## Facts

- A private developer applied to the U.S. Forest Service for a special-use permit to build and operate a downhill ski resort in the Okanogan National Forest in Washington.
- The Forest Service prepared an environmental impact statement (EIS) evaluating environmental effects, including wildlife and air quality impacts, including off-site effects.
- The EIS discussed possible mitigation steps but described them as conceptual and to be made more specific during later design and implementation stages, including measures dependent on state and local government actions.
- The Regional Forester approved issuance of a special-use permit for a large ski area; the Chief of the Forest Service affirmed on administrative appeal.
- Environmental groups sued under the Administrative Procedure Act, alleging the EIS failed to satisfy the National Environmental Policy Act (NEPA).
- A federal magistrate concluded the EIS was adequate.
- The Ninth Circuit reversed, holding the EIS inadequate because NEPA required a detailed mitigation plan, required a worst-case analysis when information was incomplete, and because the Forest Service allegedly violated its own regulations by not developing a complete mitigation plan.
- The Supreme Court granted certiorari.

## Issues

1. Whether NEPA imposes a substantive duty to mitigate adverse environmental effects, requiring an EIS to include a fully developed mitigation plan and specific mitigation commitments.
2. Whether NEPA requires a “worst case analysis” when environmental effects information is incomplete or uncertain.
3. Whether the Forest Service’s own regulations required a complete mitigation plan as a precondition to issuing the special-use permit.

## Decision

- The Supreme Court reversed the Ninth Circuit.
- The Court held NEPA does not impose a substantive duty to mitigate adverse environmental effects and does not require a fully developed mitigation plan in every EIS.
- The Court held NEPA does not require a worst-case analysis; agencies may follow the then-governing CEQ approach for incomplete or unavailable information using credible scientific evidence and accepted methods.
- The Court concluded the EIS’s mitigation discussion was adequate as a procedural matter because it provided a reasonably thorough discussion of possible mitigation measures, even if conceptual and subject to later refinement.
- The Court deferred to the Forest Service’s reasonable interpretation of its own regulations that they required discussion of mitigation but not a complete, finalized mitigation plan, particularly for off-site measures dependent on other governments.

## Legal Principles

- NEPA primarily imposes procedural duties: agencies must take a “hard look” at environmental consequences to prevent uninformed decisionmaking, but NEPA does not mandate particular substantive outcomes.
- An EIS must include a detailed discussion of possible mitigation measures sufficient to inform decisionmakers and the public, but NEPA does not require the agency to adopt mitigation or to finalize a complete mitigation plan at the EIS stage.
- NEPA does not independently require a “worst case analysis”; compliance is measured against applicable Council on Environmental Quality regulations governing treatment of incomplete or unavailable information.
- An agency’s reasonable interpretation of its own regulations is controlling unless plainly erroneous or inconsistent with the regulation.

## Conclusion

The Court upheld the EIS and permit approval by reaffirming NEPA’s procedural character: agencies must disclose and consider environmental impacts, including possible mitigation, but need not commit to a complete mitigation plan or perform worst-case analysis absent regulatory requirement.


Robertson v. Methow Valley Citizens Council, 490 U.S. 332 (1989)