Standing Rock Sioux Tribe v. United States Army Corps ( 2021 )


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  •  United States Court of Appeals
    FOR THE DISTRICT OF COLUMBIA CIRCUIT
    Argued November 4, 2020            Decided January 26, 2021
    No. 20-5197
    STANDING ROCK SIOUX TRIBE , ET AL.,
    APPELLEES
    v.
    UNITED STATES A RMY CORPS OF ENGINEERS,
    APPELLANT
    DAKOTA A CCESS LLC,
    INTERVENOR
    Consolidated with 20-5201
    Appeals from the United States District Court
    for the District of Columbia
    (No. 1:16-cv-01534)
    James A. Maysonett, Attorney, U.S. Department of Justice,
    argued the cause for appellant United States Army Corps of
    Engineers. With him on the briefs were Jeffrey Bossert Clark,
    Assistant Attorney General, Jonathan D. Brightbill, Principal
    Deputy Assistant Attorney General, Eric A. Grant, Deputy
    2
    Assistant Attorney General, and Andrew C. Mergen and Erica
    M. Zilioli, Attorneys.
    Miguel A. Estrada argued the cause for appellant Dakota
    Access LLC. With him on the briefs were William S. Scherman
    and David J. Debold.
    Wayne K. Stenehjem, Attorney General, Office of the
    Attorney General for the State of North Dakota, and Matthew
    A. Sagsveen, Solicitor General, were on the brief for amicus
    curiae the State of North Dakota.
    Tim Fox, Attorney General, Office of the Attorney General
    for the State of Montana, Curtis T. Hill, Jr., Attorney General,
    Office of the Attorney General for the State of Indiana, Thomas
    M. Fisher, Solicitor General, Tom Miller, Attorney General,
    Office of the Attorney General for the State of Iowa, Derek
    Schmidt, Attorney General, Office of the Attorney General for
    the State of Kansas, Daniel Cameron, Attorney General, Office
    of the Attorney General for the Commonwealth of Kentucky,
    Jeff Landry, Attorney General, Office of the Attorney General
    for the State of Louisiana, Doug Peterson, Attorney General,
    Office of the Attorney General for the State of Nebraska, Dave
    Yost, Attorney General, Office of the Attorney General for the
    State of Ohio, Jason Ravnsborg, Attorney General, Office of
    the Attorney General for the State of South Dakota, Patrick
    Morrisey, Attorney General, Office of the Attorney General for
    the State of West Virginia, and Bridget Hall, Attorney General,
    Office of the Attorney General for the State of Wyoming, were
    on the brief for amici curiae the States of Indiana, Montana,
    and 9 other states in support of appellants.
    David H. Coburn, Joshua H. Runyan, Richard S.
    Moskowitz, Tyler J. Kubik, Stephen J. Obermeier, Wesley E.
    Weeks, John P. Wagner, Steven M. Kramer, Steven P.
    3
    Lehotsky, and Michael B. Schon, were on the brief for amici
    curiae American Fuel & Petrochemical Manufacturers, et al. in
    support of appellants.
    Jared R. Wigginton and Kent Mayo were on the brief for
    amici curiae North Dakota Farm Bureau, et al.
    Christopher O. Murray was on the brief for amicus curiae
    for appellant North Dakota Water Users Association in support
    of appellants.
    Jan Hasselman argued the cause for appellees Standing
    Rock Sioux Tribe, et al. With him on the brief were Patti A.
    Goldman, Nicole E. Ducheneaux, Jennifer S. Baker, Rollie E.
    Wilson, Jeffrey Rasmussen, Michael L. Roy, Jennifer P.
    Hughes, and Elliott A. Milhollin. Jeremy J. Patterson entered
    an appearance.
    Joel West Williams was on the brief for amici curiae the
    Great Plains Tribal Chairmen’s Association, et al. in support of
    appellees.
    Maura Healey, Attorney General, Office of the Attorney
    General for the Commonwealth of Massachusetts, Seth G.
    Schofield, Senior Appellate Counsel, Xavier Becerra, Attorney
    General, Office of the Attorney General for the State of
    California, Jamie B. Jefferson and Joshua R. Purtle, Deputy
    Attorneys General, Kathleen Jennings, Attorney General,
    Office of the Attorney General for the State of Delaware,
    Christian Douglas Wright, Director of Impact Ligitation,
    Aaron M. Frey, Attorney General, Office of the Attorney
    General for the State of Maine, William Tong, Attorney
    General, Office of the Attorney General for the State of
    Connecticut, Clare Kindall, Solicitor General, Kwame Raoul,
    Attorney General, Office of the Attorney General for the State
    4
    of Illinois, Brian E. Frosh, Attorney General, Office of the
    Attorney General for the State of Maryland, Dana Nessel,
    Attorney General, Office of the Attorney General for the State
    of Michigan, Elizabeth Morrisseau, Assistant Attorney
    General, Gurbir S. Grewal, Attorney General, Office of the
    Attorney General for the State of New Jersey, Letitia James,
    Attorney General, Office of the Attorney General for the State
    of New York, Aaron Ford, Attorney General, Office of the
    Attorney General for the State of Nevada, Hector Balderas,
    Attorney General, Office of the Attorney General for the State
    of New Mexico, Ellen Rosenblum, Attorney General, Office of
    the Attorney General for the State of Oregon, Paul Garrahan,
    Attorney-in-Charge, Steven Novick, Special Assistant Attorney
    General, Peter F. Neronha, Attorney General, Office of the
    Attorney General for the State of Rhode Island, Tricia K.
    Jedele, Special Assistant Attorney General, Robert W.
    Ferguson, Attorney General, Office of the Attorney General
    for the State of Washington, Noah Guzzo Purcell, Solicitor
    General, Leevin T. Camacho, Attorney General, Office of the
    Attorney General for the Territory of Guam, Thomas J.
    Donovan, Jr., Attorney General, Office of the Attorney
    General for the State of Vermont, Nicholas F. Persampieri,
    Assistant Attorney General, Karl A. Racine, Attorney General,
    Office of the Attorney General for the District of Columbia,
    Loren L. AliKhan, Solicitor General, Jacqueline R. Bechara,
    Appellant Litigation Fellow, and Sarah Utley were on the brief
    for amici curiae States of Massachusetts, et al. in support of
    appellees.
    Douglas P. Hayes was on the brief for amici curiae Sierra
    Club, et al. in support of appellees.
    Kenneth Rumelt and James G. Murphy were on the brief
    for amicus curiae Members of Congress in support of
    appellees.
    5
    Mary Kathryn Nagle was on the brief for amicus curiae
    National Indigenous Women’s Resource Center, Inc. in
    support of appellees.
    Before: TATEL and MILLETT, Circuit Judges, and
    SENTELLE , Senior Circuit Judge.
    Opinion for the Court filed by Circuit Judge TATEL.
    TATEL, Circuit Judge: Lake Oahe, created when the
    United States Army Corps of Engineers flooded thousands of
    acres of Sioux lands in the Dakotas by constructing the Oahe
    Dam on the Missouri River, provides several successor tribes
    of the Great Sioux Nation with water for drinking, industry,
    and sacred cultural practices. Passing beneath Lake Oahe’s
    waters, the Dakota Access Pipeline transports crude oil from
    North Dakota to Illinois. Under the Mineral Leasing Act, 
    30 U.S.C. § 185
    , the pipeline could not traverse the federally
    owned land at the Oahe crossing site without an easement from
    the Corps. The question presented here is whether the Corps
    violated the National Environmental Policy Act, 
    42 U.S.C. § 4321
    , by issuing that easement without preparing an
    environmental impact statement despite substantial criticisms
    from the Tribes and, if so, what should be done about that
    failure. We agree with the district court that the Corps acted
    unlawfully, and we affirm the court’s order vacating the
    easement while the Corps prepares an environmental impact
    statement. But we reverse the court’s order to the extent it
    directed that the pipeline be shut down and emptied of oil.
    I.
    “In order to ‘create and maintain conditions under which
    man and nature can exist in productive harmony,’ the National
    Environmental Protection Act (NEPA), 
    42 U.S.C. § 4331
    (a),
    6
    requires any federal agency issuing a construction permit,
    opening new lands to drilling, or undertaking any other ‘major’
    project to take a hard look at the project’s environmental
    consequences, 
    id.
     § 4332(2)(C) . . . .” National Parks
    Conservation Association v. Semonite, 
    916 F.3d 1075
    , 1077
    (D.C. Cir. 2019). “To this end, the agency must develop an
    environmental impact statement (EIS) that identifies and
    rigorously appraises the project’s environmental effects, unless
    it finds that the project will have ‘no significant impact.’” 
    Id.
    (quoting 
    40 C.F.R. § 1508.9
    (a)(1)). “If any ‘significant’
    environmental impacts might result from the proposed agency
    action[,] then an EIS must be prepared before agency action is
    taken.” Grand Canyon Trust v. FAA, 
    290 F.3d 339
    , 340 (D.C.
    Cir. 2002) (quoting Sierra Club v. Peterson, 
    717 F.2d 1409
    ,
    1415 (D.C. Cir. 1983)). Preparing an EIS is a significant
    undertaking, requiring the agency to “consult with and obtain
    the comments of” other relevant agencies and publish a
    “detailed statement” about the action’s environmental effects.
    
    42 U.S.C. § 4332
    (2)(C).
    “Whether a project has significant environmental impacts,
    thus triggering the need to produce an EIS, depends on its
    ‘context’ (regional, locality) and ‘intensity’ (‘severity of
    impact’).” National Parks, 916 F.3d at 1082 (quoting 
    40 C.F.R. § 1508.27
     (2018)). The operative regulations (since amended,
    Update to the Regulations Implementing the Procedural
    Provisions of the National Environmental Policy Act, 
    85 Fed. Reg. 43,304
     (July 16, 2020)) enumerate ten factors that “should
    be considered” in assessing NEPA’s “intensity” element. 
    40 C.F.R. § 1508.27
    (b) (2019). “Implicating any one of the factors
    may be sufficient to require development of an EIS.” National
    Parks, 916 F.3d at 1082. This case concerns the fourth factor—
    “[t]he degree to which the effects on the quality of the human
    environment are likely to be highly controversial.” 
    40 C.F.R. § 1508.27
    (b)(4) (2019).
    7
    The Dakota Access Pipeline (DAPL), nearly 1,200 miles
    long, is designed to move more than half a million gallons of
    crude oil from North Dakota to Illinois each day. Standing
    Rock Sioux Tribe v. U.S. Army Corps of Engineers (Standing
    Rock III), 
    255 F. Supp. 3d 101
    , 114 (D.D.C. 2017). DAPL
    crosses many waterways, including Lake Oahe, an artificial
    reservoir in the Missouri River created when the Corps
    constructed a dam in 1958. The dam’s construction and Lake
    Oahe’s creation flooded 56,000 acres of the Standing Rock
    Reservation and 104,420 acres of the Cheyenne River Sioux
    Tribe’s trust lands. 
    Id.
     The Tribes now rely on Lake Oahe’s
    water for drinking, agriculture, industry, and sacred religious
    and medicinal practices. 
    Id.
     As the Standing Rock Sioux Tribe
    explained:
    Lake Oahe is the source of life for the Tribe. It
    provides drinking water for over 4,200 people on the
    Reservation. It is the source of water for irrigation and
    other economic pursuits central to the Tribal
    economy. And it provides the habitat for fish and
    wildlife on the Reservation upon which tribal
    members rely for subsistence, cultural, and
    recreational purposes. Moreover, the Tribe’s
    traditions provide that water is more than just a
    resource, it is sacred—as water connects all of nature
    and sustains life.
    Letter from Dave Archambault II, Chairman, Standing Rock
    Sioux Tribe, to Lowry A. Crook, Principal Deputy Assistant
    Secretary for Civil Works, Office of the Assistant Secretary for
    the Army, and Col. John Henderson, P.E., District
    Commander, U.S. Army Corps of Engineers—Omaha District
    (Mar. 24, 2016), Appendix (A.) 318.
    8
    Oil pipelines crossing federally regulated waters like Lake
    Oahe require federal approval. See Standing Rock III, 255 F.
    Supp. 3d at 114. In June 2014, Dakota Access, formed to
    construct and own DAPL, notified the Corps that it intended to
    construct a portion of DAPL under Lake Oahe, just half a mile
    north of the Standing Rock Reservation. Id. To do so, Dakota
    Access needed, among other things, a real-estate easement
    from the Corps under the Mineral Leasing Act (MLA), 
    30 U.S.C. § 185
    .
    In December 2015, the Corps published and sought public
    comment on a Draft Environmental Assessment (EA) finding
    that the construction would have no significant environmental
    impact. Standing Rock III, 255 F. Supp. 3d at 114–15. The
    Tribes submitted comments voicing a range of concerns.
    Relevant here, the Tribes contended that the Corps had
    insufficiently analyzed the risks and consequences of an oil
    spill.
    Two federal agencies also raised concerns. The
    Department of the Interior requested that the Corps prepare an
    EIS given the pipeline’s potential impact on trust resources,
    criticizing the Corps for “not adequately justify[ing] or
    otherwise support[ing] its conclusion that there would be no
    significant impacts upon the surrounding environment and
    community.” Letter from Lawrence S. Roberts, Acting
    Assistant Secretary—Indian Affairs, U.S. Department of the
    Interior, to Brent Cossette, U.S. Army Corps of Engineers,
    Omaha District (Mar. 29, 2016), A. 385–86. The
    Environmental Protection Agency (EPA) registered its concern
    that the Draft EA “lack[ed] sufficient analysis of direct and
    indirect impacts to water resources,” though it requested
    additional information and mitigation in the EA rather than
    preparation of an EIS. Letter from Philip S. Strobel, Director,
    NEPA Compliance and Review Program Office of Ecosystems
    9
    Protection and Remediation, EPA, to Brent Cossette, U.S.
    Army Corps of Engineers, Omaha District (Jan. 8, 2016),
    Reply Supplemental Appendix 1. But after becoming aware of
    the pipeline’s proximity to the Standing Rock reservation, EPA
    supplemented its comments to note that, while it agreed with
    the Corps that there was “minimal risk of an oil spill,” it
    worried, based on its “experience in spill response,” that a
    break or leak could nonetheless significantly affect water
    resources. Letter from Philip S. Strobel, Director, NEPA
    Compliance and Review Program, Office of Ecosystems
    Protection and Remediation, EPA, to Brent Cossette, U.S.
    Army Corps of Engineers, Omaha District (Mar. 11, 2016), A.
    389–90.
    On July 25, 2016, the Corps published its Final EA and a
    “Mitigated Finding of No Significant Impact” (Mitigated
    FONSI). The Mitigated FONSI explained that, given the
    Corps’s adoption of various mitigation measures, including
    horizontal directional drilling, the Lake Oahe crossing would
    not “significantly affect the quality of the human environment”
    and that an EIS was therefore unnecessary.
    Shortly after the Final EA’s release, Standing Rock sued
    the Corps for declaratory and injunctive relief under NEPA
    (and several other federal laws not at issue in this appeal).
    Standing Rock III, 255 F. Supp. 3d at 116–17. Dakota Access
    and the Cheyenne River Sioux Tribe intervened on opposing
    sides, and Cheyenne River filed a separate complaint adding
    additional claims. Id. at 117. Though the district court denied
    the Tribes’ request for a preliminary injunction on September
    9, 2016, the Departments of Justice, Interior, and the Army
    immediately issued a joint statement explaining that the Corps
    would not issue an MLA easement and that construction would
    not move forward until the Army could determine whether
    10
    reconsideration of any of its previous decisions was necessary.
    Id.
    Following that statement, Standing Rock submitted
    several letters to the Assistant Secretary of the Army for Civil
    Works, who oversees the portion of the Corps’s mission that
    includes issuing permits for pipelines like DAPL. Those letters
    raised concerns about the EA’s spill risk analysis. The tribe
    also submitted an expert review of the EA from an experienced
    pipeline consultant who concluded that the assessment was
    “seriously deficient and [could not] support the finding of no
    significant impact, even with the proposed mitigations.”
    Accufacts Review of the U.S. Army Corps of Engineers
    Environmental Assessment for the Dakota Access Pipeline
    (Oct. 28, 2016), A. 837–46. Following the Corps’s internal
    review, the Assistant Secretary stood by her prior decision, but
    nonetheless concluded that the historical relationship between
    the affected tribes and the federal government “merit[ed]
    additional analysis, more rigorous exploration and evaluation
    of reasonable siting alternatives, and greater public and tribal
    participation and comments.” Memorandum from Jo-Ellen
    Darcy, Assistant Secretary of the Army (Civil Works) (Dec. 4,
    2016), A. 260; see Standing Rock III, 255 F. Supp. 3d at 117–
    18.
    During the ensuing review, both Standing Rock and the
    Oglala Sioux Tribe submitted additional comments and
    analysis. The Corps solicited Interior’s opinion on the pipeline,
    Interior’s Solicitor responded with a recommendation that the
    Corps prepare an EIS, and the Secretary of the Army for Civil
    Works issued a memorandum directing the Army not to grant
    an easement prior to preparation of an EIS. See Standing Rock
    III, 255 F. Supp. 3d at 118–19. On January 18, 2017, the
    Assistant Secretary of the Army for Civil Works published in
    the Federal Register a notice of intent to prepare an EIS. See
    11
    Notice of Intent to Prepare an EIS in Connection with Dakota
    Access, LLC’s Request for an Easement to Cross Lake Oahe,
    North Dakota, 
    82 Fed. Reg. 5,543
     (Jan. 18, 2017).
    Two days later, a new administration took office, and the
    government’s position changed significantly. In a January 24
    memorandum, the President directed the Secretary of the Army
    to instruct the Corps and the Assistant Secretary for Civil
    Works to expedite DAPL approvals and consider whether to
    rescind or modify the Notice of Intent to Prepare an EIS.
    Memorandum of January 24, 2017, Construction of the Dakota
    Access Pipeline, 
    82 Fed. Reg. 8,661
     (Jan. 30, 2017). The Army
    in turn concluded that the record supported granting an
    easement and that no EIS or further supplementation was
    necessary.
    The Corps granted the easement on February 8, 2017, and
    after the district court denied Cheyenne River’s motion for a
    preliminary injunction and temporary restraining order, both
    the Tribes and the Corps moved for partial summary judgment
    on several claims. The district court concluded that the Corps’s
    decision not to issue an EIS violated NEPA by failing to
    adequately consider three issues: whether the project’s effects
    were likely to be “highly controversial,” the impact of a
    hypothetical oil spill on the Tribes’ fishing and hunting rights,
    and the environmental-justice effects of the project. Standing
    Rock III, 255 F. Supp. 3d at 111–12. It accordingly remanded
    the matter to the agency to address those three issues. Id. at
    160–61.
    After the Corps completed its remand analysis in February
    2019, the parties again moved for summary judgment, with the
    Tribes arguing that the Corps failed to remedy its NEPA
    violations and pressing several other non-NEPA claims.
    Standing Rock Sioux Tribe v. U.S. Army Corps of Engineers
    12
    (Standing Rock V), 
    440 F. Supp. 3d 1
    , 11 (D.D.C. 2020). Based
    on its examination of four topics of criticism out of “many
    . . . to choose from,” 
    id. at 17
    , the district court concluded that
    “many commenters in this case pointed to serious gaps in
    crucial parts of the Corps’[s] analysis,” demonstrating that the
    easement’s effects were “likely to be highly controversial,” 
    id. at 26
     (internal quotation marks omitted). It therefore remanded
    to the agency for it to complete an EIS but reserved the question
    whether the easement should be vacated during the remand. 
    Id.
    at 29–30. Following additional briefing, the court concluded
    that vacatur was warranted, Standing Rock Sioux Tribe v. U.S.
    Army Corps of Engineers (Standing Rock VII), 
    471 F. Supp. 3d 71
    , 87 (D.D.C. 2020), and ordered that “Dakota Access shall
    shut down the pipeline and empty it of oil by August 5, 2020,”
    Order, Standing Rock Sioux Tribe v. U.S. Army Corps of
    Engineers, No. 16-cv-01534-JEB, at 2 (D.D.C. July 6, 2020),
    ECF No. 545.
    The Corps and Dakota Access now appeal the district
    court’s order remanding for preparation of an EIS, as well as
    its separate order granting vacatur of the pipeline’s MLA
    easement and ordering that the pipeline be shut down. While
    this appeal was pending, a motions panel denied the Corps’s
    request to stay the vacatur of the easement but granted its
    request to stay the district court’s order to the extent it enjoined
    the pipeline’s use. Order, Standing Rock Sioux Tribe v. U.S.
    Army Corps of Engineers, No. 20-5197, at 1 (D.C. Cir. Aug. 5,
    2020) (August 5 Order).
    II.
    The Corps, together with Dakota Access, challenges the
    district court’s conclusion that the effects of the Corps’s
    easement decision were “likely to be highly controversial”
    under NEPA. A decision is “highly controversial,” we
    explained in National Parks Conservation Association v.
    13
    Semonite, if a “substantial dispute exists as to the size, nature,
    or effect of the major federal action.” 916 F.3d at 1083 (internal
    quotation marks omitted). But not just any criticism renders the
    effects of agency action “highly controversial.” Rather,
    “something more is required for a highly controversial finding
    besides the fact that some people may be highly agitated and
    be willing to go to court over the matter.” Id. (internal quotation
    marks omitted).
    In National Parks, we clarified what more is required.
    There, we considered the Corps’s decision to forgo an EIS
    before approving a permit authorizing an electrical
    infrastructure project in a historically significant area. “[T]he
    Corps’s assessment of the scope of the Project’s effects ha[d]
    drawn consistent and strenuous opposition, often in the form of
    concrete objections to the Corps’s analytical process and
    findings, from agencies entrusted with preserving historical
    resources and organizations with subject-matter expertise.” Id.
    at 1086. Because those criticisms reflected “the considered
    responses . . . of highly specialized governmental agencies and
    organizations” rather than “the hyperbolic cries of . . . not-in-
    my-backyard neighbors,” we found the effects of the Corps’s
    decision “highly controversial.” Id. at 1085–86. “[R]epeated
    criticism from many agencies who serve as stewards of the
    exact resources at issue, not to mention consultants and
    organizations with on-point expertise, surely rises to more than
    mere passion.” Id. at 1085. And while the Corps “did
    acknowledge and try to address [those] concerns,” that was not
    enough to put the controversy to rest. Id. at 1085–86. “The
    question is not whether the Corps attempted to resolve the
    controversy, but whether it succeeded.” Id. Indeed, an EIS is
    perhaps especially warranted where an agency explanation
    confronts but fails to resolve serious outside criticism, leaving
    a project’s effects uncertain. “Congress created the EIS process
    to provide robust information in situations . . . where, following
    14
    an environmental assessment, the scope of a project’s impacts
    remains both uncertain and controversial.” Id. at 1087–88.
    The Corps and Dakota Access advance two arguments:
    that, in relying on National Parks, the “district court applied
    the wrong legal standard,” Appellant’s Br. 14, and that the
    Corps adequately addressed the four specific disputes on which
    the district court relied in finding the effects of the Corps’s
    easement decision likely to be highly controversial. We
    disagree as to both.
    The Corps offers two bases for distinguishing this case
    from National Parks. First, it argues that here, in contrast to in
    National Parks, “the Corps’[s] efforts to respond to the Tribes’
    criticisms were not ‘superficial.’” Appellant’s Br. 19. That
    distinction, however, rests on an inaccurate description of
    National Parks. Contrary to the Corps’s claim that we deemed
    “superficial and inadequate” the Corps’s response to criticisms,
    we pointedly explained that we took “no position on the
    adequacy of the Corps’s alternatives analyses.” National
    Parks, 916 F.3d at 1088. Instead, we noted only that other
    agencies had expressed concerns about the superficiality and
    inadequacy of the Corps’s efforts. Id. Furthermore, the Corps’s
    position that a response to criticism suffices so long as it is not
    “superficial” is hard to square with our statement in National
    Parks that “[t]he question is not whether the Corps attempted
    to resolve the controversy, but whether it succeeded.” Id. at
    1085–86. The decisive factor is not the volume of ink spilled
    in response to criticism, but whether the agency has, through
    the strength of its response, convinced the court that it has
    materially addressed and resolved serious objections to its
    analysis, a matter requiring us to delve into the details of the
    Tribes’ criticisms—to which we shall turn momentarily.
    15
    As a second basis for distinguishing National Parks, the
    Corps emphasizes that the “opposition here has come from the
    Tribes and their consultants, not from disinterested public
    officials.” Appellant’s Br. 20. But the Tribes are not, as Dakota
    Access suggested at oral argument, “quintessential . . . not-in-
    my-backyard neighbors.” Oral Arg. Tr. 97:17–18. They are
    sovereign nations with at least some stewardship responsibility
    over the precise natural resources implicated by the Corps’s
    analysis. “Indian tribes within Indian country are,” the
    Supreme Court has declared, “a good deal more than private,
    voluntary organizations.” Merrion v. Jicarilla Apache Tribe,
    
    455 U.S. 130
    , 140 (1982) (internal quotation marks omitted).
    Rather, they are “domestic dependent nations that exercise
    inherent sovereign authority over their members and
    territories” and the resources therein. Oklahoma Tax
    Commission v. Citizen Band Potawatomi Indian Tribe of
    Oklahoma, 
    498 U.S. 505
    , 509 (1991) (internal quotation marks
    omitted); see also New Mexico v. Mescalero Apache Tribe, 
    462 U.S. 324
    , 335 (1983) (“We have held that tribes have the power
    to manage the use of [their] territory and resources by both
    members and nonmembers . . . .”); Merrion, 
    455 U.S. at 140
    (“Indian tribes . . . . are unique aggregations possessing
    attributes of sovereignty over both their members and their
    territory.” (internal quotation marks omitted)).
    The Tribes’ unique role and their government-to-
    government relationship with the United States demand that
    their criticisms be treated with appropriate solicitude. Of
    course, as the Corps points out, the Tribes are not the federal
    government. But in National Parks, we emphasized the
    important role played by entities other than the federal
    government. There, criticism came from “highly specialized
    governmental agencies and organizations,” including the
    Virginia Department of Historic Resources and several
    conservation groups. National Parks, 916 F.3d at 1084–85; see
    16
    also North Carolina v. Federal Aviation Administration, 
    957 F.2d 1125
    , 1131–33 (4th Cir. 1992) (finding “legitimate
    controversy” present where “[s]tate, local and federal officials,
    interested individuals,” and a federal agency “expressed
    concern”); Foundation for North American Wild Sheep v. U.S.
    Department of Agriculture, 
    681 F.2d 1172
    , 1182 (9th Cir.
    1982) (finding that criticism from “conservationists,
    biologists,” two state agencies, and “other knowledgeable
    individuals” demonstrated the existence of “precisely the type
    of ‘controversial’ action for which an EIS must be prepared”);
    Friends of the Earth, Inc. v. U.S. Army Corps of Engineers, 
    109 F. Supp. 2d 30
    , 43 (D.D.C. 2000) (finding that a project was
    “genuinely and extremely controversial” where “three federal
    agencies,” “one state agency,” and the public “all disputed the
    Corps[’s] evaluation”). The Tribes are of at least equivalent
    status.
    With the proper legal framework in mind, we turn to the
    four disputed facets of the Corps’s analysis that the district
    court found involved unresolved scientific controversies for
    purposes of NEPA’s “highly controversial” factor.
    DAPL’s Leak Detection System
    The district court found that serious unresolved
    controversy existed concerning the effectiveness of DAPL’s
    leak detection system. Specifically, it found that the 2012
    Pipeline and Hazardous Materials Safety Administration
    (PHMSA) study submitted with Standing Rock’s expert report
    “indicated an 80% failure rate in the type of leak-detection
    system employed by DAPL.” Standing Rock V, 440 F. Supp.
    3d at 18. The court went on to note that “the system was not
    even designed to detect leaks that constituted 1% or less of the
    pipe’s flow rate,” which could amount to 6,000 barrels a day.
    Id. Because the Corps “failed entirely to respond to” those
    deficiencies, the court found that the Corps had not succeeded
    17
    in resolving the controversy presented by the study. Id. at
    17–18.
    On appeal, the Corps correctly points out that the 2012
    PHMSA study does not reflect an 80% “failure rate.” Rather,
    the study indicates that in 80% of all incidents where it was in
    use and “functional,” the “computational pipeline monitoring”
    (CPM) system used by DAPL was not the first system to detect
    a leak. That the CPM system was commonly eclipsed by visual
    identification, however, casts serious, unaddressed doubt on
    the Corps’s statement that the system will “detect the pressure
    drop from a pipeline rupture within seconds.” Appellant’s Br.
    21 (internal quotation marks omitted). As the PHMSA study
    explains, “CPM systems by themselves did not appear to
    respond more often than personnel . . . or members of the
    public passing by the release incident.” U.S. Department of
    Transportation, Pipeline and Hazardous Materials Safety
    Administration, Final Report Leak Detection Study 2-11 (Dec.
    10, 2012). The Corps has failed to address the apparent
    disconnect, suggested by the PHMSA study, between the CPM
    system’s historic performance and the agency’s representations
    about its future utility. Indeed, the Corps acknowledges that it
    “did not explicitly discuss the 2012 PHMSA report” in its
    review. Appellant’s Br. 22. The consequences of that oversight
    are especially significant since DAPL is buried deep
    underground and visual identification is therefore unlikely to
    make up for deficiencies in the CPM system, as it apparently
    has in the incidents included in the PHMSA study.
    Attempting to discount the significance of the Corps’s
    failure to consider the 2012 PHMSA study, the Corps and
    Dakota Access observe that the study included older pipelines
    and that the type of pinhole leaks the study suggests the CPM
    system might initially miss are rare. But as the district court
    noted, the Tribes’ expert observed that “more recent
    18
    investigations” corroborated the study’s leak detection data.
    Standing Rock V, 440 F. Supp. 3d at 17 (internal quotation
    marks omitted). The Corps’s failure to address the study cannot
    be justified by the mere fact that the study’s data set includes
    some older pipelines.
    As for the rarity of pinhole leaks, the Tribes pointed to
    “numerous examples of pipelines that leaked for hours or days
    after similar detection systems failed.” Appellees’ Br. 27. In
    one such instance, DAPL’s own operator spilled 8,600 barrels
    of oil during a 12-day-long slow leak in 2016, even though the
    monitoring system in use there showed the exact same type of
    “detectable meter imbalance” that the Corps here claims will
    quickly alert DAPL’s operators to a slow leak. See
    Supplemental Appendix (S.A.) 317–18. That same year, at
    another pipeline buried deep underground in North Dakota, an
    operator’s leak detection system “registered an imbalance” and
    “notified the control room”—but the control room
    “misinterpreted its own data[.]” PHMSA, Post-Hearing
    Decision Confirming Corrective Action Order, Belle Fourche
    Pipeline Co. 5 (Mar. 24, 2017), https://primis.phmsa.dot.gov/
    comm/reports/enforce/documents/520165013H/520165013H_
    HQ%20Post%20Hearing%20Decision%20Confirming%20C
    AO_03242017.pdf. That led to a slow release of more than
    12,600 barrels of oil into a nearby creek over at least a two-day
    period, until it was discovered by a rancher at the release site.
    Id. at 1–2; S.A. 711. So there is ample reason to believe that
    the magnitude of harm from such a leak could be substantial.
    Appearing to acknowledge those troubling examples, the
    Corps discounts their significance by asserting that leaks will
    eventually be found. But how rapidly such leaks would be
    detected and their potential severity are key factors underlying
    the Corps’s EA and precisely the issues called into question by
    the Tribes’ unaddressed criticism. We also note that the volume
    19
    of a one percent spill from a pinhole leak would double if the
    volume of oil placed in the pipeline were itself to double. And
    DAPL’s operator has represented to its investors that it intends
    to double the amount of oil it places in the pipeline as early as
    this coming summer. See Illinois approves expansion of
    Dakota Access oil pipeline, Reuters, Oct. 15, 2020,
    https://www.reuters.com/article/us-energy-transfer-oil-
    pipeline-illinois-idUSKBN2702DL. In any event, when asked
    why the EA did not evaluate the potential consequences of an
    undetected slow pinhole leak, the Corps responded that “there
    was no particular reason” it did not do so. Oral Arg. Tr. 12:8–9,
    Standing Rock Sioux Tribe v. U.S. Army Corps of Engineers,
    No. 16-cv-01534-JEB (D.D.C. Mar. 18, 2020), ECF No. 498.
    The Tribes’ criticisms therefore present an unresolved
    controversy requiring the Corps to prepare an EIS.
    DAPL’s Operator Safety Record
    The district court found that the Corps’s decision to rely in
    its risk analysis on general pipeline safety data, rather than
    DAPL’s operator’s specific safety record, rendered the effects
    of the Corps’s decision highly controversial. We agree.
    To analyze the Corps’s risk assessment, Standing Rock
    retained as an expert “an attorney, investigator, and process
    safety practitioner with many decades of experience.”
    Holmstrom Decl. ¶ 1, S.A. 79–80. The expert explained that
    “PHMSA data shows Sunoco,” DAPL’s operator, “has
    experienced 276 incidents in 2006–2016,” which the expert
    described as “one of the lower performing safety records of any
    pipeline operator in the industry for spills and releases.” Id. ¶ 9.
    Here, as in the district court, “[t]he Corps focuse[s] its
    responses on defending the operator’s performance record
    itself rather than on justifying its decision to not incorporate
    that record into its analysis.” Standing Rock V, 
    440 F. Supp. 3d 20
    at 19. In so doing, the Corps and Dakota Access make two
    arguments.
    First, the Corps emphasizes that “70% of [DAPL’s]
    operator’s reported accidents on other pipelines were minor
    and limited to the operator’s property.” Appellant’s Br. 31. But
    that does nothing to address the “[t]wo central concerns” on
    which the district court based its decision: “(1) the 30% of
    spills—about 80 of them—that were not limited to operator
    property; and (2) the criticism that the spill analysis should
    have incorporated the operator’s record.” Standing Rock V, 440
    F. Supp. 3d at 20. For its part, Dakota Access argues that while
    Sunoco’s number of leaks is high, its number of spills per mile
    of pipeline operated “is in line with industry averages.”
    Intervenor’s Br. 22. Not only has Dakota Access failed to
    identify record evidence supporting that assertion, the relevant
    evidence that does exist suggests a serious risk that Sunoco’s
    record is worse than the industry average. The Corps’s own
    analysis concluded that, industry-wide, there were 0.953
    onshore crude oil accidents per 1,000 miles of pipeline in 2016
    and 0.848 in 2017. U.S. Army Corps of Engineers, Analysis of
    the Issues Remanded by the U.S. District Court for the District
    of Columbia Related to the Dakota Access Pipeline Crossing
    at Lake Oahe 13 (Aug. 31, 2018). By contrast, Dakota Access’s
    expert explained that Energy Transfer, Sunoco’s parent
    company following a merger, experienced 1.42 “reportable
    incidents per 1,000 miles of pipeline”—after a 50% decline in
    incidents on Sunoco lines since 2017. Second Godfrey Decl.
    ¶ 7, A. 1612. If anything, comparing that figure to the industry-
    wide average understates the safety gap between Sunoco and
    other operators because, as Dakota Access and its expert
    observe, Sunoco is “one of the largest pipeline operators,”
    Intervenor’s Br. 22, and its own incidents are included in the
    average. See Appellant’s Br. 32 (“The Corps also considered
    21
    PHMSA’s historical data on oil spills, which necessarily
    includes this operator’s safety record.”).
    Nor are we persuaded by the Corps’s second argument,
    that it had no need at all to address the operator safety
    controversy. Though the Corps may have considered “other
    objective measures of the operator’s safety practices,”
    Appellant’s Br. 31, the cited materials—industry-wide spill
    data and a questionnaire about Sunoco’s safety practices—fall
    short of resolving the controversy. The Corps contends that its
    “decision to use all data on oil spills, and not just the operator’s
    safety record, is the kind of technical judgment that is entrusted
    to the agency and entitled to deference from the Court.”
    Appellant’s Br. 32. That is not at all clear. For example, it
    would be strange indeed if we were to defer to the Federal
    Aviation Administration’s decision to renew the operating
    certificate of an airline with an extremely poor safety record on
    the basis that the airline industry, on average, is safe. The
    Supreme Court, moreover, has “frequently reiterated that an
    agency must cogently explain why it has exercised its
    discretion in a given manner,” Motor Vehicle Manufacturers
    Ass’n of the United States, Inc. v. State Farm Mutual
    Automobile Insurance Co., 
    463 U.S. 29
    , 48 (1983), and the
    Corps has made no effort to do so here. To treat the Corps’s
    unadorned plea for deference as a sufficient basis for ignoring
    well-reasoned expert criticism would vitiate National Parks.
    Winter Conditions
    The district court found the Corps’s response insufficient
    to resolve criticism of the agency’s “failure to consider the
    impact of harsh North Dakota winters on response efforts in the
    event of a spill.” Standing Rock V, 440 F. Supp. 3d at 20. In
    particular, the Tribes’ experts explained that shut-off valves
    might be more prone to failure and response efforts hindered
    by freezing conditions. Elaborating, Oglala’s expert explained
    22
    that “winter conditions create significant difficulties” because,
    among other things, “workers require more breaks and move
    slower due to the bundling of clothing,” “daylight hours are
    shorter,” and “slip-trip-fall risk increases significantly.”
    Earthfax Report at 7, A. 830.
    The Corps argues that it had no need to engage in a
    quantitative evaluation of a winter spill scenario because its
    non-quantitative response was adequate. Appellant’s Br.
    29–30. In the Corps’s view, it adequately considered winter
    conditions by noting that ice coverage could “have a mixed
    effect on efforts to contain an oil spill” and by ordering DAPL’s
    operator to conduct winter spill response training exercises at
    Lake Oahe as a condition of the easement. Appellant’s Br. 29.
    But the Corps’s passing reference to winter conditions’
    “mixed” effects, without more, provides little comfort. The
    Corps’s point might have been more forceful had the agency
    estimated just how much time during a spill would be saved by
    the oil-containing properties of ice and compared that to the
    additional time required to identify oil pockets and adjust work
    methods to extreme conditions. Indeed, it seems that such an
    analysis is precisely what the Tribes believe the Corps ought to
    have done, and such a reasoned weighing of the evidence
    would have been entitled to substantial deference. But instead,
    faced with serious expert criticism, the Corps simply declared
    the evidence “mixed” and offered no attempt at explaining its
    apparent conclusion that winter’s countervailing effects
    measured out to zero. Moreover, we agree with the district
    court that while winter response training may be “prudent and
    perhaps a good avenue for producing data as to how exactly
    winter conditions would delay response efforts,” such exercises
    do “not get to the point of addressing the concern that the spill
    model does not currently take that kind of data into account.”
    Standing Rock V, 440 F. Supp. 3d at 21.
    23
    The Corps next argues that the Tribes failed to present a
    “specific alternative methodology” for incorporating winter
    conditions into its spill response modeling. Appellant’s Br. 30.
    But the fact that an established methodology for assessing the
    consequences of a unique type of risk is not readily apparent to
    commenters hardly means an agency can discount relevant,
    serious criticism of its method of analysis. Although the Corps
    emphasizes in its brief that “no one has identified any way to
    calculate exactly how much more difficult” a clean-up would
    be during winter, Appellant’s Br. 30, our review “is limited to
    the grounds that the agency invoked when it took the action,”
    Department of Homeland Security v. Regents of the University
    of California, 
    140 S. Ct. 1891
    , 1907 (2020) (internal quotation
    marks omitted), and the Corps does not suggest that, during its
    environmental review process, it actually applied its technical
    expertise to consider whether it was possible to identify such a
    method. Had the Corps considered the problem and concluded
    that no comprehensive analysis was possible, that might have
    amounted to “successfully” resolving the controversy. But the
    Corps cannot foist its duty to consider such technical matters
    onto commenters who point out valid deficiencies.
    Worst Case Discharge
    The district court considered the “largest area of scientific
    controversy” to be “the worst-case-discharge estimate for
    DAPL used in the spill-impact analysis.” Standing Rock V, 440
    F. Supp 3d at 21. The regulations set forth a detailed formula
    for calculating the worst-case discharge, 
    49 C.F.R. § 194.105
    (b)(1), but we need not delve into its specifics here.
    “The idea,” the district court succinctly explained, “is to
    calculate the maximum amount of oil that could possibly leak
    from the pipeline before a spill is detected and stopped.”
    Standing Rock V, 440 F. Supp. 3d at 21.
    24
    According to the Corps, we need not consider the Tribes’
    criticisms because “an accident leading to a full-bore rupture
    of the pipeline is extremely unlikely” and, in any event, no
    statute or regulation required the Corps to calculate the worst-
    case discharge at all. Appellant’s Br. 26. The thrust of both
    arguments is that because the Corps need not have calculated a
    worst-case discharge in the first place, it is unimportant
    whether it did so in a reasonable manner. But we agree with the
    district court that because the Corps chose to perform such a
    calculation and then relied on it throughout its analysis, it
    cannot dispel serious doubts about its methods by explaining
    that it could have forgone such a calculation in the first place.
    See Sierra Club v. Sigler, 
    695 F.2d 957
    , 966 (5th Cir. 1983)
    (“The purpose of judicial review under NEPA is to ensure the
    procedural integrity of the agency’s consideration of
    environmental factors in the EIS and in its decision to issue
    permits. If the agency follows a particular procedure, it is only
    logical to review the agency’s adherence to that procedure, not
    to some altogether different one that was not used.”). We
    therefore turn to the Tribes’ criticisms of the Corps’s
    calculations.
    The Corps estimated that, for purposes of a worst-case
    discharge, it would take 9 minutes to detect a leak and 3.9
    minutes to close the shut-down valves. Appellant’s Br. 26–27.
    Before the district court, the Corps suggested that its nine-
    minute figure included one minute of detection time, with the
    remaining eight minutes devoted to shutting down the mainline
    pumps. Standing Rock V, 440 F. Supp. 3d at 23. But as the
    district court observed, the Tribes pointed to “many experts
    who commented that hours, rather than minutes, were more
    accurate figures for the [worst-case discharge].” Id. The Tribes’
    expert explained that “[m]ajor spill incidents typically occur
    with multiple system causes, when people, or equipment, or
    systems do not function exactly as they are expected to.”
    25
    Holmstrom Decl. ¶ 11, S.A. 83. The Corps’s explanation that
    its response time estimates were mildly conservative does not
    begin to explain its choice to ignore the real-world possibility
    of significant human errors or technical malfunctions, see
    supra at 18–19, in calculating what it claimed was a worst-case
    estimate. Although the PHMSA formula did not require the
    Corps to model a complete doomsday scenario in which every
    possible human error and technical malfunction occurs
    simultaneously, we agree with the district court that the Corps’s
    failure to explain why it declined to consider any such
    eventualities leaves unresolved a substantial dispute as to its
    worst-case discharge calculation.
    The Corps also argues that, even if, as the Tribes claim,
    some aspects of the model are unduly optimistic, the model is
    nonetheless sufficiently conservative because it assumes the
    pipeline lies directly on top of the water rather than beneath
    ninety-two feet of overburden. Appellant’s Br. 25–26. In effect,
    the Corps tries to defend its decision to develop a model that
    assumes away significant risks by explaining that, despite those
    omissions, it analyzed an imaginary pipeline of roughly
    equivalent risk to DAPL—one laying directly on top of Lake
    Oahe, but with superior leak detection and shut-down valve
    systems. The Corps, however, never explains why its one
    conservative assumption accurately counterbalances the
    particular risks the Tribes identify. Accordingly, the model’s
    assumption that DAPL lies directly on the water fails to resolve
    the controversies raised by the Tribes’ criticisms.
    ***
    Having determined that several serious scientific disputes
    mean that the effects of the Corps’s easement decision are
    likely to be “highly controversial,” we turn to one other issue
    before considering the appropriate remedy. The Corps and
    26
    Dakota Access repeatedly urge that, whatever the merits of the
    Tribes’ criticisms, the Corps’s easement decision cannot be
    highly controversial because the risk of a spill is exceedingly
    low and because the pipeline’s location deep underground
    provides protection against the consequences of any spill. That
    argument faces two major hurdles.
    First, the claimed low risk of a spill rests, in part, on the
    Corps’s use of generalized industry safety data and its
    optimism concerning its ability to respond to small leaks before
    they worsen—precisely what the Tribes’ unresolved criticisms
    address. Second, as our court made clear in New York v.
    Nuclear Regulatory Commission, 
    681 F.3d 471
    , 478–79 (D.C.
    Cir. 2012), “[u]nder NEPA, an agency must look at both the
    probabilities of potentially harmful events and the
    consequences if those events come to pass.” 
    Id. at 148
    . A
    finding of no significant impact is appropriate only if a grave
    harm’s “probability is so low as to be remote and speculative,
    or if the combination of probability and harm is sufficiently
    minimal.” 
    Id.
     at 147–48 (internal quotation marks omitted).
    Doing away with the obligation to prepare an EIS whenever a
    project presents a low-probability risk of very significant
    consequences would wall off a vast category of major projects
    from NEPA’s EIS requirement. After all, the government is not
    in the business of approving pipelines, offshore oil wells,
    nuclear power plants, or spent fuel rod storage facilities that
    have any material prospect of catastrophic failure. In this case,
    although the risk of a pipeline leak may be low, that risk is
    sufficient “‘that a person of ordinary prudence would take it
    into account in reaching a decision’” to approve the pipeline’s
    placement, and its potential consequences are therefore
    properly considered here. Sierra Club v. FERC, 
    827 F.3d 36
    ,
    47 (D.C. Cir. 2016) (quoting City of Shoreacres v. Waterworth,
    
    420 F.3d 440
    , 453 (5th Cir. 2005)).
    27
    III.
    This brings us to the Corps’s challenge to the district
    court’s remedy, and specifically to its orders (1) requiring that
    the Corps prepare an EIS, (2) vacating the easement pending
    preparation of an EIS, and (3) ordering that the pipeline be shut
    down and emptied of oil.
    As already explained, “[i]mplicating any one of the
    [intensity] factors may be sufficient to require development of
    an EIS.” National Parks, 916 F.3d at 1082. Dakota Access
    argues that because implicating the “highly controversial”
    factor does not itself mandate preparation of an EIS, the district
    court erred in ordering the Corps to prepare one. In National
    Parks, however, we ordered the Corps to prepare an EIS where,
    as here, it “failed to make a ‘convincing case’ that an EIS is
    unnecessary.” Id. at 1087 (quoting Myersville Citizens for a
    Rural Community, Inc. v. FERC, 
    783 F.3d 1301
    , 1322 (D.C.
    Cir. 2015)). National Parks thus forecloses the idea that we
    must ordinarily remand to the agency to weigh the intensity
    factors anew whenever we find that it improperly analyzed one
    of them.
    That National Parks involved multiple intensity factors is
    at most a superficial distinction between this case and National
    Parks. For one thing, as explained above, the effects of the
    Corps’s easement decision are “highly controversial” in four
    distinct respects, and we see no good reason for treating
    differently a decision that implicates multiple significance
    factors and a decision that implicates a single factor in several
    important ways. Moreover, both National Parks and this case
    present “precisely” the circumstances in which Congress
    intended to require an EIS, namely “where, following an
    environmental assessment, the scope of a project’s impacts
    remains both uncertain and controversial.” 
    Id.
     at 1087–88.
    Finally, as in National Parks, the “context” of this case—“a
    28
    place of extraordinary importance to the Tribes, a landscape of
    profound cultural importance, and the water supply for the
    Tribes and millions of others”—weighs in favor of requiring an
    EIS. Appellees’ Br. 40–41. And in at least one sense, the case
    for ordering production of an EIS is stronger here than in
    National Parks or the cases on which Dakota Access relies,
    Intervenor’s Br. 29–30, given that, unlike in those cases, the
    district court has already given the Corps an opportunity to
    resolve the Tribes’ serious criticisms and it failed to do so.
    The Corps and Dakota Access next argue that, even if the
    district court properly ordered the Corps to prepare an EIS, the
    court abused its discretion by vacating the pipeline’s easement
    in the interim. “The ordinary practice,” however, “is to vacate
    unlawful agency action,” United Steel v. Mine Safety & Health
    Administration, 
    925 F.3d 1279
    , 1287 (D.C. Cir. 2019) (citing 
    5 U.S.C. § 706
    (2)), and district courts in this circuit routinely
    vacate agency actions taken in violation of NEPA. See, e.g.,
    Humane Society of the United States v. Johanns, 
    520 F. Supp. 2d 8
    , 37 (D.D.C. 2007) (observing that vacatur is the “standard
    remedy” for an “action promulgated in violation of NEPA”);
    Greater Yellowstone Coalition v. Bosworth, 
    209 F. Supp. 2d 156
    , 163 (D.D.C. 2002) (“[P]laintiffs . . . seek a vacatur of the
    permit . . . until the [agency] complies with NEPA. As a
    general matter, an agency action that violates the APA must be
    set aside. . . . Based on this authority, I shall vacate the permit
    . . . .”).
    “While unsupported agency action normally warrants
    vacatur, [a] court is not without discretion” to leave agency
    action in place while the decision is remanded for further
    explanation. Advocates for Highway and Auto Safety v.
    Federal Motor Carrier Safety Administration, 
    429 F.3d 1136
    ,
    1151 (D.C. Cir. 2005) (citation omitted). In Allied-Signal,
    Inc. v. U.S. Nuclear Regulatory Commission, 
    988 F.2d 146
    29
    (D.C. Cir. 1993), our court set forth the two factors governing
    that exercise of discretion: “The decision whether to vacate
    depends on the seriousness of the order’s deficiencies (and thus
    the extent of doubt whether the agency chose correctly) and the
    disruptive consequences of an interim change that may itself be
    changed.” 
    Id.
     at 150–51 (internal quotation marks omitted).
    The “seriousness” of a deficiency, we have explained, is
    determined at least in part by whether there is “a significant
    possibility that the [agency] may find an adequate explanation
    for its actions” on remand. Williston Basin Interstate Pipeline
    Co. v. FERC, 
    519 F.3d 497
    , 504 (D.C. Cir. 2008). “We review
    the district court’s decision to vacate . . . for abuse of
    discretion.” Nebraska Department of Health & Human
    Services v. Department of Health & Human Services, 
    435 F.3d 326
    , 330 (D.C. Cir. 2006).
    As to the first factor, the district court concluded that the
    Corps was unlikely to resolve the controversies on remand
    because the court had previously remanded without vacatur for
    just that purpose and the Corps had nonetheless failed to
    resolve them. Standing Rock VII, 471 F. Supp. 3d at 79–80. The
    court also explained that the Corps focused on the wrong
    question: whether, on remand, it would be able to justify its
    easement decision rather than its decision to forgo an EIS. Id.
    at 81. (“Looking at the first Allied-Signal factor, the Court does
    not assess the deficiency of the ultimate decision itself—the
    choice to issue the permit—but rather the deficiency of the
    determination that an EIS was not warranted.” (internal
    quotation marks omitted)).
    With respect to the disruptive consequences of vacatur, the
    district court understood that shutting down pipeline operations
    would cause Dakota Access and other entities significant
    economic harm. But for four reasons it concluded that those
    effects did not justify remanding without vacatur. First, the
    30
    Corps’s expedited timeline for preparing an EIS “would cabin
    the economic disruption of a shutdown.” Id. at 84. Second,
    though economic disruption is properly considered, it is not
    commonly a basis, standing alone, for declining to vacate
    agency action. Id. at 84–85. Third, Dakota Access’s approach
    would subvert NEPA’s objectives. “[I]f you can build first and
    consider environmental consequences later, NEPA’s action-
    forcing purpose loses its bite.” Id. at 85. And finally, the
    countervailing risk of a spill—difficult to quantify in part
    because of the Corps’s failure to prepare an EIS—counseled in
    favor of vacatur. Id. at 85–86. The district court discounted as
    “inconclusive” Dakota Access’s evidence that if DAPL were
    inoperative, more oil would be transported by rail, a riskier
    alternative. Id. at 87.
    On appeal, Dakota Access takes primary responsibility for
    arguing against vacatur. It contends first that the Corps can
    “easily substantiate its easement decision on remand even if it
    must prepare an EIS.” Intervenor’s Br. 33. But that is not the
    question. As the district court explained, the question is
    whether the Corps is likely to justify its issuance of a FONSI
    and refusal to prepare an EIS. Dakota Access argues that
    Heartland Regional Medical Center v. Sebelius, 
    566 F.3d 193
    (D.C. Cir. 2009), supports its contrary view that the Allied-
    Signal factors look to whether an agency can justify the action
    the court is considering whether to vacate, rather than the
    challenged procedural decision. There, we sought to determine
    whether an earlier district court decision had, by declaring a
    regulatory requirement invalid for failing to consider certain
    public comments, necessarily vacated the regulation. In
    making that determination, we concluded that the Allied-Signal
    factors would have directed remand without vacatur. 
    Id.
     at
    197–98. But because the agency had not elected to forgo a
    procedural requirement (in that case, notice and comment),
    only one agency action—the decision to promulgate the
    31
    challenged rule—was implicated at all. Heartland Regional
    therefore says nothing one way or the other about the proper
    focus of the Allied-Signal inquiry in cases, like this one, where
    we confront a distinct challenge to an agency’s decision to
    forgo a major procedural step in its path to its ultimate action.
    Cf. 
    id. at 199
     (“Failure to provide the required notice and to
    invite public comment—in contrast to the agency’s failure here
    adequately to explain why it chose one approach rather than
    another for one aspect of an otherwise permissible rule—is a
    fundamental flaw that normally requires vacatur of the rule.”
    (internal quotation marks omitted)). Besides, the district
    court’s view is more sensible.
    Consider the consequences of Dakota Access’s contrary
    approach. If, when an agency declined to prepare an EIS before
    approving a project, courts considered only whether the agency
    was likely to ultimately justify the approval, it would subvert
    NEPA’s purpose by giving substantial ammunition to agencies
    seeking to build first and conduct comprehensive reviews later.
    If an agency were reasonably confident that its EIS would
    ultimately counsel in favor of approval, there would be little
    reason to bear the economic consequences of additional delay.
    For similar reasons, an agency that bypassed required notice
    and comment rulemaking obviously could not ordinarily keep
    in place a regulation while it completed that fundamental
    procedural prerequisite. See Daimler Trucks North America
    LLC v. EPA, 
    737 F.3d 95
    , 103 (D.C. Cir. 2013) (“[T]he court
    typically vacates rules when an agency ‘entirely fail[s]’ to
    provide notice and comment . . . .” (quoting Shell Oil Co. v.
    EPA, 
    950 F.2d 741
    , 752 (D.C. Cir. 1991))). When an agency
    bypasses a fundamental procedural step, the vacatur inquiry
    asks not whether the ultimate action could be justified, but
    whether the agency could, with further explanation, justify its
    decision to skip that procedural step. Otherwise, our cases
    32
    explaining that vacatur is the default response to a fundamental
    procedural failure would make little sense.
    Even were we to consider the Corps’s odds of ultimately
    approving the easement, our case law still instructs that a
    failure to prepare a required EIS should lead us to doubt that
    the ultimate action will be approved. In Oglala Sioux Tribe v.
    U.S. Nuclear Regulatory Commission, 
    896 F.3d 520
     (D.C. Cir.
    2018), we explained that because NEPA is a “purely procedural
    statute,” where an agency’s NEPA review suffers from “a
    significant deficiency,” refusing to vacate the corresponding
    agency action would “vitiate” the statute. 
    Id. at 536
     (internal
    quotation marks omitted). As we made clear, “[p]art of the
    harm NEPA attempts to prevent in requiring an EIS is that,
    without one, there may be little if any information about
    prospective environmental harms and potential mitigating
    measures.” 
    Id.
     (internal quotation marks omitted). Put another
    way, Oglala strongly suggests that where an EIS was required
    but not prepared, courts should harbor substantial doubt that
    “‘the agency chose correctly’” regarding the substantive action
    at issue—in this case, granting the easement. 
    Id. at 538
    (quoting Allied-Signal, 
    988 F.2d at
    150–51). The Corps resists
    the proposition that Oglala cautions against applying Allied-
    Signal in NEPA cases, but that is not the point. The point is that
    Oglala’s application of those factors suggests that NEPA
    violations are serious notwithstanding an agency’s argument
    that it might ultimately be able to justify the challenged action.
    As for vacatur’s consequences, Dakota Access contends
    that while the district court “acknowledged the severe
    economic disruption that vacatur would cause,” it “wrongly
    discounted those severe consequences” and “credit[ed] remote,
    unsubstantiated harms.” Intervenor’s Br. 35. But in reviewing
    for abuse of discretion, we “consider whether the decision
    maker failed to consider a relevant factor, whether he [or she]
    33
    relied on an improper factor, and whether the reasons given
    reasonably support the conclusion.” Kickapoo Tribe of Indians
    of Kickapoo Reservation in Kansas v. Babbitt, 
    43 F.3d 1491
    ,
    1497 (D.C. Cir. 1995) (alteration in original) (internal
    quotation marks omitted). In doing so, we may not “substitute
    our judgment for that of the trial court, so we cannot decide the
    issue by determining whether we would have reached the same
    conclusion.” United States v. Mathis–Gardner, 
    783 F.3d 1286
    ,
    1288 (D.C. Cir. 2015) (citation omitted) (internal quotation
    marks omitted). Dakota Access believes that the district court’s
    assessment of a shutdown’s economic impacts was far too rosy
    and that the court “ignored” a shutdown’s environmental
    consequences. But the court considered all important aspects
    of the issue and reasonably concluded that the harms were less
    severe than the Corps and Dakota Access suggested. In view of
    the discretion owed the district court and the seriousness of the
    NEPA violation, Dakota Access has given us no basis for
    concluding that the district court abused its discretion in
    applying the Allied-Signal factors. See National Parks
    Conservation Association v. Semonite, 
    925 F.3d 500
    , 502
    (D.C. Cir. 2019) (“[The district] court is best positioned to . . .
    make factual findings[] and determine the remedies necessary
    to protect the purpose and integrity of the EIS process.”); Stand
    Up for California! v. U.S. Department of Interior, 
    879 F.3d 1177
    , 1190 (D.C. Cir. 2018) (“[T]he district court acted well
    within its discretion in finding vacatur unnecessary to address
    any harm the defect had caused.”).
    In any event, Dakota Access’s assessment of vacatur’s
    consequences is undercut significantly by the fact that we agree
    that the district court’s shutdown order cannot stand.
    On August 5, 2020, a motions panel of this court ordered
    that “to the extent the district court issued an injunction by
    ordering Dakota Access LLC to shut down the Dakota Access
    34
    Pipeline and empty it of oil by August 5, 2020, the injunction
    be stayed.” August 5 Order at 1. Relying on the Supreme
    Court’s decision in Monsanto Co. v. Geertson Seed Farms, 
    561 U.S. 139
     (2010), the panel explained that “[t]he district court
    did not make the findings necessary for injunctive relief.”
    August 5 Order at 1 (“[B]efore issuing an injunction in a
    [NEPA] case, ‘a court must determine that an injunction should
    issue under the traditional four-factor test.’” (quoting
    Monsanto, 
    561 U.S. at 158
    )).
    The Tribes argue that an injunction was unnecessary
    because vacatur itself “invalidat[ed] the underlying easement,”
    thus requiring the “suspension of pipeline operations pending
    compliance with NEPA.” Appellees’ Br. 73–74. That is the
    view the district court appeared to adopt, Standing Rock VII,
    471 F. Supp. 3d at 88 (requiring, after vacating the pipeline’s
    easement, “the oil to stop flowing and the pipeline to be
    emptied within 30 days”), and that approach finds some
    support in our case law. For instance, in Sierra Club v. FERC,
    
    867 F.3d 1357
    , 1379 (D.C. Cir. 2017), we vacated a pipeline
    authorization due to a NEPA violation and appeared to assume
    that vacatur encompassed an end to construction. Likewise in
    National Parks, we appeared to accept the parties’ assumption
    that vacating Corps-issued construction permits would require
    ceasing construction of the challenged electrical towers or
    tearing them down. See National Parks, 925 F.3d at 502.
    The Tribes’ approach, however, cannot be squared with
    Monsanto, which should caution against reading too far into
    our tacit approval of shutdown orders in prior cases. If a district
    court could, in every case, effectively enjoin agency action
    simply by recharacterizing its injunction as a necessary
    consequence of vacatur, that would circumvent the Supreme
    Court’s instruction in Monsanto that “a court must determine
    that an injunction should issue under the traditional four-factor
    35
    test.” 
    561 U.S. at 158
    . In fact, the Tribes have already moved
    for a permanent injunction in the district court during the
    pendency of this appeal, and that motion is fully briefed.
    Furthermore, Sierra Club and National Parks differ from
    this case in a subtle but important way. Those cases involved
    challenges to agency authorizations of the very activities the
    court assumed would end. Vacating a construction permit in
    National Parks, for instance, naturally implied an end to
    construction. Here, in contrast, we affirm the vacatur of an
    easement authorizing the pipeline to cross federal lands. With
    or without oil flowing, the pipeline will remain an
    encroachment, leaving the precise consequences of vacatur
    uncertain. In fact, the parties have identified no other
    instance—and we have found none—in which the sole issue
    before a court was whether an easement already in use (rather
    than a construction or operating permit) must be vacated on
    NEPA grounds. That makes this case quite unusual and cabins
    our decision to the facts before us.
    It may well be—though we have no occasion to consider
    the matter here—that the law or the Corps’s regulations oblige
    the Corps to vindicate its property rights by requiring the
    pipeline to cease operation and that the Tribes or others could
    seek judicial relief under the APA should the Corps fail to do
    so. But how and on what terms the Corps will enforce its
    property rights is, absent a properly issued injunction, a matter
    for the Corps to consider in the first instance, though we would
    expect it to decide promptly. To do otherwise would be to issue
    a de facto outgrant without engaging in the NEPA analysis that
    the Corps concedes such an action requires. See Oral Arg. Tr.
    36:14–15 (“The Corps’[s] regulations contemplate that an
    outgrant would require a NEPA analysis.”). Although the
    district court was attuned to the discretion owed the Corps, see
    Standing Rock VII, 471 F. Supp. 3d at 88 (“Not wishing to
    36
    micromanage the shutdown, [the court] will not prescribe the
    method by which DAPL must [make the flow of oil cease].”),
    we nonetheless conclude that it could not order the pipeline to
    be shut down without, as required by Monsanto, making the
    findings necessary for injunctive relief.
    IV.
    For the foregoing reasons, we affirm the district court’s
    order vacating DAPL’s easement and directing the Corps to
    prepare an EIS. We reverse to the extent the court’s order
    directs that the pipeline be shut down and emptied of oil.
    So ordered.