Congress authorized the Rogue River Basin Project in Oregon, which included construction of three dams—Lost Creek, Applegate, and Elk Creek—managed by the U.S. Army Corps of Engineers. The Corps prepared a programmatic EIS and later supplemental environmental analysis for aspects of the project. As the project progressed, new studies emerged regarding impacts on salmon populations in the Rogue River system, including evidence of dissolved oxygen and temperature effects and gas supersaturation associated with dam operations that could affect fish survival. Environmental groups, including the Oregon Natural Resources Council (ONRC), argued that these data constituted significant new information relevant to environmental concerns and to the incremental effects of the yet-to-be-completed Elk Creek Dam. They sought to compel the Corps to prepare a supplemental EIS before continuing with construction and related actions. The Corps reviewed the new studies, consulted with fisheries experts, considered mitigation measures and operational changes at related facilities, and concluded that a supplemental EIS was not required because the new information did not present significant impacts beyond those already analyzed and because mitigation and operational refinements would address identified concerns. ONRC sued under NEPA and the APA. The district court, applying a "reasonableness" standard, ordered the Corps to prepare a SEIS; the Ninth Circuit affirmed. The Supreme Court granted certiorari to resolve the proper standard of review and to assess whether the Corps' decision was lawful.
Under NEPA and the CEQ regulations, and reviewed pursuant to the APA, what is the proper standard for judicial review of an agency's decision not to prepare a supplemental EIS in light of new information, and was the Corps' decision here unlawful under that standard?
NEPA requires agencies to take a "hard look" at environmental consequences, but it does not mandate particular outcomes. CEQ regulations require supplementation of an EIS if there are significant new circumstances or information relevant to environmental concerns and bearing on the proposed action or its impacts, where major federal action remains to occur. 40 C.F.R. § 1502.9(c). Judicial review of an agency's decision not to supplement is governed by the APA, 5 U.S.C. § 706(2)(A), which asks whether the decision was arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law. Courts must be highly deferential to an agency's reasoned evaluation of technical and scientific data, ensuring only that the agency considered the relevant factors and articulated a rational connection between the facts found and the decision made. Review is generally confined to the administrative record, though courts may consider explanatory affidavits or testimony to illuminate the agency's reasoning without accepting post hoc rationalizations.
The Supreme Court held that an agency's decision not to prepare a supplemental EIS is reviewed under the APA's arbitrary-and-capricious standard, not a de novo or "reasonableness" standard. Applying that standard, the Court concluded the Corps' decision not to prepare a SEIS for the Elk Creek Dam—after taking a hard look at the new studies and expert opinions—was not arbitrary or capricious. The Court also held that it is permissible for a reviewing court to consider explanatory affidavits from agency officials to clarify technical issues and the agency's reasoning.
The Court began by reaffirming that NEPA imposes procedural duties to ensure informed decision-making, not to dictate substantive outcomes. The CEQ regulation at 40 C.F.R. § 1502.9(c) requires supplementation when significant new information or circumstances emerge while major federal action remains to occur. Because deciding what counts as "significant" and how new information bears on impacts involves complex, predictive, and technical judgments, the APA's arbitrary-and-capricious standard governs judicial review. The Ninth Circuit's "reasonableness" test invited courts to substitute their judgment for that of the agency on technical matters. The Supreme Court rejected that approach, emphasizing deference to the agency's expertise. Citing precedents such as Kleppe and State Farm, the Court explained that reviewing courts must confirm the agency considered relevant factors and took a "hard look," but may not resolve scientific disputes themselves or prefer one set of experts over another absent a clear failure of reasoned decision-making. Turning to the record, the Court found that the Corps engaged with the new fishery studies, sought input from relevant experts, and considered mitigation and operational strategies that would address potential impacts. The Corps explained why the incremental effects of the Elk Creek Dam, in light of existing analyses and mitigation at related dams, did not amount to significant new impacts requiring a SEIS. Although scientific opinions varied, the Corps was entitled to rely on the reasonable views of its own qualified experts. The Court concluded that the Corps articulated a rational connection between the evidence and its decision not to supplement. Finally, the Court addressed the scope of the record and the propriety of extra-record materials. While judicial review generally is confined to the administrative record, courts may consider contemporaneous or explanatory affidavits from agency personnel to clarify technical aspects of the decision or the agency's reasoning, provided they do not constitute post hoc rationalizations advanced by counsel. The district court therefore did not err in considering explanatory materials that illuminated the basis for the Corps' non-supplement decision.
Marsh cements the APA's arbitrary-and-capricious standard as the lens through which courts review agency decisions concerning supplemental NEPA analysis. It teaches that NEPA's "hard look" is rigorous but deferential, especially on scientific and technical issues, and that courts must avoid second-guessing agencies where the record demonstrates reasoned evaluation. For litigants, Marsh underscores the centrality of the administrative record and the strategic importance of building, and then challenging, the agency's documented reasoning. For agencies, it confirms that thorough engagement with new information—documented in the record and supported by expert analysis—can sustain a decision not to prepare a SEIS.
Marsh v. Oregon Natural Resources Council is a touchstone for understanding how NEPA's procedural mandate operates within the APA's framework of deferential judicial review. It clarifies that an agency's decision whether to prepare a supplemental EIS is a technical judgment call reviewed for arbitrariness, not for correctness in a scientific sense, and that courts should respect agency expertise when the record shows a reasoned evaluation of new information.