                             PUBLISHED

UNITED STATES COURT OF APPEALS
                   FOR THE FOURTH CIRCUIT


OHIO VALLEY ENVIRONMENTAL                 
COALITION; COAL RIVER MOUNTAIN
WATCH; NATURAL RESOURCES
DEFENSE COUNCIL,
                Plaintiffs-Appellees,
                    v.
WILLIAM BULEN, District Engineer,
U.S. Army Corps of Engineers,
Huntington District; ROBERT B.
FLOWERS, Lieutenant General, Chief
of Engineers and Commander of the
                                          
U.S. Army Corps of Engineers,
             Defendants-Appellants,           No. 04-2129
                   and
WEST VIRGINIA COAL ASSOCIATION;
KENTUCKY COAL ASSOCIATION; OHIO
COAL ASSOCIATION; COAL
OPERATIONS AND ASSOCIATES,
INCORPORATED; NATIONAL MINING
ASSOCIATION; GREEN VALLEY COAL
COMPANY,
             Intervenors-Defendants,
CONSOL   OF   KENTUCKY, INCORPORATED,
                     Party in Interest.
                                          
2                 OHIO VALLEY ENVIRONMENTAL v. BULEN



OHIO VALLEY ENVIRONMENTAL                 
COALITION; COAL RIVER MOUNTAIN
WATCH; NATURAL RESOURCES
DEFENSE COUNCIL,
                Plaintiffs-Appellees,
                    v.
WEST VIRGINIA COAL ASSOCIATION;
KENTUCKY COAL ASSOCIATION; OHIO
COAL ASSOCIATION; COAL
OPERATIONS AND ASSOCIATES,
INCORPORATED; NATIONAL MINING
ASSOCIATION,
             Intervenors-Defendants-
                         Appellants,           No. 04-2137
                   and
WILLIAM BULEN, District Engineer,
U.S. Army Corps of Engineers,
Huntington District; ROBERT B.
FLOWERS, Lieutenant General, Chief
of Engineers and Commander of the
U.S. Army Corps of Engineers,
                        Defendants,
GREEN VALLEY COAL COMPANY,
             Intervenor-Defendant,
CONSOL   OF   KENTUCKY, INCORPORATED,
                     Party in Interest.
                                          
                  OHIO VALLEY ENVIRONMENTAL v. BULEN                3



OHIO VALLEY ENVIRONMENTAL                 
COALITION; COAL RIVER MOUNTAIN
WATCH; NATURAL RESOURCES
DEFENSE COUNCIL,
                Plaintiffs-Appellees,
                    v.
GREEN VALLEY COAL COMPANY,
            Intervenor-Defendant-
                       Appellant,
                   and
WILLIAM BULEN, District Engineer,
U.S. Army Corps of Engineers,
Huntington District; ROBERT B.                       No. 04-2402
FLOWERS, Lieutenant General, Chief
of Engineers and Commander of the
U.S. Army Corps of Engineers,
                        Defendants,
CONSOL   OF   KENTUCKY, INCORPORATED,
                     Party in Interest,
WEST VIRGINIA COAL ASSOCIATION;
KENTUCKY COAL ASSOCIATION; OHIO
COAL ASSOCIATION; COAL
OPERATIONS AND ASSOCIATES,
INCORPORATED; NATIONAL MINING
ASSOCIATION,
             Intervenors-Defendants.
                                          
           Appeals from the United States District Court
     for the Southern District of West Virginia, at Huntington.
              Joseph Robert Goodwin, District Judge.
                          (CA-03-2281-3)

                         Argued: September 19, 2005

                         Decided: November 23, 2005
4              OHIO VALLEY ENVIRONMENTAL v. BULEN
      Before NIEMEYER and LUTTIG, Circuit Judges, and
    Robert J. CONRAD, Jr., United States District Judge for the
     Western District of North Carolina, sitting by designation.



Affirmed in part, vacated in part, and remanded by published opinion.
Judge Luttig wrote the opinion, in which Judge Niemeyer and Judge
Conrad joined.


                            COUNSEL

ARGUED: Elizabeth Ann Kessler, UNITED STATES DEPART-
MENT OF JUSTICE, Environment & Natural Resources Division,
Washington, D.C.; Michael Randolph Shebelskie, HUNTON & WIL-
LIAMS, Richmond, Virginia, for Appellants. James Millard Hecker,
TRIAL LAWYERS FOR PUBLIC JUSTICE, Washington, D.C., for
Appellees. ON BRIEF: Earl H. Stockdale, Chief Counsel, Lance D.
Wood, Russell W. Petit, U.S. ARMY CORPS OF ENGINEERS,
Washington, D.C.; Ann R. Klee, General Counsel, UNITED STATES
ENVIRONMENTAL PROTECTION AGENCY, Washington, D.C.;
Sue Ellen Wooldridge, Solicitor, UNITED STATES DEPARTMENT
OF THE INTERIOR, Washington, D.C.; Thomas L. Sansonetti,
Assistant Attorney General, Jeffrey Bossert Clark, Deputy Assistant
Attorney General, Mary B. Neumayr, Timothy Racicot, John A. Bry-
son, Steve Rusak, John T. Stahr, UNITED STATES DEPARTMENT
OF JUSTICE, Environment & Natural Resources Division, Washing-
ton, D.C., for Appellants William Bulen, District Engineer, U.S.
Army Corps of Engineers, Huntington District, and Robert B. Flow-
ers, Lieutenant General, Chief of Engineers and Commander of the
U.S. Army Corps of Engineers. Robert G. McLusky, Blair M. Gard-
ner, Lindsey K. Griffith, JACKSON KELLY, P.L.L.C., Charleston,
West Virginia; William H. Wright, Jr., HUNTON & WILLIAMS,
L.L.P., Richmond, Virginia; Harold P. Quinn, Jr., NATIONAL MIN-
ING ASSOCIATION, Washington, D.C., for Appellants West Vir-
ginia Coal Association, Kentucky Coal Association, Ohio Coal
Association, Coal Operations and Associates, Incorporated, National
Mining Association, Green Valley Coal Company. Joseph M. Lovett,
                OHIO VALLEY ENVIRONMENTAL v. BULEN                     5
APPALACHIAN CENTER FOR THE ECONOMY AND THE
ENVIRONMENT, Lewisburg, West Virginia, for Appellees.


                              OPINION

LUTTIG, Circuit Judge:

   This case presents the question whether the United States Army
Corps of Engineers ("the Corps") exceeded its authority under the
Clean Water Act ("CWA") when it promulgated Nationwide Permit
21 ("NWP 21"), a general permit for the discharge of dredged or fill
material into the waters of the United States that allows projects to
proceed only after receiving individualized authorization from the
Corps. We conclude that the Corps complied with the CWA when it
promulgated NWP 21. The contrary judgment of the district court is
therefore vacated.

                                   I.

    The Clean Water Act prohibits the discharge of any "pollutant" into
the waters of the United States without a permit. See 33 U.S.C.
§ 1311(a). The Army Corps of Engineers has authority under the
CWA to issue two types of permits for the discharge of dredged or
fill material: individual permits and general permits. The Corps issues
individual permits under section 404(a) on a case-by-case basis for
discharges at "specified disposal sites," after providing notice and
opportunity for public hearing. Id. § 1344(a). The Corps issues gen-
eral permits, which authorize "categories of activities" rather than
individual projects, under section 404(e). That section provides, in
relevant part, that:

    the [Corps] may, after notice and opportunity for public
    hearing, issue general permits on a State, regional, or nation-
    wide basis for any category of activities involving dis-
    charges of dredged or fill material if the [Corps] determines
    that the activities in such category are similar in nature, will
    cause only minimal adverse environmental effects when
    performed separately, and will have only minimal cumula-
6               OHIO VALLEY ENVIRONMENTAL v. BULEN
    tive adverse effect on the environment. Any general permit
    issued under this subsection shall . . . set forth the require-
    ments and standards which shall apply to any activity autho-
    rized by such general permit.

Id. § 1344(e)(1).

   Pursuant to section 404(e), the Corps has promulgated a number of
general permits, all but one of which authorize projects that comply
with the permits’ terms to proceed without prior approval by the
Corps. The exception, NWP 21 — which authorizes discharges of
dredged or fill material associated with surface coal mining and recla-
mation projects — requires that projects be individually authorized by
the Corps. NWP 21 authorizes:

    [d]ischarges of dredged or fill material into waters of the US
    associated with surface coal mining and reclamation opera-
    tions provided the coal mining activities are authorized by
    the DOI, Office of Surface Mining (OSM), or by states with
    approved programs under Title V of the Surface Mining
    Control and Reclamation Act of 1977 and provided the per-
    mittee notifies the District Engineer in accordance with the
    "Notification" General Condition. In addition, to be autho-
    rized by this NWP, the District Engineer must determine
    that the activity complies with the terms and conditions of
    the NWP and that the adverse environmental effects are
    minimal both individually and cumulatively and must notify
    the project sponsor of this determination in writing.

Issuance of Nationwide Permits, 67 Fed. Reg. 2020, 2081 (Jan. 15,
2002).

   In this litigation, plaintiffs, a coalition of environmental groups,
have raised various challenges to NWP 21. The district court did not
reach most of those challenges, holding simply that NWP 21 is
facially invalid under Chevron U.S.A. Inc. v. Natural Resources
Defense Council, Inc., 467 U.S. 837 (1984), because it conflicts with
the unambiguous meaning of section 404(e). J.A. 1070. The district
court accordingly suspended existing authorizations under NWP 21
and enjoined the Corps from issuing further NWP 21 authorizations
                OHIO VALLEY ENVIRONMENTAL v. BULEN                    7
in the Southern District of West Virginia. Id. at 1079. This appeal fol-
lowed.

                                  II.

   The district court concluded that NWP 21 conflicts with the unam-
biguous meaning of section 404(e) for essentially four reasons. First,
it concluded that NWP 21 "defines a procedure instead of permitting
a category of activities." J.A. 1070. Second, it concluded that section
404(e) "unambiguously requires determination of minimal impact
before, not after, the issuance of a nationwide permit," and that, in
violation of this requirement, "NWP 21 provides for a post hoc, case-
by-case evaluation of environmental impact." Id. Third, it concluded
that section 404(e) unambiguously requires that general permits
authorize discharges to proceed without further involvement from the
Corps, and NWP 21 violates this requirement because it authorizes
projects to proceed only after receiving individualized approval from
the Corps. Id. Finally, it concluded that NWP 21 violates the statutory
requirement that the Corps provide notice and opportunity for public
hearing before issuing a permit. Id. None of these conclusions with-
stands scrutiny.

   At the outset, we note that, while our review of the district court’s
construction of section 404 is de novo, our review of the Corps’ con-
struction of section 404 is governed by the Supreme Court’s decision
in Chevron. Under Chevron, if the requirements of section 404 are
unambiguous, "that is the end of the matter; for the court, as well as
the agency, must give effect to the unambiguously expressed intent
of Congress." Chevron, 467 U.S. at 842-43. However, if section 404
is ambiguous or silent with respect to the questions at issue, we "must
defer, under Chevron, to [the Corps’ interpretation of its governing
statute], so long as that interpretation is permissible in light of the
statutory text and reasonable." Asika v. Ashcroft, 362 F.3d 264, 267
(4th Cir. 2004) (per curiam).

                                  A.

  The district court first concluded that NWP 21 fails to comply with
section 404(e) because it "defines a procedure instead of permitting
a category of activities." J.A. 1070. We disagree. NWP 21 plainly
8               OHIO VALLEY ENVIRONMENTAL v. BULEN
authorizes a "category of activities." The category of activities autho-
rized by NWP 21 consists of those discharges of dredged or fill mate-
rial that (1) are associated with surface coal mining and reclamation
operations, so long as those operations are authorized by the Depart-
ment of Interior or by states with approved programs under the Sur-
face Mining Control and Reclamation Act of 1977, (2) are preceded
by notice to the Corps, and (3) are approved by the Corps after the
Corps concludes that the activity complies with the terms of NWP 21
and that its adverse environmental effects are minimal both individu-
ally and cumulatively. See 67 Fed. Reg. at 2081.

   The district court erroneously reasoned that NWP 21 does not
authorize a "category of activities" because it is defined by procedural
requirements "rather than objective requirements or standards." J.A.
1072 ("NWP 21 imposes no limit on the number of linear feet of a
stream, for example, that might be impacted by a valley fill or surface
impoundment. It does not limit the total acreage of a watershed that
might be impacted."). As an initial matter, we note that, by virtue of
its incorporation of the requirements of the Surface Mining Control
and Reclamation Act ("SMCRA"), NWP 21 does contain substantive
requirements.1 More importantly, nothing in section 404(e) or in logic
prohibits, much less unambiguously prohibits, the use of procedural,
in addition to substantive, parameters to define a "category." The dis-
trict court therefore erred when it concluded that NWP 21 does not
define a "category of activities."2
    1
     SMCRA imposes a host of "performance standards" on "all surface
coal mining and reclamation operations." 30 U.S.C. § 1265(b). For
example, under SMCRA, all surface coal mining operations must "mini-
mize the disturbances to the prevailing hydrologic balance at the mine-
site and in associated offsite areas and to the quality and quantity of
water in surface and ground water systems both during and after surface
coal mining operations and during reclamation." Id. § 1265(b)(10).
   2
     For the same reasons, the district court erred when it held that NWP
21 violates section 404(e) because it does not set forth the "requirements
and standards" that apply to the activities it authorizes. J.A. 1072. NWP
21 plainly sets forth both substantive and procedural requirements and
standards that apply to the activities it authorizes. And nothing in the
statute or in the plain meaning of the words dictates that "requirements"
and "standards" must be substantive rather than procedural.
                OHIO VALLEY ENVIRONMENTAL v. BULEN                      9
                                   B.

   The district court next concluded that NWP 21 violates the unam-
biguous terms of section 404(e) because it allows the Corps to defer
the statutorily-required minimal-environmental-impact determinations
until after issuance of the nationwide permit. J.A. 1070 ("The statute
unambiguously requires determination of minimal impact before, not
after, the issuance of a nationwide permit.").3 Section 404(e) allows
the Corps to issue a general permit only "if [it] determines . . . that
the activities in [the subject] category . . . will cause only minimal
adverse environmental effects when performed separately, and will
have only minimal cumulative adverse effect on the environment." 33
U.S.C. § 1344(e). The district court concluded that the Corps did not
make the required minimal-impact determinations before issuing
NWP 21, but instead opted to make those determinations on a case-
by-case basis after issuance of the permit. J.A. 1072-73.

   It is clear from the record before us that the Corps did make the
required minimal-impact determinations before it issued NWP 21. See
id. at 512 (stating that the Corps "determined that NWP 21 . . . will
authorize only those activities with minimal adverse effects on the
aquatic environment, individually or cumulatively"). The Decision
Document for NWP 21 and the supplement to that document, set forth
at pages 469-512 of the Joint Appendix, contain the Corps’ pre-
issuance analysis of the anticipated environmental impact of the activ-
ities authorized by NWP 21. Id. at 462 ("This document contains a
general assessment of the foreseeable effects of the individual activi-
ties authorized by this NWP, the anticipated cumulative effects of
  3
   The Corps argues that section 404(e) does not unambiguously require
that the minimal-impact determinations be made before issuance of a
nationwide permit. The Corps believes that the statute allows it to issue
a nationwide permit so long as it makes the minimal-impact determina-
tions before the permit is actually used to authorize discharges, even if
after issuance of the permit. At the very least, the Corps argues, section
404(e)’s minimal impact determination requirement is temporally ambig-
uous, and the Corps’ reading is a permissible construction entitled to
Chevron deference. Because we conclude that the Corps made the
required minimal-impact determinations before issuing NWP 21, we do
not reach these contentions.
10              OHIO VALLEY ENVIRONMENTAL v. BULEN
those activities, and the potential future losses of waters of the United
States that are estimated to occur until the expiration date of the
NWP."). The Corps’ impact analysis took account of a variety of fac-
tors, including public commentators’ opinions, id. at 464-65 (stating
that the Corps "uses substantive public comments on proposed NWPs
to assess the expected impacts" and considering several suggestions
made by public commentators); NWP 21’s incorporation of
SMCRA’s requirements, id. at 470 (stating that the review required
under SMCRA will ensure that the permit will not authorize activities
that affect historic properties), 476 (stating that the review required
under SMCRA will ensure that the permit will not authorize activities
that will jeopardize any endangered species); the nature of the coal-
mining activities authorized by NWP 21, id. at 475 ("The activities
authorized by this NWP are unlikely to adversely affect salinity gradi-
ents, since the NWP authorizes surface coal mining activities that are
typically located in inland areas."), 482 ("The nature and scope of the
work authorized by the NWP will most likely restrict the extent of the
beneficial and detrimental effects to the area immediately surrounding
the surface coal mining and reclamation activity."); the applicability
of a variety of General Conditions to NWP 21, id. at 483 ("No toxic
discharges will be authorized by this NWP. General Condition 18
specifically states that the material must be free from toxic pollutants
in toxic amounts."), 484 ("General Condition 19 requires permittees
to avoid and minimize discharges of dredged or fill material into
waters of the United States to the maximum extent practicable on the
project site."); and data about usage of previous versions of NWP 21,
id. at 481 (estimating, based on usage of previous versions of NWP
21, that, over the five-year duration of this issuance of NWP 21, "ap-
proximately 1,625 activities could be authorized . . . resulting in
impacts to approximately 875 acres of waters of the United States"),
511 ("From February 11, 1997 to February 11, 2002, approximately
539 individual activities were authorized under approximately 156
NWP verifications, impacting approximately 42.58 acres of riverine
and palustrine waters within West Virginia. Based on the types of
activities authorized within West Virginia during previous years, the
Districts anticipate similar use of this NWP in the future. NWP 21
generally does not cause the loss of wetlands in the state of West Vir-
ginia."). Based on these considerations and others, the Corps con-
cluded that the activities authorized by NWP 21 "will not result in
significant degradation of the aquatic environment." Id. at 485. This
                 OHIO VALLEY ENVIRONMENTAL v. BULEN                     11
determination was sufficient to meet the requirements of section
404(e).

   The district court held that the Corps did not satisfy section 404(e)
because it did not provide an ex ante guarantee that the activities
authorized by NWP 21 would have only a minimal impact. The dis-
trict court reasoned that, under section 404(e), "[t]he issuance of a
nationwide permit . . . functions as a guarantee ab initio that every
instance of the permitted activity will meet the minimal impact stan-
dard," and that, by permitting the Corps to engage in "post hoc, case-
by-case evaluation of environmental impact," NWP 21 "runs afoul of
the statutory requirement of initial certainty." Id. at 1070.

   The district court erred. It is simply not the case that issuance of
a general permit functions as a guarantee ab initio that every instance
of the permitted activity will have only a minimal impact. Neither the
phrase "guarantee ab initio" nor the phrase "initial certainty" appears
in section 404(e). And neither that section nor any other provision of
the CWA specifies how the Corps must make the minimal-impact
determinations, the degree of certainty that must undergird them, or
the extent to which the Corps may rely on post-issuance procedures
in making them.

   For two reasons, we do not believe that an interpretation of section
404(e) that would require initial certainty is tenable. First, section
404(e)(2) gives the Corps authority to revoke or modify a general per-
mit if, after issuing the permit, it "determines that the activities autho-
rized by such general permit have an adverse impact on the
environment." 33 U.S.C. § 1344(e)(2). This provision demonstrates
that Congress anticipated that the Corps would make its initial
minimal-impact determinations under conditions of uncertainty and
that those determinations would therefore sometimes be inaccurate,
resulting in general permits that authorize activities with more-than-
minimal impacts. It also demonstrates that Congress expected that the
Corps would engage in post-issuance policing of the activities autho-
rized by general permits in order to ensure that their environmental
impacts are minimal.4
  4
   We recognize that one possible inference to draw from section
404(e)(2) is that, if the Corps discovers that any of the activities autho-
12               OHIO VALLEY ENVIRONMENTAL v. BULEN
   Second, it is impossible for the Corps’ ex ante determinations of
minimal impact to be anything more than reasoned predictions. Even
under the paradigmatic general permit envisioned by the district court,
where the parameters of the authorized activities are delineated in
objective, measurable terms, the Corps’ minimal-impact determina-
tions would necessarily be a forecast only. This is so because the
environmental impact of the activities authorized by a general permit
depends on factors that, as a practical matter, are outside the Corps’
ability to predict with certainty ex ante. This uncertainty is especially
acute when the Corps issues a nationwide permit like NWP 21
because the Corps must attempt to forecast the environmental effects
the authorized activities could have if undertaken anywhere in the
country under any set of circumstances. As the intervenors contend,
"[i]t is impossible to conceive of a class of discharges that . . . ‘invari-
ably’ could have no adverse environmental effects, individually or
cumulatively. Any individual fill, depending on its location, might
have more than a minimal adverse effect. It could, for instance, be
located at a site that is a critical habitat for an endangered species.
Moreover, any class of fills could have more than a minimal adverse
effect on a cumulative basis, depending on the number and location
of the fills." Intervenors’ Br. at 52 (citation omitted).

   Given section 404(e)(2)’s recognition of the possibility that activi-
ties authorized by a general permit could result in a more-than-
minimal impact, as well as the impossibility of making an ex ante
guarantee that the authorized activities could never result in a more-
than-minimal impact, we cannot agree with the district court’s conclu-

rized by a general permit has a more-than-minimal adverse impact, the
Corps’ only option is to revoke or modify the entire permit. However,
section 404(e)(2) does not unambiguously require that result. In fact, that
provision is most naturally read as addressing the Corps’ options when
it concludes that the activities authorized by a general permit have sys-
tematic adverse impacts. It does not necessarily address instances in
which the Corps concludes that a particular project authorized by the
terms of a general permit will have a more-than-minimal impact. As
explained below, we conclude that the Corps’ interpretation — that it
may use more tailored means to prevent adverse impacts on a project-by-
project basis — is a permissible construction of the statute and is reason-
able.
                 OHIO VALLEY ENVIRONMENTAL v. BULEN                      13
sion that section 404(e) allows the Corps to issue general permits only
for those activities that "will invariably have only minimal effects on
the environment." J.A. 1070.

   Nor can we agree with the district court’s implicit conclusion that
the Corps may not rely on the availability of post-issuance proce-
dures, such as NWP 21’s requirement of post-issuance individualized
authorization, when it makes its pre-issuance minimal-impact deter-
minations. See id. at 1072-73. The statute is silent on the question
whether the Corps may make its pre-issuance minimal impact deter-
minations by relying in part on the fact that its post-issuance proce-
dures will ensure that the authorized projects will have only minimal
impacts. We must therefore defer to the Corps’ conclusion that it may
do so if that conclusion is permissible in light of the statutory lan-
guage and is reasonable. It is both. Again, nothing in the statute speci-
fies how the Corps must make the minimal-impact determinations.
And, given the inevitable ex ante uncertainty the Corps confronts
when issuing a nationwide permit, its reliance on post-issuance proce-
dures is a reasonable, if not the only possible, way for it to cement
its determination that the projects it has authorized will have only
minimal environmental impacts.5
  5
    NWP 21 is not unique in its reliance on post-issuance procedures to
ensure minimal impacts. Under General Condition 13, for example, most
nationwide permits (24 of the 43) are subject to a pre-construction notifi-
cation ("PCN") requirement. See 67 Fed. Reg. at 2090-92. Pursuant to
this requirement, prospective permittees must notify the Corps of the
projects they intend to undertake under the general permit and their
anticipated environmental effects. Id. at 2090. A project cannot proceed
until either the Corps notifies the prospective permittee that it may pro-
ceed or forty-five days have passed since submission of the PCN without
receipt of notice from the Corps. Id. In reviewing a PCN, the Corps must
"determine whether the activity . . . will result in more than minimal indi-
vidual or cumulative adverse environmental effects." Id. at 2091. If so,
the Corps must either require that the project undergo mitigation or mod-
ifications to "reduce the adverse effects on the aquatic environment to the
minimal level," or notify the applicant that "the project does not qualify
for authorization under the NWP" and require that it receive an individ-
ual permit under section 404(a). Id. at 2092.
 This procedure is materially indistinguishable from that required by
NWP 21. In both cases, the Corps engages in post-issuance review of
14              OHIO VALLEY ENVIRONMENTAL v. BULEN
   In concluding that section 404(e) permits the Corps to rely in part
on post-issuance procedures to make its pre-issuance minimal-impact
determinations, we do not suggest that section 404(e) permits the
Corps completely to defer the minimal-impact determinations until
after issuance of the permit. We would have substantial doubts about
the Corps’ ability to issue a nationwide permit that relied solely on
post-issuance, case-by-case determinations of minimal impact, with
no general pre-issuance determinations. In such a case, the Corps’
"determinations" would consist of little more than its own promise to
obey the law.

   Apparently, the district court believed that such was the case here.
See J.A. 1073 ("The ‘category’ of activities authorized by NWP 21
. . . is nothing more than the collection of activities that the Corps
determines, during reviews that take place long after the issuance of
NWP 21, will have minimal effects."). It is true that the Corps
acknowledged in its decision document that the PCN and prior-
authorization requirements would operate to ensure that NWP 21’s
adverse effects on the environment would be minimal. See, e.g., id.
at 466 (noting that NWP 21 requires "case-by-case review of all activ-
ities, to ensure that the NWP authorizes only those surface coal min-
ing activities that have minimal adverse effects on the aquatic
environment"), 478-79 (stating that, with respect to "special aquatic
sites," including sanctuaries and refuges, wetlands, mud flats, vege-
tated shallows, coral reefs, and riffle and pool complexes, minimal
impact will be ensured through individualized review pursuant to the
PCN requirement), 480 ("All activities authorized by this NWP
require notification to the [Corps], which will allow review of each
activity to ensure that adverse effects to economically important fish
and shellfish are minimal.").

individual projects to ensure that they have only minimal impacts. The
only difference is the default rule. Under the standard PCN requirement,
projects are authorized unless the Corps prohibits them. Under NWP 21,
projects are prohibited unless the Corps authorizes them. This strikes us
as a difference immaterial to the question we address herein. If permits
that rely on the PCN requirement to ensure minimal impacts are permis-
sible — and not even appellees contend that they are not — then so is
NWP 21.
                 OHIO VALLEY ENVIRONMENTAL v. BULEN                      15
   However, we are satisfied, based on our review of the Corps’ deci-
sion document, that the Corps did actually make, in advance, the
minimal-impact determinations required by the statute. It made those
determinations after undertaking a good-faith, comprehensive, pre-
issuance review of the anticipated environmental effects of the activi-
ties authorized by NWP 21, and its partial reliance on post-issuance
procedures to ensure minimal impacts did not make those determina-
tions any less valid. As explained above, section 404(e) does not
unambiguously forbid the Corps from making the minimal-
environmental-impact determinations by relying in part on the avail-
ability of post-issuance procedures, and such reliance is a reasonable
way for the Corps to ensure that the projects it authorizes under gen-
eral permits will have only minimal impacts. We therefore conclude
that the Corps’ minimal-impact determinations, made prior to the
issuance of NWP 21, complied with section 404(e).6

                                    C.

   The district court’s third basis for invalidating NWP 21 was its
conclusion that section 404 unambiguously prohibits the Corps from
creating a general permit that authorizes activities to proceed only
after receiving individualized approval from the Corps. See J.A. 1070.
The district court based its conclusion on the structure of section 404
— which contains separate provisions for individual permits (section
404(a)) and general permits (section 404(e)) — and on its belief that
"Congress intended for a potential discharger whose project fits into
one of [the categories of activities authorized by a general permit] to
begin discharging with no further involvement from the Corps, no
uncertainty, and no red tape." Id. at 1070, 1073-76. The district court
thus held that NWP 21, which allows projects to proceed only after
receiving post-issuance individualized authorization from the Corps,
violates section 404.
  6
    It is of course open to the plaintiffs on remand to reassert their argu-
ment that the Corps’ minimal-impact determination was arbitrary and
capricious because the Corps relied on erroneous premises or ignored rel-
evant data (and we note that this argument concedes that there was a
determination). We express no view on that matter. Our holding today
is simply that the Corps did in fact make the determinations required by
section 404(e).
16              OHIO VALLEY ENVIRONMENTAL v. BULEN
   Contrary to the district court’s conclusion, nothing in section 404
prohibits the Corps from issuing a general permit that contains a
requirement of post-issuance individualized consideration or authori-
zation by the Corps. Neither section 404(e) nor any other provision
of the CWA defines the term "general permit," so there is no explicit
textual basis for such a conclusion. Moreover, the structure of the
statute is not, as the district court believed, dispositive on this score.
One cannot conclude that, simply because section 404 has separate
provisions for individual and general permits, it unambiguously (or
even arguably) forbids the creation of a general permit that includes
a requirement of individualized consideration or approval. And even
if section 404(e) was intended to create a streamlined process for
authorizing insignificant discharges, NWP 21 is not inconsistent with
that intent. The process for obtaining authorization under a general
permit — even one requiring individualized review by the Corps —
is significantly more expeditious than the process for obtaining an
individual permit under section 404(a). See 67 Fed. Reg. at 2024 (stat-
ing that "[t]he average time to verify a NWP activity is 19 days" and
that "the NWP process is faster than the standard permit process"
because "[a]n individual activity authorized by an NWP does not
require a public notice or the same level of review required for a stan-
dard permit activity").

   Given that section 404 does not speak to this issue, the question is
whether the Corps’ interpretation — that general permits may contain
requirements of individualized review or approval — is a reasonable
one. We conclude that it is. As explained above, the Corps’ post-
issuance, case-by-case policing of the activities authorized by general
permits is a reasonable, if not the only possible, means of ensuring
that general permits are used to authorize only activities with minimal
environmental effects. The Corps’ construction is therefore entitled to
Chevron deference.

                                   D.

   The district court’s final basis for invalidating NWP 21 was its
conclusion that NWP 21 impermissibly allows the Corps to issue indi-
vidual authorizations without providing notice and an opportunity for
public hearing. See J.A. 1073-74. This, the district court reasoned, is
inconsistent with the fact that section 404(a) requires the Corps to
                 OHIO VALLEY ENVIRONMENTAL v. BULEN                      17
provide notice and an opportunity for public hearing before issuing an
individual permit, and it "eliminates public involvement in decision-
making at a stage where meaningful input in the minimal impact
determination is possible." Id. at 1074. According to the district court,
Congress intended "that the Corps consider the public’s concerns
before making decisions about whether individual projects or catego-
ries of activities will have only minimal adverse effects on the envi-
ronment." Id.

   The fatal flaw in the district court’s analysis is that the Corps did
provide notice and opportunity for pubic hearing before making the
determination that the category of activities authorized by NWP 21
would have only minimal adverse effects on the environment. See
Proposal to Reissue and Modify Nationwide Permits, 66 Fed. Reg.
42,070, 42,076 (Aug. 9, 2001). Moreover, when the Corps made its
minimal-impact determinations, it expressly addressed several of the
concerns raised by public commentators. See J.A. 464-65. To the
extent that the district court’s conclusion was based on the belief that
section 404 requires notice and a hearing before the Corps authorizes
an individual project under a general permit, it erred. There is no stat-
utory requirement that notice and opportunity for public hearing be
provided before individual projects can proceed under a general per-
mit, and one cannot infer such a requirement from the fact that indi-
vidual permits can issue only after notice and opportunity for public
hearing. Section 404(e)’s only requirement is that there be notice and
opportunity for public hearing before the general permit itself issues,
and that requirement was clearly satisfied here.7

                                    III.

  One issue remains for our consideration in this appeal.8 Appellants
  7
    We also note that the public is in fact entitled to notice and an oppor-
tunity to comment on the individual activities authorized by NWP 21.
This is because NWP 21 incorporates SMCRA, and SMCRA authorizes
activities only after public notice and comment. See 30 U.S.C. § 1263.
  8
    Appellees have raised numerous other challenges to the validity of
NWP 21 which, although raised below, were not passed on by the district
court and have not been adequately briefed by the parties. We leave
those challenges to be adjudicated in the first instance by the district
court on remand.
18              OHIO VALLEY ENVIRONMENTAL v. BULEN
argue that the district court abused its discretion in refusing to join
holders of NWP 21 authorizations as necessary parties to this case
under the Federal Rules of Civil Procedure. Rule 19(a) provides that
a person shall be joined if, among other things, "the person claims an
interest relating to the subject of the action and is so situated that the
disposition of the action in the persons’s absence may as a practical
matter impair or impede the persons’s ability to protect that interest."
Fed. R. Civ. P. 19(a). Appellants argue that holders of NWP 21 autho-
rizations have an interest in the subject matter of this case — prevent-
ing their authorizations from being invalidated — and that their
absence will impede them from protecting that interest because they
will be unable to present evidence about the extent of each of their
investment and reliance interests in their authorizations, evidence
appellants claim is relevant to the equitable considerations the district
court must take into account in crafting the scope of injunctive relief.

   The district court held that joinder was not required because the
parties are capable of representing the interests of the authorization
holders. J.A. 1068. A litigant may serve as a proxy for an absent party
if the interests of the two are identical. See Nat’l Union Fire Ins. Co.
v. Rite Aid of South Carolina, Inc., 210 F.3d 246, 250-51 (4th Cir.
2000). Here, the district court concluded that the interests of the cur-
rent parties are identical to the interests of the authorization holders
because the current parties include coal associations who are arguing
on behalf of their members, including members with existing opera-
tions dependent on NWP 21. J.A. at 1069. In other words, the district
court concluded that the interests of the current parties are identical
to the interests of the authorization holders — and that the former
would therefore adequately represent the interests of the latter —
because both seek to protect investment and reliance interests that
would be upset by invalidation of NWP 21. The district court did not
abuse its discretion in so concluding and in refusing to join the autho-
rization holders as necessary parties.

                            CONCLUSION

   In sum, we conclude that the Corps complied with section 404(e)
when it issued NWP 21. The Corps identified a category of activities,
it determined that those activities would have a minimal environmen-
tal impact both separately and cumulatively, and it provided notice
               OHIO VALLEY ENVIRONMENTAL v. BULEN                  19
and opportunity for public hearing before issuing the permit. The
Corps’ issuance of NWP 21 thus fell within its authority under section
404(e). The contrary judgment of the district court and the injunction
against NWP 21 authorizations are vacated, and the case is remanded
for further proceedings not inconsistent with this opinion.

                        AFFIRMED IN PART, VACATED IN PART,
                                            AND REMANDED
