a proposed action;
(b) there are significant new circumstances, discovered prior to final agency decision, including information bearing on the proposed action or its impacts that change the basis for the decision; or
(c) following preparation of a draft EIS and prior to completion of a final EIS, the agency determines that there is a need for substantial, additional information to evaluate the impacts of a proposed action or reasonable alternatives.
(2) A supplement must include, but is not limited to, a description of the following:
(a) an explanation of the need for the supplement;
(b) the proposed action; and
(c) any impacts, alternatives or other items required by ARM 17.4.617 for a draft EIS or ARM 17.4.619 for a final EIS that were either not covered in the original statement or that must be revised based on new information or circumstances concerning the proposed action.
(3) The same time periods applicable to draft and final EISs apply to the circulation and review of supplements.