 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued October 13, 2017            Decided January 12, 2018

                        No. 16-5327

            STAND UP FOR CALIFORNIA!, ET AL.,
                      APPELLANTS

   PICAYUNE RANCHERIA OF THE CHUKCHANSI INDIANS, A
         FEDERALLY RECOGNIZED INDIAN TRIBE,
                    APPELLANT

                             v.

   UNITED STATES DEPARTMENT OF THE INTERIOR, ET AL.,
                     APPELLEES

        NORTH FORK RANCHERIA OF MONO INDIANS,
                INTERVENOR-APPELLEE


                 Consolidated with 16-5328


        Appeals from the United States District Court
                for the District of Columbia
                    (No. 1:12-cv-02039)


    Sean M. Sherlock argued the cause for appellants Stand Up
for California!, et al. With him on the briefs were Todd E.
Lundell and Benjamin Sharp. Jennifer A. MacLean entered an
appearance.
                               2

     Michael A. Robinson argued the cause for appellant
Picayune Rancheria of the Chukchansi Indians. With him on
the briefs was James Qaqundah. Merrill C. Godfrey entered an
appearance.

     Brian C. Toth, Attorney, U.S. Department of Justice,
argued the cause for appellees. With him on the brief were
Jeffrey H. Wood, Acting Assistant Attorney General, and
Eileen T. McDonough, Attorney. Mary G. Sprague, Attorney,
entered an appearance.

    Seth P. Waxman argued the cause for intervenor-appellee
North Fork Rancheria of Mono Indians. With him on the brief
were Danielle Spinelli, Christopher E. Babbitt, Jonathan A.
Bressler, John T. Byrnes, and John M. Schultz.

   Before: GARLAND, Chief Judge, TATEL, Circuit Judge, and
EDWARDS, Senior Circuit Judge.

    Opinion for the Court filed by Circuit Judge TATEL.

     TATEL, Circuit Judge: Following a nearly seven-year
administrative process, the Interior Department took a tract of
land into trust for the North Fork Rancheria of Mono Indians,
a federally recognized Indian tribe based in California, and
authorized it to operate a casino there. Several entities,
including nearby community groups and an Indian tribe with a
competing casino, challenged the Department’s decision in
United States district court, raising a host of statutory,
regulatory, and procedural challenges. In a thorough and
persuasive opinion, the district court granted summary
judgment to the Department on most claims and dismissed the
remainder. For the reasons set forth in this opinion, we affirm.
                               3
                               I.
     Facing high unemployment, inadequate public services,
and an uncertain revenue stream, the North Fork Rancheria of
Mono Indians (the “North Fork”) proposed in March 2005 to
stimulate economic development by building a large-scale
casino complex. Because the North Fork’s existing land was
ill-suited to the purpose, it asked the U.S. Department of the
Interior (the “Department”) to exercise its authority under the
Indian Reorganization Act (IRA), 25 U.S.C. § 5101 et seq., to
acquire land “for Indians,” id. § 5108, by taking a largely
undeveloped, 305-acre tract of land in Madera County into trust
for the tribe. But because a different statute—the Indian
Gaming Regulatory Act (IGRA), 25 U.S.C. § 2701 et seq.—
generally prohibits gaming on newly acquired Indian trust land,
see id. § 2719(a), the tribe also asked the Department to
determine that it qualified for a statutory exception, available
where the Department “determines [1] that a gaming
establishment on newly acquired lands would be in the best
interest of the Indian tribe and its members, and [2] would not
be detrimental to the surrounding community,” and “[3] the
Governor of the State in which the gaming activity is to be
conducted concurs in the [Department’s] determination,” id.
§ 2719(b)(1)(A). The Department made the requested
determination in September 2011, and California’s governor
concurred soon after. See U.S. Department of the Interior,
Secretarial Determination Pursuant to the Indian Gaming
Regulatory Act for the 305.49-Acre Madera Site in Madera
County, California, for the North Fork Rancheria of Mono
Indians 89 (2011) (“IGRA Decision”), Joint Appendix (J.A.)
3961; Letter from Edmund G. Brown, Jr., Governor of
California, to Kenneth L. Salazar, U.S. Secretary of the Interior
(Aug. 30, 2012), J.A. 4014–15.

   Before it could take the land into trust, however, the
Department had to ensure that the project was consistent with
                               4
the Clean Air Act, 42 U.S.C. § 7401 et seq. That Act provides
that “[n]o department, agency, or instrumentality of the Federal
Government shall engage in, support in any way or provide
financial assistance for, license or permit, or approve, any
activity which does not conform” to a state’s plan for achieving
federally mandated air quality standards. Id. § 7506(c). Prior to
making a final “conformity determination,” the agency must
provide 30-day advance notice to the public, 40 C.F.R.
§ 93.156(b), and to tribal and governmental entities specified
in Environmental Protection Agency (EPA) regulations, see id.
§ 93.155(a). EPA regulations also require that the conformity
determination be based on “the latest and most accurate
emission estimation techniques available.” Id. § 93.159(b).
Having given advance notice to the public and to most—but
not all—entities expressly entitled to receive it, the Department
in June 2011 determined that, under California’s latest
available emissions model, the casino would conform to the
state’s plan for achieving and maintaining the Clean Air Act’s
federal air quality standards.

     Based, among other things, on its findings that the
proposed casino complied with IGRA and the Clean Air Act,
the Department in November 2012 agreed to take the tract of
land into trust for the North Fork. See U.S. Department of the
Interior, Trust Acquisition of the 305.49-Acre Madera Site in
Madera County, California, for the North Fork Rancheria of
Mono Indians 1 (2012) (“Trust Decision”), J.A. 4041. Stand
Up for California!—a nonprofit organization focusing on the
community effects of gambling—along with five other casino
opponents (collectively, “Stand Up”), all appellants here, sued
the Department and the Bureau of Indian Affairs. Another
appellant, the Picayune Rancheria of the Chukchansi Indians
(the “Picayune”), which operates a casino expected to compete
with the North Fork’s, filed a similar suit. The district court
consolidated the cases and the North Fork intervened as a
                                5
defendant. See Stand Up for California! v. U.S. Department of
the Interior, 204 F. Supp. 3d 212, 234 (D.D.C. 2016).

     Stand Up and the Picayune argued that the Department’s
trust decision violated the IRA, IGRA, the Clean Air Act, and
the Administrative Procedure Act, 5 U.S.C. § 551 et seq. Most
directly, they argued that the North Fork is not an Indian tribe
for which the Department has IRA authority to acquire land.
They also argued that the acquisition rested on faulty
predicates, namely, the Department’s determinations that the
proposed casino complied with the Clean Air Act and qualified
for the IGRA exception, as well as the California governor’s
concurrence in the latter determination.

     After the district court remanded the Clean Air Act
conformity determination without vacatur so that the
Department could correct its initial failure to notify all entities
entitled to notice under EPA regulations, see Stand Up for
California!, 204 F. Supp. 3d at 236, the parties filed cross-
motions for summary judgment. The district court, Chief Judge
Howell, denied summary judgment to Stand Up and the
Picayune, dismissed Stand Up’s claims for failure to join an
indispensable party—California—insofar as those claims
challenged the California governor’s concurrence in the
Department’s IGRA determination, and granted the federal
defendants and the North Fork summary judgment on all other
relevant claims. Id. at 323.

     Stand Up and the Picayune now appeal. We review the
district court’s summary judgment rulings de novo, evaluating
the administrative record directly and invalidating the
Department’s actions only if, based on that record, they are
“arbitrary, capricious, an abuse of discretion, or otherwise not
in accordance with law.” District Hospital Partners, L.P. v.
Burwell, 786 F.3d 46, 54 (D.C. Cir. 2015) (quoting 5 U.S.C.
                                6
§ 706(2)). In doing so, we defer to the Department’s reasonable
interpretation of ambiguities in statutes it is tasked with
implementing and give “substantial deference” to the
Department’s “interpretation of its own regulations unless it is
contrary to the regulation[s’] plain language.” Confederated
Tribes of Grand Ronde Community of Oregon v. Jewell, 830
F.3d 552, 558–59 (D.C. Cir. 2016). We accept the
Department’s factual findings so long as they are supported by
substantial evidence in the record. See Center for Auto Safety
v. Federal Highway Administration, 956 F.2d 309, 313 (D.C.
Cir. 1992).

                               II.
    We begin with Stand Up’s threshold argument that the
Department lacked statutory authority to take land into trust for
the North Fork. The IRA provision pursuant to which the
Department acted, 25 U.S.C. § 5108, authorizes it to acquire
land “for Indians,” id., defined as “all persons of Indian descent
who are members of any recognized Indian tribe” that was
“under Federal jurisdiction” at the time of the IRA’s 1934
enactment, id. § 5129; see Carcieri v. Salazar, 555 U.S. 379,
395 (2009) (interpreting IRA’s “Indian” definition to include
only tribes that were under federal jurisdiction in 1934).

     Conceding that the North Fork is now a “recognized Indian
tribe,” Stand Up Br. 6, Stand Up argues that the Department
lacked substantial evidence to find, as the IRA requires, that
the North Fork was a tribe “under Federal jurisdiction” in 1934.
The Department rested that finding primarily on its earlier
decision, roughly contemporaneous with the IRA’s enactment,
to hold a special election at the North Fork’s reservation, the
North Fork Rancheria, pursuant to an IRA provision
authorizing the Department to give reservations the
opportunity to vote within a year of the IRA’s passage on
whether to accept the statute’s coverage. See 25 U.S.C. § 5125
                                7
(authorizing the Department to call special elections). Stand Up
concedes that such an election, called a section 18 election, is,
for IRA purposes, sufficient to establish federal jurisdiction
over a participating tribe. Oral Arg. at 9:33–10:48; cf.
Confederated Tribes of Grand Ronde, 830 F.3d at 563–64
(upholding IRA interpretation that finds “federal jurisdiction”
over a tribe if governmental actions in or before 1934 “reflect
federal obligations, duties, responsibility for or authority over
the tribe”). In its view, however, the record here was
insufficient to establish, broadly, that the participants in the
North Fork’s section 18 election belonged to any one tribe or,
more narrowly, that they belonged to a tribe with any
connection to today’s North Fork Indians. We consider each of
these arguments in turn.

                               A.
     The IRA authorized “reservation[s]” to hold section 18
elections within a year of its enactment. 25 U.S.C. § 5125
(emphasis added). Stand Up argues that although a section 18
election can demonstrate that the voters in such an election
resided on a single reservation falling under federal jurisdiction
in 1934, it cannot demonstrate that they belonged to a single
“Indian tribe [then] under Federal jurisdiction,” id. § 5129
(emphasis added), eligible to receive trust land today. This
argument ignores the IRA’s plain text. The statute provides that
“[t]he term ‘tribe’ wherever used in this Act shall be construed
to refer to any Indian tribe, organized band, pueblo, or the
Indians residing on one reservation.” Id. (emphasis added).
Because the North Fork Rancheria, eligible to hold a section 18
election, was a “reservation” at the IRA’s enactment, id.
§ 5125, the voters—whose Indian or resident status Stand Up
nowhere disputes—were “Indians residing on one reservation”
at that time and so, by the IRA’s own terms, constituted a
“tribe,” id. § 5129.
                                 8
     According to Stand Up, we may not now rely on the IRA’s
definition of “tribe” because the Department failed to cite it
when concluding that the North Fork was a tribe subject to
federal jurisdiction in 1934. But the Department cited the
section 18 election held “at the [North Fork’s] Reservation” as
evidence of the North Fork’s 1934 tribal status, Trust Decision
at 55, J.A. 4095, and nothing suggests that in doing so the
Department departed from the straightforward textual reading
it has given the IRA’s “tribe” definition in prior cases. See, e.g.,
United Auburn Indian Community v. Sacramento Area
Director, 24 IBIA 33, 41–42 (1993) (agency opinion citing
IRA’s “tribe” definition in finding section 18 election
established tribal existence). Although we will “not supply a
reasoned basis for [an] agency’s action that the agency itself
has not given,” we may affirm “if the agency’s path may
reasonably be discerned.” Bowman Transportation, Inc. v.
Arkansas-Best Freight System, Inc., 419 U.S. 281, 285–86
(1974). Here, that path is clear: a section 18 election on a
reservation establishes that the Indian residents qualify as a
tribe subject to federal jurisdiction.

     Undaunted, Stand Up points to Department documents
supposedly establishing that, notwithstanding the IRA’s text,
residency is distinct from tribal affiliation. Specifically, two
1934 interpretive opinions by the Department’s Solicitor
mention that certain reservation residents typically ineligible to
participate in tribal affairs could nonetheless vote in their
reservation’s section 18 election and that a tribe split over
multiple reservations could organize as a single tribe. Cf. 25
U.S.C. § 5123(a) (allowing a tribe, rather than a reservation,
“to organize for its common welfare”). Stand Up also cites a
2013 court filing in which the Department acknowledges that
some organized tribes lack a designated reservation.
                                9
     Of course, such agency statements cannot overcome the
IRA’s clear text: “the Indians residing on one reservation”
comprise a “tribe” under the Act. Id. § 5129. Besides, the
materials Stand Up cites are fully consistent with the
proposition that the residents of a single reservation constitute
a tribe under the IRA. At most, they suggest that a reservation
resident might also belong to another tribe that is not
territorially defined. Nothing suggests that Congress precluded
the possibility of holding dual tribal identities, one based on
cultural or genealogical ties and another on residency. Cf. Act
of Aug. 11, 1964, Pub. L. No. 88-419, 78 Stat. 390, 391
(clarifying that a prior statute stripping Indian status from
certain reservation residents left those affected wholly bereft of
Indian status only if they were “not members of any other tribe
or band”). As the district court aptly noted, “nothing in the text
of [the IRA] requires a tribe” within the meaning of the statute
“to be ‘single,’ ‘unified,’ or comprised of members of the same
historically cohesive or ethnographically homogenous tribe.”
Stand Up for California!, 204 F. Supp. 3d at 289. Stand Up’s
response—that yoking residency to tribal identity contravenes
tribal autonomy by artificially lumping heterogeneous
populations together as tribes—is best addressed to Congress.

     Moreover, beyond the section 18 election, other record
evidence confirms the North Fork’s longstanding tribal
existence. Specifically, in 1916, the Department used
congressionally appropriated funds to buy the North Fork
Rancheria for the tribe’s use. See Act of June 30, 1913, Pub. L.
No. 63-4, 38 Stat. 77, 86 (appropriating funds “[f]or support
and civilization of Indians in California”). Stand Up insists that
we may not consider this purchase because the Department
treated the section 18 election alone as “conclusively
establish[ing] that the [North Fork] was under Federal
jurisdiction” in 1934. Trust Decision at 55, J.A. 4095. Stand Up
misreads the Department’s decision. Although the Department
                               10
treated the election held “at the Tribe’s Reservation” as
dispositive of the government’s jurisdictional relationship with
the reservation’s residents, it presupposed that the reservation
was a “Tribe’s.” Id. The source of that presupposition becomes
clear in the decision’s very next section, where the Department
characterized the 1916 Rancheria purchase as establishing the
North Fork’s “tribal land.” Id.

     According to Stand Up, the beneficiary of the Rancheria’s
purchase was not a cohesive tribal entity, but rather a set of
diverse Indian groups occupying the geographic North Fork
region. Ample record evidence, however, including the 1916
purchase authorization itself, supports the Department’s
contrary conclusion. See Bethel-Fink Decl. exh. A, ECF
No. 33-1 at 10 (authorizing purchase of land “for the use of the
North Fork band of landless Indians”), quoted in Bureau of
Indian Affairs Decision Package, Administrative Record
NF_AR_0000776, J.A. 527; Letter from John J. Terrell,
Special Indian Agent, to Commissioner of Indian Affairs 1
(Apr. 4, 1916), J.A. 532 (referring to a member “of th[e] band”
of “the Indians of Northfork and v[i]cinity”); id. at 3, J.A. 534
(“[T]here is likely more than 200 Indians properly belonging to
the Northfork and v[i]cinity band.”). Nothing more is required.
See FPL Energy Maine Hydro LLC v. FERC, 287 F.3d 1151,
1160 (D.C. Cir. 2002) (substantial evidence standard “requires
more than a scintilla, but can be satisfied by something less
than a preponderance of the evidence”).

                                 B.
     Stand Up next argues that even if record evidence
establishes that the North Fork Rancheria’s 1934 residents
belonged to an identifiable tribe “under Federal jurisdiction,”
25 U.S.C. § 5129, the evidence is insufficient to connect the
present-day North Fork to that historic group. Our examination
of the North Fork’s history, however, demonstrates that even
                                11
though the tribe has had its ups and downs, substantial record
evidence supports the Department’s conclusion that today’s
North Fork traces its roots to the Indians who lived on the
Rancheria in 1934.

      When the Department purchased the North Fork
Rancheria in 1916, some 200 Indians lived in the vicinity. By
1933, the population had dwindled to seven, and by 1955 only
one adult Indian, Susan Johnson, lived at the Rancheria. Three
years later, in 1958, Congress passed the California Rancheria
Act, Pub. L. No. 85-671, 72 Stat. 619 (1958), which ended the
federal government’s trust relationship with forty-one
California reservations and Rancherias, among them the North
Fork Rancheria, see id. §§ 1, 9, and effectively divested certain
residents, including Ms. Johnson, of Indian status, see id.
§ 10(b). But years later, in 1983, as part of a stipulated
judgment in a case challenging the government’s termination
of its trust relationship with certain Rancherias, Hardwick v.
United States, No. C-79-1710-SW (N.D. Cal.), the government
reversed course, agreeing to “restore[] and confirm[]” Indian
status for some who had lost it under the California Rancheria
Act; to “recognize the Indian Tribes, Bands, Communities or
groups of” seventeen listed Rancherias, including the North
Fork Rancheria, “as Indian entities with the same status as they
possessed” prior to the 1958 Act; and to list those entities as
federally recognized tribes, Stipulation for Entry of Judgment,
Hardwick, No. C-79-1710-SW, ¶¶ 2–4 (Aug. 3, 1983)
(“Hardwick Stipulation”), J.A. 549–51.

     Although acknowledging that the Hardwick stipulation
restored the North Fork to its 1958 status and that it retains that
status today, Stand Up Reply Br. 11, Stand Up insists that
nothing in the record establishes that the North Fork had any
tribal status in 1958 capable of restoration through the
stipulation. This is incorrect. Substantial record evidence
                               12
supports the Department’s conclusion that the North Fork
continued to exist in 1958. Most obviously, Congress’s 1958
decision to terminate the federal trust relationship with the
North Fork via the California Rancheria Act demonstrates that
there was in fact a relationship to terminate. Stand Up believes
that the Act ended the government’s relationship with the
North Fork Rancheria, not with any tribe. But as explained
above, under the IRA, the “Indians residing on one reservation”
are a tribe. 25 U.S.C. § 5129; see also Amador County v.
Salazar, 640 F.3d 373, 375 (D.C. Cir. 2011) (describing the
California Rancheria Act as “authoriz[ing] the [Department] to
terminate the federal trust relationship with several California
tribes”). Moreover, and again as explained above, substantial
evidence supports the conclusion that the Rancheria was itself
purchased for a discernible band of North Fork Indians that
included, but was not necessarily limited to, the residents of the
land that became the Rancheria. The fact that only one adult
member of this band—Ms. Johnson—lived at the Rancheria in
1958 is as easily attributable to the fact that the Rancheria was
“poorly located and absolutely worthless as a place to build
homes on” as it is to tribal dissolution. Lipps-Michaels Survey
of Landless Nonreservation Indians of California 1919–1920,
at 50 (July 15, 1920), J.A. 4029.

     Furthermore, the Hardwick stipulation reinstated “the
Indian Tribes, Bands, Communities or groups of” seventeen
named Rancherias, including the North Fork, “as Indian
entities with the same status as they possessed” in 1958.
Hardwick Stipulation ¶ 4, J.A. 550. Stand Up reads this
bargained-for provision as a nullity with respect to the North
Fork. The Department, however, quite reasonably understood
the provision to establish that the North Fork had a 1958 status
worth restoring. Stand Up cites a Ninth Circuit decision,
Williams v. Gover, 490 F.3d 785 (9th Cir. 2007)—in which the
descendants of a terminated Rancheria’s pre-1958 members
                                13
unsuccessfully challenged the Rancheria’s post-Hardwick
decision to exclude them from full tribal membership, see id.
at 787–88—for the proposition that, as Stand Up sees it, “there
is no inevitable connection between a tribe that emerged from
the Hardwick Stipulation and those residing on a Rancheria”
prior to the California Rancheria Act, Stand Up Br. 31. This
misreads Williams. The Ninth Circuit held only that a reinstated
tribe retains “power to define membership as it chooses,” even
if in doing so the tribe elects not to privilege individual Indians’
pre-1958 tribal ties. Williams, 490 F.3d at 789–90.

     Having failed to undermine the Department’s perfectly
reasonable reliance on the Hardwick stipulation as evidence
that the North Fork existed in 1958, Stand Up grasps at isolated
bits of the record that, in its view, nonetheless compel the
opposite conclusion. It first points to a Federal Register notice
terminating Ms. Johnson’s Indian status pursuant to the
California Rancheria Act and purporting to “affect[] only
Indians who are not members of any tribe or band of Indians.”
Notice of Termination of Federal Supervision Over Property
and Individual Members, 31 Fed. Reg. 2911, 2911 (Feb. 18,
1966). According to Stand Up, the notice’s disclaimer means
that Ms. Johnson—who, as the North Fork Rancheria’s only
1958 adult Indian inhabitant, belonged to the North Fork tribe
if such a tribe existed—had no 1958 tribal affiliation. True to
form, Stand Up misreads the disclaimer. By its own terms, the
disclaimer was expressly linked to a “provision[] in [a] 1964
Act” amending the California Rancheria Act, id., and that
amendment clarified that the original 1958 Act’s provision
voiding certain residents’ Indian status was meant to apply to
only those Indians “who [were] not members of any other tribe
or band of Indians,” Act of Aug. 11, 1964, Pub. L. No. 88-419,
78 Stat. 390, 391 (emphasis added). Put simply, the Federal
Register notice indicates not that Ms. Johnson had been
unaffiliated prior to 1958, but rather that she would lose Indian
                               14
status thereafter only if she belonged to no tribe other than the
North Fork.

     Stand Up next cites a 1960 opinion by the Solicitor of the
Bureau of Indian Affairs quoting a portion of the California
Rancheria Act’s legislative history that characterizes “the
groups” occupying the Rancherias subject to the Act as “not
well defined,” Rancheria Act of August 18, 1958, Department
of the Interior, Opinions of the Solicitor 1884 (Aug. 1, 1960)
(“Solicitor Opinion”), J.A. 324, as well as a Senate Report
stating that the North Fork had “no approved membership roll”
in 1958, S. Rep. No. 85-1874, at 33 (1958), J.A. 306. Stand Up
draws the wrong conclusion from the cited legislative history.
That the Solicitor’s opinion associated the Rancherias with
groups that were “not well defined” is far less significant than
that it associated them with “groups,” thereby supporting the
Department’s conclusion that the North Fork Rancheria was
connected to an identifiable North Fork tribal entity. Solicitor
Opinion at 1884, J.A. 324. Likewise, that the North Fork failed
to keep membership records in 1958 hardly undermines the
Department’s finding that the tribe existed at that time.

     Finally, Stand Up argues that even if substantial evidence
establishes the North Fork’s 1958 existence, nothing connects
the tribe’s 1958 iteration to the voters in the North Fork
Rancheria’s 1934 section 18 election. Enough is enough! Stand
Up demands an unnecessary—indeed impossible—
genealogical exercise. Barring affirmative evidence of tribal
discontinuity between 1934 and 1958, the Department was
entitled to rely on the unremarkable assumption that a political
entity, even as its membership evolves over time, retains its
essential character.
                               15
                               III.
     Now joined by the Picayune, Stand Up contends that, even
if the Department had IRA authority to acquire trust land for
the North Fork, it could not exercise that authority in
connection with the North Fork’s proposed casino project
because the Department’s determinations that the proposal
complied with IGRA and the Clean Air Act were fatally
flawed. We disagree.

                               A.
    Although IGRA generally bars gaming on newly acquired
Indian trust land, 25 U.S.C. § 2719(a), it creates an exception
where the Department “determines that a gaming establishment
on newly acquired lands [1] would be in the best interest of the
Indian tribe and its members, and [2] would not be detrimental
to the surrounding community,” provided that “[3] the
Governor of the State in which the gaming activity is to be
conducted concurs in the [Department’s] determination,” id.
§ 2719(b)(1)(A). In this case, the Department made the
required determinations, and California’s governor concurred.

     Neither Stand Up nor the Picayune disputes that the first
of the exception’s requirements—that the proposed casino is in
the North Fork’s best interests—was satisfied here. Instead,
they challenge the Department’s finding that “[t]he proposed
Resort would not be detrimental to the surrounding
community.” IGRA Decision at 84, J.A. 3956. The Picayune
also challenges the gubernatorial concurrence as invalid under
California law.

     Although the former argument requires some discussion,
we can easily dispose of the latter, as it is twice forfeited. The
district court concluded that the Picayune, having “nowhere in
its ample briefing on summary judgment even mention[ed]” the
gubernatorial concurrence’s supposed invalidity, abandoned
                                16
any challenge to the concurrence. Stand Up for California!,
204 F. Supp. 3d at 247 n.16. The district court further ruled that
no such challenge could proceed in any event, as California
was not a party. See id. at 254. Because the Picyaune
challenged neither of these independently dispositive findings
in its opening brief, it has forfeited its opportunity to do so. See
Russell v. Harman International Industries, Inc., 773 F.3d 253,
255 n.1 (D.C. Cir. 2014) (argument not raised in opening brief
on appeal is forfeited).

     We turn, then, to the Department’s non-detriment finding.
Stand Up first attacks the Department for considering the
casino’s benefits as well as its detriments to the surrounding
community, arguing that “benefits that are not connected to and
will not mitigate [a] casino’s undisputed detrimental impacts
cannot simply cancel out those detrimental impacts.” Stand Up
Br. 37. As Stand Up sees it, IGRA’s requirement that a casino
“not be detrimental to the surrounding community,” 25 U.S.C.
§ 2719(b)(1)(A), requires that a casino have no unmitigated
negative impacts whatsoever, not that it, on balance, have a
positive or at least neutral net effect on the surrounding
community.

     The district court rejected this “cramped reading” of
IGRA, which, it found, “would result in barring any new
gaming establishments,” given that “[a]ll new commercial
developments are bound to entail some [unmitigated] costs.”
Stand Up for California!, 204 F. Supp. 3d at 262 (first
alteration in original) (quoting Stand Up for California! v. U.S.
Department of the Interior, 919 F. Supp. 2d 51, 74 (D.D.C.
2013)). We do too. Stand Up points to nothing in IGRA that
forecloses the Department, when making a non-detriment
finding, from considering a casino’s community benefits, even
if those benefits do not directly mitigate a specific cost imposed
by the casino. Indeed, Stand Up never even challenges IGRA
                              17
regulations that expressly allow the Department to consider
“[a]ny . . . information that may provide a basis for a . . .
[d]etermination whether the proposed gaming establishment
would or would not be detrimental to the surrounding
community.” 25 C.F.R. § 292.18(g) (emphasis added); see also
id. § 292.21(a) (cataloguing the information the Department is
to consider). The Department reads this regulation as
authorizing it to consider a casino’s community benefits—even
those that do not directly remediate a specific detriment—and
we defer to this perfectly reasonable reading. See Confederated
Tribes of Grand Ronde, 830 F.3d at 559 (“[W]e give substantial
deference to an agency’s interpretation of its own regulations
unless it is contrary to the regulation[s’] plain language.”).

    Finding no defect in the Department’s overall
methodology, we move on to Stand Up’s argument that the
Department’s non-detriment finding is unsupported by
substantial evidence. Stand Up offers two reasons for this
position, neither persuasive.

     Stand Up first claims that the finding rests on an
assumption that the North Fork will adopt mitigation measures
set out in an environmental impact statement the Department
prepared to comply with the National Environmental Policy
Act of 1969 (NEPA), 42 U.S.C. § 4321 et seq. This assumption
is untenable, Stand Up argues, because “NEPA imposes no
substantive requirement that mitigation measures actually be
taken.” Robertson v. Methow Valley Citizens Council, 490 U.S.
332, 353 n.16 (1989). But even if NEPA itself imposes no such
requirement, the North Fork signed memoranda of
understanding (MOUs) with local governments, in which it
agreed to undertake the contemplated measures. Stand Up
insists that the Department could not rely on the MOUs as
evidence that the North Fork would undertake mitigation
because the MOUs, by their terms, would go into effect only
                              18
after the North Fork had entered a compact with California
governing the terms of gaming at the proposed casino, and
because the Department had no guarantee that such a compact
would ever materialize. Unchallenged IGRA regulations,
however, obliged the Department to consider the MOUs. See
25 C.F.R. § 292.18(g) (application to qualify for IGRA
exception must contain information on “memoranda of
understanding . . . with affected local governments”); id.
§ 292.21(a) (Department must consider this information). And
it was reasonable for the Department to assume that the
mitigation measures spelled out in the MOUs would take effect
if necessary, even if the MOUs would not become binding
absent a tribal-state compact. In most instances, such a compact
is a statutory precondition to gaming on Indian land, see 25
U.S.C. § 2710(d)(1)(C), and, accordingly, a precondition to
any casino-related harms the MOUs sought to mitigate. And
although, absent a tribal-state compact, IGRA allows the
Department to conditionally authorize gaming under
prescribed conditions, see id. § 2710(d)(7)(B)(vii), the
Department justifiably declined to allow its predictive
judgment as to the casino’s probable effects to be governed by
the outside possibility that the North Fork would secure
authorization to operate the casino without also abiding by the
MOUs, see Rural Cellular Association v. FCC, 588 F.3d 1095,
1105 (D.C. Cir. 2009) (“The ‘arbitrary and capricious’ standard
is particularly deferential in matters implicating predictive
judgments . . . .”).

     Stand Up next argues that even with the mitigation
measures in place, the Department lacked a sufficient basis for
making a non-detriment finding because record evidence
estimated that the casino would add 531 new problem gamblers
to Madera County’s adult population. Well aware of that
consequence, the Department relied on the North Fork’s
promise to, among other things, cover the estimated $63,600
                                19
annual treatment costs attributable to new gamblers through an
annual $50,000 earmarked contribution to Madera County and
an additional catchall sum specifically calculated to cover the
remaining $13,600. According to Stand Up, this mitigation
does not address problem gamblers who never seek treatment,
and the record suggests treatment “may,” rather than “will,”
attenuate problem gambling in any event. U.S. Department of
the Interior, Bureau of Indian Affairs, Final Environmental
Impact Statement: North Fork Casino 4.7-9 (2009), J.A. 711.
Perhaps so, but Stand Up has failed to show that any residual
harms the North Fork’s mitigation efforts leave unaddressed
will be so substantial that the Department, permissibly viewing
the casino’s net effects holistically, was obliged to find that the
casino would be detrimental.

     The Picayune likewise focuses on a narrow subset of the
casino’s effects—specifically, the competitive threat to its own
gaming operations. The Department acknowledged that it
“must accord weight to [the] Picayune’s concerns,” IGRA
Decision at 86, J.A. 3958, but due to “the relative proximity of
[the] Picyaune’s lands, headquarters, and existing class III
gaming facility” to the site of the North Fork’s proposed
casino, id. at 85, J.A. 3957, it determined, pursuant to IGRA
regulations unchallenged by the Picayune, that the tribe was not
part of the “surrounding community,” 25 C.F.R. § 292.2, and
so assigned its concerns “less weight than comments submitted
by communities and tribes that f[e]ll within the definition of
‘surrounding community’ in [the] regulations,” IGRA Decision
at 85, J.A. 3957. Appropriately weighed, the Department
concluded, the proposed casino’s competitive effects on the
Picayune’s own operations were insufficient to mandate a
finding that the casino would be detrimental to the surrounding
community. See id. The Picayune raises three challenges to the
Department’s reasoning.
                                20
     First, the Picayune argues that the Department erred in
concluding that it was not part of the surrounding community.
But under IGRA regulations—again unchallenged by the
Picyaune—“[s]urrounding           community       means       local
governments and nearby Indian tribes located within a 25-mile
radius of the site of the proposed gaming establishment,” 25
C.F.R. § 292.2, and the Picayune concedes that it is located
outside the relevant 25-mile radius, Picayune Br. 12 n.1.
Insisting that it nonetheless constitutes part of the surrounding
community, the Picayune cites a portion of the IGRA
regulation that allows a “nearby Indian tribe located beyond the
25-mile radius” to “petition for consultation if it can establish
that its governmental functions, infrastructure or services will
be directly, immediately and significantly impacted by the
proposed gaming establishment.” 25 C.F.R. § 292.2.
Specifically, it contends that the Department, in finding that
“the relative proximity of [the] Picayune’s lands, headquarters,
and existing . . . gaming facility to the [proposed casino’s] Site”
counseled in favor of considering the Picayune’s concerns,
IGRA Decision at 85, J.A. 3957, necessarily concluded that its
“governmental functions, infrastructure or services will be
directly, immediately and significantly impacted by” the North
Fork’s casino, 25 C.F.R. § 292.2, and so was obliged to treat it
as part of the surrounding community.

     The Picayune has given us no basis for upsetting the
Department’s reasonable interpretation of its own regulation as
excluding from the “surrounding community” all communities
outside the 25-mile radius—even those that may otherwise
petition for consultation. See Thomas Jefferson University v.
Shalala, 512 U.S. 504, 512 (1994) (deferring to agency
interpretation of its own regulation unless the interpretation is
“plainly erroneous or inconsistent with the regulation” (quoting
Udall v. Tallman, 380 U.S. 1, 16 (1965))). This interpretation
follows readily from the regulation’s text and, contrary to the
                               21
Picayune’s argument, comports with the Department’s
characterization—in commentary contemporaneous with the
regulation’s promulgation—of the 25-mile radius as a
“rebuttable presumption.” Gaming on Trust Lands Acquired
After October 17, 1988, 73 Fed. Reg. 29,354, 29,357 (May 20,
2008). A community outside the radius may, by showing that
it will be “directly, immediately and significantly impacted by”
a casino, rebut the presumption that it is not entitled to
consultation, even while remaining outside the “surrounding
community.” 25 C.F.R. § 292.2.

     Second, the Picyaune argues that even if the Department
properly considered it to be outside the “surrounding
community,” nothing in IGRA’s regulations “hints at the
[Department] having any discretion to discount the weight”
afforded to input from any community included in the
consultation process. Picayune Br. 30. Contrary to the tribe’s
improbable assumption, however, nothing in the regulations so
much as suggests that the Department must treat differently
situated communities identically. To be sure, a casino might
have substantial effects on even far-flung communities, but
Congress was concerned only with the “surrounding
community,” 25 U.S.C. § 2719(b)(1)(A), and given Congress’s
choice to speak in geographic terms, the Department
reasonably concluded that “[t]he weight accorded to the
comments of tribes and local governments outside the
definition of ‘surrounding community’ will naturally diminish
as the distance between their jurisdictions and the proposed off-
reservation gaming site increases,” IGRA Decision at 86, J.A.
3958.

    Lastly, the Picayune claims that the Department ignored
evidence that competition from the North Fork’s proposed
casino would reduce its revenues, causing job loss and reduced
public services. Expressly acknowledging this evidence, the
                                22
Department nonetheless concluded that because the Picayune’s
casino “has proven to be a successful operation in a highly
competitive gaming market,” any “competition from the [North
Fork] Tribe’s proposed gaming facility in an overlapping
gaming market is not sufficient, in and of itself, to conclude
that it would result in a detrimental impact to [the] Picayune.”
Id. Contrary to the Picayune’s suggestion, the Department did
not discount an anticipated competitive injury merely because
“the source of the injury was competition,” Picayune Br. 34;
instead, the Department concluded that the Picayune’s casino
could successfully absorb the expected competitive effects.
Given the reduced weight the Department permissibly assigned
the Picayune’s concerns, it concluded—appropriately in our
view—that the casino’s potential effects on the tribe were
insufficient to render the casino detrimental to the surrounding
community overall.

                                B.
     Rounding out the bevy of challenges to the predicate
determinations underlying the trust decision, Stand Up attacks
the Department’s finding that the proposed casino project
conformed to California’s plan for achieving compliance with
federal air quality standards under the Clean Air Act. See 42
U.S.C. § 7410 (describing requirements for state
implementation plans for achieving air quality standards); id.
§ 7506(c) (placing “affirmative responsibility” on federal
agency heads to ensure certain projects’ conformity to the
relevant state implementation plans prior to approval). The
Department concedes that it is unable to prove that, prior to
issuing its conformity determination in June 2011, it gave prior
notice to each and every governmental and tribal entity entitled
to such notice as required by Clean Air Act regulations. See 40
C.F.R. § 93.155(a) (listing entities entitled to notice). When
this defect was first brought to the district court’s attention, it
responded by allowing a limited remand, without vacatur, so
                               23
that the Department could belatedly issue the required notice
and consider any responsive comments. See Stand Up for
California!, 204 F. Supp. 3d at 236. After taking these steps in
2014, the Department reissued its original determination
unchanged.

     Stand Up argues that the Department’s notice violation
was incapable of after-the-fact cure and so required the district
court to vacate the conformity determination. In initially
granting remand without vacatur, however, the district court
observed that the procedural flaw was minimal because the
Department had given prior public notice of its determination
in 2011, as well as specific notice targeting the entities “most
likely to have substantive comments,” and because “the much
broader Environmental Impact Statement required under the
[NEPA] was widely publicized and heavily commented upon.”
Stand Up for California! v. U.S. Department of the Interior,
No. 12-2039, 2013 WL 12203229, at *3 & n.2 (D.D.C. Dec.
16, 2013). Given the notice defect’s relative insignificance, as
well as the potentially “disruptive consequences” of rolling
back an essential predicate to the trust decision, the district
court acted well within its discretion in finding vacatur
unnecessary to address any harm the defect had caused. Sugar
Cane Growers Co-operative of Florida v. Veneman, 289 F.3d
89, 98 (D.C. Cir. 2002) (“[T]he decision whether to vacate
depends on ‘the seriousness of the order’s deficiencies . . . and
the disruptive consequences of an interim change that may
itself be changed.’” (quoting Allied-Signal, Inc. v. United
States Nuclear Regulatory Commission, 988 F.2d 146, 150–51
(D.C. Cir. 1993))); see also State of Nebraska Department of
Health & Human Services v. Department of Health & Human
Services, 435 F.3d 326, 330 (D.C. Cir. 2006) (reviewing
district court’s choice of equitable remedy for abuse of
discretion).
                              24
     Stand Up contends that “[e]ven if the district court could
properly remand without vacating the [Department’s] initial
[conformity determination], the [Department’s] actions on
remand—which treated the notice as perfunctory and simply
rubber-stamped [its] earlier decision—were inadequate to meet
the Clean Air Act’s requirements.” Stand Up Reply Br. 23. But
in ordering remand without vacatur, the district court
considered it “substantially likely” that the Department would
“reach the same conclusion and reinstitute the same action” on
remand, given that the Department had initially made the
conformity determination only after considerable participation
from multiple stakeholders. Stand Up for California!, 2013 WL
12203229, at *3. Stand Up identifies no new facts or
considerations raised on remand that required the Department
to part ways with its earlier conclusion.

     Finally, Stand Up argues that the conformity
determination, contrary to EPA regulations, was not “based on
the latest and most accurate emission estimation techniques,”
40 C.F.R. § 93.159(b), and in particular on “the most current”
available motor vehicle emissions model specified by the
agency, id. § 93.159(b)(1). When first issued in 2011, the
determination here undisputedly complied with this
requirement. But because EPA updated the relevant emissions
model for California in 2013, see Official Release of
EMFAC2011 Motor Vehicle Emission Factor Model for Use
in the State of California, 78 Fed. Reg. 14,533 (Mar. 6, 2013),
Stand Up argues that the Department, when reissuing the 2011
conformity determination on remand in 2014, should have
done its calculations in accordance with the 2013 emissions
model.

    The parties dispute whether the reissued conformity
determination falls into a regulatory safe harbor that allows
“[c]onformity analyses for which the analysis was begun [three
                              25
months after] or no more than 3 months before” announcement
of a new emissions model to rely on the prior model. 40 C.F.R.
§ 93.159(b)(1)(ii). We need not address this issue, however,
because the relevant date for compliance with the regulatory
emissions modeling requirement was 2011, when the
Department initially made its conformity determination.
Although the determination was subject to a limited remand on
an unrelated notice issue, it was never vacated. In withholding
vacatur, the district court expressly rejected Stand Up’s
“argument that the remand should require the [Department] to
perform the entire Clean Air Act conformity determination
again,” Stand Up for California!, 2013 WL 12203229, at *4,
instead viewing the remand as giving the Department an
opportunity to “remedy a minor procedural defect,” id. at *1.
As we have already concluded, the district court acted well
within its discretion in determining that the appropriate remedy
for the Department’s notice violation was a narrow remand for
a single purpose. Under such circumstances, the Department
had no obligation to rebuild the conformity determination from
the ground up. Cf. Allied-Signal, 988 F.2d at 151 (agency need
not refund fees collected under an inadequately supported rule
where district court remands without vacatur to allow agency
to “develop a reasoned explanation based on an alternative
justification”).

     To be clear, we agree with Stand Up that an agency “is
bound to enforce administrative guidelines in effect when it
takes final action.” Sierra Club v. EPA, 762 F.3d 971, 980 (9th
Cir. 2014). Here, the Department’s “final action” took place in
2011 and complied fully with the relevant regulatory
requirement. Since then, the Department has done nothing
more than ratify that final action in response to a narrow
remand order that not only declined to vacate the 2011
conformity determination, but also affirmatively found it
unnecessary for the agency to redo its prior analysis.
                              26
                              IV.
     After reviewing thousands of pages of evidence over the
span of seven years, the Interior Department took the tract of
land at issue into trust for the North Fork and approved the
tribe’s proposed casino. Viewing the same extensive record
and affording the appropriate measure of deference to the
Department’s supportable judgments, we, like the district
court, conclude that this decision was reasonable and consistent
with applicable law. We affirm.

                                                    So ordered.
