                 FOR PUBLICATION
 UNITED STATES COURT OF APPEALS
      FOR THE NINTH CIRCUIT

HELLS CANYON PRESERVATION             
COUNCIL, an Oregon non-profit
corporation,
                                           No. 03-35579
               Plaintiff-Appellant,
               v.                           D.C. No.
                                          CV-02-01138-FR
UNITED STATES FOREST SERVICE, an
                                            OPINION
agency of the United States
Department of Agriculture,
              Defendant-Appellee.
                                      
       Appeal from the United States District Court
                for the District of Oregon
         Helen J. Frye, District Judge, Presiding

                 Argued and Submitted
            March 10, 2005—Portland, Oregon

                    Filed April 5, 2005

     Before: Stephen Reinhardt, Marsha S. Berzon, and
               Jay S. Bybee, Circuit Judges.

                 Opinion by Judge Berzon




                           3995
3998        HELLS CANYON v. U.S. FOREST SERVICE


                        COUNSEL

William H. Sherlock, Hutchinson, Cox, Coons & DuPriest,
P.C., Eugene, Oregon, and Brett Brownscombe, Hells Canyon
Preservation Council, La Grande, Oregon, for the plaintiff-
appellant.

Michael W. Mosman, United States Attorney, and Jeffrey K.
Handy, Assistant United States Attorney, District of Oregon,
Portland, Oregon, Thomas L. Sansonetti, Assistant Attorney
General, and James Kilborn, Todd S. Aagard, Clay Samford,
and Katherine W. Hazard, Environmental & Natural
Resources Division, United States Department of Justice,
Washington, D.C., for the defendant-appellee.


                        OPINION

BERZON, Circuit Judge:

  When a party withdraws one of its claims before the trial
court enters judgment and the action is subsequently dis-
              HELLS CANYON v. U.S. FOREST SERVICE                3999
missed on the merits, does the trial court’s failure to indicate
that the withdrawn claim was dismissed without prejudice
necessarily render its decision a “final judgment on the mer-
its” as to that claim? Because we answer this question in the
negative, we reverse the district court’s dismissal of this suit
on res judicata grounds and remand for further proceedings.

                                   I

   The Hells Canyon National Recreation Area (HCNRA),
located on Oregon’s border with Idaho, was established by
Congress in 1975 pursuant to the Hells Canyon National Rec-
reation Area Act, Pub. L. No. 94-199, 89 Stat. 1117 (1975),
16 U.S.C. §§ 460gg et seq. The HCNRA includes the “Hells
Canyon Wilderness,” which is defined as the area “depicted
on the map entitled ‘Hells Canyon National Recreation Area’
dated May 1978,[1] which shall be on file and available for
public inspection in the officer of the Chief, Forest Service,
United States Department of Agriculture.” 16 U.S.C.
§ 460gg(b). The Hells Canyon Wilderness is governed by the
HCNRA Act or the Wilderness Act, 16 U.S.C. §§ 1131-36,
whichever is more restrictive. See 16 U.S.C. § 460gg-1(b).
One of the Wilderness Act’s restrictions bars the use of
motorized vehicles within designated wilderness areas “ex-
cept as necessary to meet minimum requirements for the
administration of the area.” Id. § 1133(c).

   At issue in this case is the Lord Flat Trail, a fifteen-mile
man-made motorized path on the rim of Hells Canyon. The
Trail was initially created in 1960 as a defense against a fire.
Since that time, motorized use of the Trail has steadily
increased.

  In 1989, the U.S. Forest Service discovered that a 1.5-mile
  1
   The original Act referred to a map dated “September 1975.” Congress
replaced the 1975 map in 1978. See National Parks and Recreation Act of
1978, Pub. L. No. 95-625, § 607, 92 Stat. 3467, 3520.
4000         HELLS CANYON v. U.S. FOREST SERVICE
stretch of the Lord Flat Trail was within the Hells Canyon
Wilderness. After temporarily banning motorized vehicle use
on the Trail to avoid violating the Wilderness Act, the Forest
Service decided in 1992 to relocate that part of the Trail
within the Wilderness. After the relocation, Hells Canyon
Preservation Council (“HCPC”), the plaintiff-appellant here,
brought suit, alleging that the Forest Service violated the
National Environmental Policy Act (NEPA), 42 U.S.C.
§§ 4321 et seq., by failing to file an environmental impact
statement when it relocated the 1.5 mile stretch of the Trail.
HCPC also maintained that, even after the relocation, other
parts of the Trail remained within the Hells Canyon Wilder-
ness, and that, consequently, motorized vehicle use of the
Trail continued to violate the Wilderness Act. See Hells Can-
yon Pres. Council v. U.S. Forest Serv., 883 F. Supp. 534, 535
(D. Or. 1995) (“HCPC I”).

   HCPC’s Wilderness Act claim in HCPC I rested on a “For-
est Service transportation system map” indicating a half-mile
area where the Lord Flat Trail appeared to cross the Wilder-
ness boundary. In response to HCPC’s summary judgment
motion, the Forest Service responded that the map upon
which HCPC relied was “incorrect.” In other words, HCPC’s
1994 Wilderness Act claim was premised on a Forest Service
map that the Forest Service asserted was inaccurate. During
oral argument before the district court on the parties’ cross-
motions for summary judgment, HCPC, taking into account
the Forest Service’s representation, voluntarily abandoned its
Wilderness Act claim. Accordingly, the district court, after it
rejected HCPC’s NEPA claim on the merits, dismissed the
cross-motions for summary judgment on the Wilderness Act
claim as “moot.” See id. at 539.

   It was not until 1997 that HCPC specifically documented
those locations where the Trail crossed the hydrologic divide.
The divide is, according to HCPC’s interpretation of the
HCNRA Act, the Wilderness boundary. In response to that
discovery, and after a series of exchanges with the Forest Ser-
              HELLS CANYON v. U.S. FOREST SERVICE                4001
vice over the existence and location of the “May 1978” map
referred to in the Act, HCPC brought this suit in 2002, alleg-
ing violations of the HCNRA Act, the Wilderness Act, and
the Administrative Procedure Act, 5 U.S.C. § 706. Specifi-
cally, HCPC alleged in its complaint that (1) the Service vio-
lated the HCNRA Act by failing to produce or otherwise
provide the 1978 map required by the Act to be on file in the
Forest Service Chief’s office; (2) portions of the Lord Flat
Trail remain in the Hells Canyon Wilderness, and motorized
vehicle use on the trail therefore violates the Wilderness Act;
and (3) the Service’s current definition of the western Wilder-
ness boundary as following the rim of the Canyon rather than
the hydrologic divide is arbitrary and capricious in violation
of the APA.

    In an unpublished opinion, the district court dismissed the
suit, finding each of HCPC’s claims barred by res judicata.
Specifically, the court held that all of HCPC’s claims either
were or could have been raised in the HCPC I litigation, and
that final judgment in that case therefore precluded re-
litigation of those claims here. From this ruling, HCPC timely
appeals.

                                  II

   [1] We review the district court’s dismissal on res judicata
grounds de novo. Stewart v. U.S. Bancorp, 297 F.3d 953, 956
(9th Cir. 2002). As we recently explained, “[t]he doctrine of
res judicata provides that a final judgment on the merits bars
further claims by parties or their privies based on the same
cause of action, and is central to the purpose for which civil
courts have been established, the conclusive resolution of dis-
putes within their jurisdiction.” Headwaters Inc. v. U.S. For-
est Serv., 399 F.3d 1047, 1051-52 (9th Cir. 2005) (internal
quotation marks omitted).2 “The elements necessary to estab-
  2
   Claim preclusion bars any lawsuits on any claims that were raised or
could have been raised in a prior action.” Providence Health Plan v.
4002            HELLS CANYON v. U.S. FOREST SERVICE
lish res judicata are: ‘(1) an identity of claims, (2) a final
judgment on the merits, and (3) privity between parties.’ ” Id.
at 1052 (quoting Tahoe-Sierra Pres. Council, Inc. v. Tahoe
Reg’l Planning Agency, 322 F.3d 1064, 1077 (9th Cir. 2003)).
As the parties are identical, the only issues here are whether
there is “an identity of claims” with, and “a final judgment on
the merits” in, HCPC I.

                                     A

   The district court in this case concluded that HCPC’s Wil-
derness Act claim reached a final judgment on the merits of
the action in HCPC I. It is certainly true that there was a final
judgment on the merits of the action in HCPC I, as the district
court granted the government’s motion for summary judg-
ment on HCPC’s NEPA claim. See HCPC I, 883 F. Supp. at
539; see also Fed. R. Civ. P. 56(c). But res judicata doctrine
focuses on an identity of claims, specifying that “a valid final
adjudication of a claim precludes a second action on that
claim or any part of it.” Baker ex rel. Thomas v. Gen. Motors
Corp., 522 U.S. 222, 233 n.5 (1998) (emphasis added). That
is to say, the “final judgment” prong of the res judicata test
is claim-specific. With that caveat in mind, we turn to the
somewhat thorny question whether the Wilderness Act claim
in HCPC I was included in the “final judgment on the merits”
rendered by the district court.

McDowell, 385 F.3d 1168, 1173-74 (9th Cir. 2004) (emphasis and internal
quotation marks omitted), petition for cert. filed, 73 U.S.L.W. 3401 (U.S.
Dec. 28, 2004) (No. 04-883). We note, briefly, that courts have not always
been clear that the phrase “claims that were raised or could have been
raised,” refers to legal theories arising out of the same transactional
nucleus of facts, rather than to distinct causes of action. The contrary read-
ing would suggest that any cause of action that could have been joined in
the original action would be precluded, a point we rejected over four dec-
ades ago. See Bankers Trust Co. v. Pac. Employers Ins. Co., 282 F.2d 106,
111 (9th Cir. 1960); see also Gallagher v. Frye, 631 F.2d 127, 130 (9th
Cir. 1980).
               HELLS CANYON v. U.S. FOREST SERVICE                    4003
  [2] “[F]inal judgment on the merits” is synonymous with
“dismissal with prejudice.” See, e.g., Stewart, 297 F.3d at
956; see also Semtek Int’l Inc. v. Lockheed Martin Corp., 531
U.S. 497, 505-06 (2001). HCPC I, however, is silent as to
whether HCPC’s Wilderness Act claim was dismissed by the
court at all. In full, the relevant passage of the opinion in
HCPC I provides as follows:

         In its complaint, plaintiff alleged that another por-
      tion of Lord Flat Road is located illegally within the
      wilderness area and must be closed. At the hearing
      on the parties’ motions for summary judgment,
      plaintiff withdrew that claim. Accordingly, defen-
      dant’s and plaintiff’s motions for summary judgment
      on that claim are MOOT.

883 F. Supp. at 539. The Forest Service also emphasizes the
language of the judgment in HCPC I. That judgment, how-
ever, provided only that “this action is dismissed,” (emphasis
added), without specifying which claims were still part of the
action when it was dismissed.

  The Forest Service argues that HCPC I’s silence is given
meaning by Rule 41(b) of the Federal Rules of Civil Proce-
dure, which provides in pertinent part that:

      Unless the court in its order for dismissal otherwise
      specifies, a dismissal under this subdivision and any
      dismissal not provided for in this rule, other than a
      dismissal for lack of jurisdiction, for improper
      venue, or for failure to join a party under Rule 19,
      operates as an adjudication upon the merits.[3]
  3
    Because this suit was filed in the same district court as HCPC I, we
need not address the impact of Semtek’s holding that an “adjudication
upon the merits” under Rule 41(b) does not necessarily have preclusive
effect in other courts. See, e.g., Semtek, 531 U.S. at 506; see also Headwa-
ters, 399 F.3d at 1052 n.4 (“As this case was filed in the same district
court as the one claimed to be preclusive, however, the otherwise impor-
tant distinction made in Semtek is not pertinent to this prong of the inqui-
ry.”).
4004         HELLS CANYON v. U.S. FOREST SERVICE
HCPC, in contrast, urges us to interpret the district court’s
silence by reference to Rule 41(a)(2), which allows for volun-
tary dismissals of actions with judicial consent, and provides
that “[u]nless otherwise specified in the order [of dismissal],
a dismissal under this paragraph is without prejudice.”

   Because we are reviewing the preclusive effect of the dis-
trict court’s decision in HCPC I, we must make our own
determination of the basis for the dismissal, using the avail-
able record from HCPC I. On our independent review of
HCPC I, we find neither party’s argument concerning the
impact of Rule 41 on the judgment entered in HCPC I con-
vincing. Instead, the district court’s treatment in HCPC I of
the Wilderness Act claim is best construed as approval of an
oral amendment of the complaint to excise that claim. So con-
strued, the action at the time of dismissal contained no Wil-
derness Act cause of action, and there was therefore no final
judgment on the merits with regard to that claim.

                               1

   As its title, “Dismissal of Actions,” suggests, Rule 41, or
at least Rule 41(a), governs dismissals of entire actions, not
of individual claims. Most contemporary courts, including our
own, have declined to read the rule literally as permitting the
dismissal only of an entire action against all defendants. See,
e.g., Pedrina v. Chun, 987 F.2d 608, 609 & n.1 (9th Cir.
1993). These same cases, however, have only extended the
rule to allow the dismissal of all claims against one defendant,
so that a defendant may be dismissed from the entire action.
Nothing in the case law suggests that Rule 41(a) extends to
the voluntary withdrawal of individual claims against a defen-
dant remaining in the case.

   [3] In the specific context of Rule 41(a)(1), we have held
that the Rule does not allow for piecemeal dismissals. Instead,
withdrawals of individual claims against a given defendant
are governed by Fed. R. Civ. P. 15, which addresses amend-
                HELLS CANYON v. U.S. FOREST SERVICE                    4005
ments to pleadings. See Ethridge v. Harbor House Restau-
rant, 861 F.2d 1389 (9th Cir. 1988). As we noted in Ethridge,
“a plaintiff may not use Rule 41(a)(1)(i) to dismiss, unilater-
ally, a single claim from a multi-claim complaint.” Id. at
1392. Instead, we agreed with two of our sister circuits4 that
“Federal Rule of Civil Procedure 15(a) is the appropriate
mechanism ‘[w]here a plaintiff desires to eliminate an issue,
or one or more but less than all of several claims, but without
dismissing as to any of the defendants.’ ” Id. (quoting 5 J.
MOORE ET AL., MOORE’S FEDERAL PRACTICE ¶ 41.06-1, at 41-83
to -84 (1987)) (alteration in original); see also Gen. Signal
Corp. v. MCI Telecomms. Corp., 66 F.3d 1500, 1513 (9th Cir.
1995) (“[W]e have held that Rule 15, not Rule 41, governs the
situation when a party dismisses some, but not all, of its
claims.” (citing Ethridge, 861 F.2d at 1392)); Gronholz v.
Sears, Roebuck & Co., 836 F.2d 515, 518 (Fed. Cir. 1987).5

   [4] In the seventeen years since Ethridge, we have not had
an opportunity to consider whether the same logic extends to
the other method of voluntary dismissal under Rule 41 — vol-
untary dismissal with judicial consent, under Rule 41(a)(2).6
   4
     The cases upon which we relied in Ethridge were Management Inves-
tors v. United Mine Workers of America, 610 F.2d 384, 394 & n.22 (6th
Cir. 1979); and Exxon Corp. v. Maryland Casualty Co., 599 F.2d 659, 662
(5th Cir. 1979). See Ethridge, 861 F.2d at 1392. Later cases from other cir-
cuits have adopted the same rule. See, e.g., Klay v. United Healthgroup,
Inc., 376 F.3d 1092, 1106 (11th Cir. 2004).
   5
     As the Federal Circuit explained in Gronholz, this distinction can have
jurisdictional consequences in some cases, where the only claim that
creates federal jurisdiction (or, in Gronholz, Federal Circuit jurisdiction),
is voluntarily dismissed. Construed instead as an amendment of the com-
plaint to drop the claim, such an action can, in those cases, destroy juris-
diction. See 836 F.2d at 518-19. There is no question, however, that the
same was not true in HCPC I.
   6
     Five years before Ethridge, we hinted that it does, without so holding,
in Mechmetals Corp. v. Telex Computer Products, Inc., 709 F.2d 1287
(9th Cir. 1983). There, in suggesting that the same prejudice inquiry
applies to dismissals under Rule 41 and amendments of complaints under
Rule 15, we cited an Eighth Circuit case for the proposition that “it is
immaterial whether [a] court acts pursuant to Rule 15(a) or Rule
41(a)(2)).” Id. at 1294 (citing Wilson v. Crouse-Hinds Co., 556 F.2d 870,
873 (8th Cir. 1977)).
4006         HELLS CANYON v. U.S. FOREST SERVICE
Other courts to consider this issue, however, have not sug-
gested any meaningful distinction between Rules 41(a)(1) and
41(a)(2) in this context. See, e.g., Klay, 376 F.3d at 1106
(“Rule 41 allows a plaintiff to dismiss all of his claims against
a particular defendant; its text does not permit plaintiffs to
pick and choose, dismissing only particular claims within an
action.”); Nilssen v. Motorola, Inc., 203 F.3d 782, 784 (Fed.
Cir. 2000).

   [5] We agree that there is no reason to make such a distinc-
tion here. The only functional difference between the two pro-
visions is dictated by timing. Dismissal is available under
Rule 41(a)(1) without the court’s consent, so long as it is
accomplished “before a responsive pleading is served or, if
there is none, before the introduction of evidence at the trial
or hearing.” FED. R. CIV. P. 41(c). In contrast, dismissal under
Rule 41(a)(2) may only be achieved through “order of the
court and upon such terms and conditions as the court deems
proper.”

   Rule 15(a), however, includes precisely the same distinc-
tion. That is to say, both timing possibilities are also
accounted for in Rule 15(a), and Rule 15(a), like Rule
41(a)(2), specifies judicial approval after a responsive plead-
ing is filed absent stipulation by the opposing party. In perti-
nent part, the Rule provides that:

       A party may amend the party’s pleading once as
    a matter of course at any time before a responsive
    pleading is served or, if the pleading is one to which
    no responsive pleading is permitted and the action
    has not been placed upon the trial calendar, the party
    may so amend it at any time within 20 days after it
    is served. Otherwise a party may amend the party’s
    pleading only by leave of court or by written consent
    of the adverse party; and leave shall be freely given
    when justice so requires.
               HELLS CANYON v. U.S. FOREST SERVICE                    4007
FED. R. CIV. P. 15(a).

   We therefore disagree with HCPC that we should construe
its withdrawal of its Wilderness Act claim in HCPC I as a
voluntary dismissal under Rule 41(a)(2), and therefore as a
dismissal without prejudice.

                                     2

   [6] Unlike Rule 41(a), Rule 41(b) does distinguish between
“actions” and “claims.”7 Therefore, the Forest Service appears
to be on firmer footing in arguing that we may separately con-
strue a dismissal of an individual claim as an involuntary dis-
missal under Rule 41(b). It is also true that, whereas the plain
language of Rule 41(b) suggests that such dismissals may
only result from a defendant’s motion, and there was not such
a motion in HCPC I, the consensus among our sister circuits,
with which we agree, is that courts may dismiss under Rule
41(b) sua sponte, at least under certain circumstances. See,
e.g., Olsen v. Mapes, 333 F.3d 1199, 1204 n.3 (10th Cir.
2003) (“[T]he Rule has long been interpreted to permit courts
to dismiss actions sua sponte for a plaintiff’s failure to prose-
cute or comply with the rules of civil procedure or court’s
orders.” (citing Link v. Wabash R.R. Co., 370 U.S. 626, 630-
31 (1962))); O’Rourke Bros. Inc. v. Nesbitt Burns, Inc., 201
F.3d 948, 952 (7th Cir. 2000).

  [7] Nevertheless, we believe that the Forest Service’s argu-
ment is premised on a fundamental misunderstanding of
HCPC I — that the district court necessarily “dismissed”
  7
    As other courts have noted, the distinction in Rule 41(b) between
actions and claims lends further support for the proposition that Rule 41(a)
does not encompass the dismissal of individual claims. See, e.g., Gron-
holz, 836 F.2d at 518 (“ ‘The reference to an “action” in Rule 41(a) con-
trasts with Rule 41(b) . . . . The language of Rule 41(b) is broader and
more comprehensive than the parallel language in Rule 41(a).’ ” (quoting
Smith, Kline & French Labs. v. A.H. Robins Co., 61 F.R.D. 24, 28 (E.D.
Pa. 1973))).
4008           HELLS CANYON v. U.S. FOREST SERVICE
HCPC’s Wilderness Act claim when it entered its final judg-
ment. Instead, as the above analysis suggests, what the district
court should have done, and what we believe it did do, was
treat HCPC’s oral withdrawal of its Wilderness Act claim as
a motion to amend its complaint under Rule 15(a).

   True, the district court did not explicitly state that it was
permitting a Rule 15(a) amendment. We agree with the Fed-
eral Circuit, however, that “[t]he true state of affairs is more
critical than mere labels. The fact that a voluntary dismissal
of a claim under Rule 41(a) is properly labeled an amendment
under Rule 15 is a technical, not a substantive, distinction.”
Nilssen, 203 F.3d at 784 (footnote omitted); see also Johnson
v. Cartwright, 355 F.2d 32, 39 (8th Cir. 1966) (“[I]t may not
be material whether the court acts under Rule 15(a) which
relates to amendments, . . . or Rule 41(a)(2).”).

   As we view HCPC I’s ruling denying the cross-motions for
summary judgment because they were “moot,” the district
court concluded that HCPC’s Wilderness Act claim was no
longer before it, because the pleadings had been amended to
eliminate it. HCPC I could not have meant to use the term
“moot” in its Article III jurisdictional sense — otherwise, it
would have had to dismiss the claim as moot, rather than deny
the motions as such.8 See, e.g., Envtl. Prot. Info. Ctr., Inc. v.
Pac. Lumber Co., 257 F.3d 1071, 1076-77 (9th Cir. 2001)
(citing Steel Co. v. Citizens for a Better Env’t, 523 U.S. 83,
94 (1998)). Consequently, the only plausible reading of
HCPC I is that the court in that case used the term “moot” to
mean that it had no reason to inquire whether there remained
a live dispute concerning the location of the Trail. This result
could only have followed if the district court understood that
  8
    This point explains why HCPC is incorrect that, even if Rule 41(b)
does govern the dismissal of its Wilderness Act claim in HCPC I, the dis-
missal was for a “lack of jurisdiction,” and was therefore not an “adjudica-
tion upon the merits.” The claim was not dismissed as moot; the cross-
motions were denied as moot. See HCPC I, 883 F. Supp. at 539.
             HELLS CANYON v. U.S. FOREST SERVICE               4009
the claim had already been withdrawn. See Klay, 376 F.3d at
1106 (recognizing that a district court’s statement that there
is no longer a “controversy” regarding a claim that was pur-
portedly dismissed can be construed as permission to amend
the complaint to eliminate that claim).

   [8] In other words, because it is clear from the record that
HCPC’s Wilderness Act claim was already withdrawn before
the district court entered judgment, the fact that the court did
not explicitly state that he was permitting an amendment of
the pleadings under Rule 15 is “a technical, not a substantive,
distinction.” We therefore construe HCPC’s oral — and, we
emphasize, unopposed — withdrawal of its Wilderness Act
claim as an amendment of its complaint, pursuant to Rule
15(a). The district court’s recognition of the withdrawal in its
opinion is, in our view, indicative of its implicit consent to the
“amendment.” Cf. Am. States Ins. Co. v. Dastar Corp., 318
F.3d 881, 888 & n.8 (9th Cir. 2003) (construing a consensual
dismissal of individual claims as a Rule 15(a) amendment). It
is axiomatic that prejudice does not attach to a claim that is
properly dropped from a complaint under Rule 15(a) prior to
final judgment. Consequently, there could not have been a
“final judgment on the merits” with regard to HCPC’s Wil-
derness Act claim in HCPC I.

                                B

   [9] In light of the foregoing analysis, the only claim that
reached a final judgment on the merits in HCPC I was
HCPC’s cause of action under NEPA. We therefore turn to
whether there is an “identity of claims” between HCPC’s
NEPA claim in that litigation and its various claims here.

  Whether there is an identity of claims turns on:

    (1) whether rights or interests established in the prior
    judgment would be destroyed or impaired by prose-
    cution of the second action; (2) whether substantially
4010         HELLS CANYON v. U.S. FOREST SERVICE
    the same evidence is presented in the two actions;
    (3) whether the two suits involve infringement of the
    same right; and (4) whether the two suits arise out of
    the same transactional nucleus of facts. The last of
    these criteria is the most important.

Costantini v. Trans World Airlines, 681 F.2d 1199, 1201-02
(9th Cir. 1982) (citation and internal quotation marks omit-
ted).

   HCPC’s claims here arise out of a different “transactional
nucleus of facts” than its NEPA claim in HCPC I. In HCPC
I, HCPC’s sole contention was that the Forest Service violated
NEPA by relocating the 1.5-mile stretch of the Lord Flat Trail
without filing an environmental impact statement. At its
broadest, then, the transactional nucleus of facts relevant to
HCPC’s NEPA claim was that the Forest Service had (a)
decided to relocate that stretch of the trail, and (b) had done
so without filing an EIS. By contrast, HCPC’s Wilderness Act
claim here arises out of its assertion that parts of the trail
remain inside the Wilderness after the relocation; its HCNRA
Act claim arises out of the Service’s failure to display the
“May 1978” map as required by the statute; and its related
APA claims arise out of the same facts.

   [10] Consequently, the district court’s final judgment on
the merits with regard to HCPC’s NEPA claim in HCPC I
does not bar HCPC’s claims on the merits here. Because there
was also no final judgment on the merits with regard to
HCPC’s Wilderness Act claim, the district court in this case
erred in dismissing HCPC’s claims on the basis of res judi-
cata.

                              III

  Although we may affirm the district court on any ground
supported by the record, Wolfe v. Strankman, 392 F.3d 358,
362 (9th Cir. 2004), this appeal comes to us on a motion to
               HELLS CANYON v. U.S. FOREST SERVICE                   4011
dismiss, and the record is therefore largely undeveloped. For
example, the government’s central alternative argument for
affirming the district court is that HCPC’s claims are time-
barred by 28 U.S.C. § 2401(a). The question of when a claim
accrues is a fact-intensive inquiry, and we have held that a
district court’s factual finding concerning when a claim
accrues is entitled to deferential review. See, e.g., Erlin v.
United States, 364 F.3d 1127, 1130 (9th Cir. 2004). The nec-
essary implication of such a holding is that, unless remand is
futile, see, e.g., Moore v. United Kingdom, 384 F.3d 1079,
1088 (9th Cir. 2004), the district court should have the first
opportunity, on a more complete record, to make such a determi-
nation.9

   We therefore REVERSE the district court’s dismissal of
this action on res judicata grounds, and REMAND for further
proceedings consistent with this opinion.

   REVERSED and REMANDED.




  9
    The same logic applies a fortiori to the Forest Service’s argument that
HCPC lacks standing to pursue its claim under the HCNRA Act. The For-
est Service’s position — that HCPC suffered no injury because the “May
1978” map was provided to them in the course of a FOIA request — nec-
essarily relies on factual evidence that the district court could not have
considered on a motion to dismiss. See FED. R. CIV. P. 12(b)(6); Jackson
v. S. Cal. Gas Co., 881 F.2d 638, 643 n.4 (9th Cir. 1989).
