## Facts

- The U.S. Army Corps of Engineers planned and began constructing Elk Creek Dam, part of a multi-dam project in Oregon affecting water quality and fish-related recreation.
- The Corps completed an EIS (1971) and later issued a final supplemental EIS (1980) evaluating, among other topics, turbidity, fish production, and angling, and concluding the dam would not have major adverse effects on fish production.
- By 1985, Congress had appropriated construction funds and the dam was about one-third complete; the Corps decided to proceed.
- After the supplemental EIS, two items of information emerged: (1) an internal memorandum by state fish and wildlife biologists suggesting greater downstream fishery harm than previously expected; and (2) a federal soil survey that could be read to imply greater downstream turbidity.
- Environmental organizations sued, asserting NEPA required additional analysis, including a second supplemental EIS to address the new information.
- The Corps evaluated the new materials in an internal report and concluded they did not materially change the prior environmental analysis.

## Issues

1. When do NEPA and implementing regulations require an agency to prepare a supplemental EIS based on new information arising after completion of an EIS and a prior supplement?
2. Under the APA, what level of judicial deference applies to an agency’s decision not to prepare a supplemental EIS, particularly where the dispute turns on technical judgments?

## Decision

- The Supreme Court unanimously reversed the court of appeals and upheld the Corps’ decision not to prepare a second supplemental EIS.
- The Court held that the agency’s non-supplementation decision is reviewed under the APA’s “arbitrary and capricious” standard and may be set aside only for a clear error of judgment.
- Applying a “rule of reason,” the Court concluded the post-supplement materials did not present a materially different environmental picture from what the agency had already considered.
- The Court rejected the appellate court’s separate criticisms that the prior supplemental EIS lacked a complete mitigation plan and a “worst case analysis,” as inconsistent with governing NEPA doctrine.

## Legal Principles

- An agency must prepare a supplemental EIS when (a) major federal action remains and (b) new information will affect environmental quality in a significant manner or to a significant extent not previously considered.
- NEPA supplementation is governed by a “rule of reason”; agencies need not issue supplements for every new study or data point, only for information that meaningfully changes the environmental assessment.
- Decisions whether to supplement are reviewed under the APA’s arbitrary-and-capricious standard; courts do not conduct de novo review and will not overturn absent a clear error of judgment.
- In technical and scientific disputes, courts defer to an agency’s reasoned reliance on its qualified experts, even where other specialists disagree.
- NEPA is procedural: the duty is to take a hard look and provide informed decisionmaking, not to reach any particular environmental outcome.

## Conclusion

The Court sustained the Corps’ refusal to issue a second supplemental EIS because the agency reasonably evaluated the asserted new information and concluded it did not significantly alter the prior analysis, and because judicial review under the APA requires substantial deference to agency technical judgments absent a clear error of judgment.


Marsh v. Or. Nat. Res. Council, 490 U.S. 360 (1989)