                FOR PUBLICATION

 UNITED STATES COURT OF APPEALS
      FOR THE NINTH CIRCUIT

AGUA CALIENTE BAND OF CAHUILLA          No. 15-55896
INDIANS,
                Plaintiff-Appellee,        D.C. No.
                                        5:13-cv-00883-
UNITED STATES OF AMERICA,                  JGB-SP
       Intervenor-Plaintiff-Appellee,

                 v.                       OPINION

COACHELLA VALLEY WATER
DISTRICT; ED PACK, in Official
Capacity as Member of the Board of
Directors of the Coachella Valley
Water District; JOHN POWELL, JR., in
Official Capacity as Member of the
Board of Directors of the Coachella
Valley Water District; PETER
NELSON, in Official Capacity as
Member of the Board of Directors of
the Coachella Valley Water District;
G. PATRICK O’DOWD, in Official
Capacity as a Member of the Board
of Directors of the Coachella Valley
Water District; CASTULO R.
ESTRADA, in Official Capacity as a
Member of the Board of Directors of
the Coachella Valley Water District;
DESERT WATER AGENCY; PATRICIA
G. OYGAR, in Official Capacity as
Member of the Board of Directors of
2                AGUA CALIENTE BAND V.
              COACHELLA VALLEY WATER DIST.


    the Desert Water Agency; THOMAS
    KIELEY, III, in Official Capacity as
    Member of the Board of Directors of
    the Desert Water Agency; JAMES
    CIOFFI, in Official Capacity as
    Member of the Board of Directors of
    the Desert Water Agency; CRAIG A.
    EWING, in Official Capacity as
    Member of the Board of Directors of
    the Desert Water Agency; JOSEPH K.
    STUART, in Official Capacity as
    Member of the Board of Directors of
    the Desert Water Agency,
                   Defendants-Appellants.



         Appeal from the United States District Court
            For the Central District of California
          Jesus G. Bernal, District Judge, Presiding

           Argued and Submitted October 18, 2016
                    Pasadena, California

                       Filed March 7, 2017

  Before: Richard C. Tallman and Morgan B. Christen,
 Circuit Judges, and Matthew F. Kennelly, * District Judge.

                   Opinion by Judge Tallman


     *
       The Honorable Matthew F. Kennelly, United States District Judge
for the Northern District of Illinois, sitting by designation.
                 AGUA CALIENTE BAND V.                               3
              COACHELLA VALLEY WATER DIST.

                          SUMMARY **


                 Water Rights / Tribal Rights

    The panel affirmed the district court’s partial summary
judgment in favor of the Agua Caliente Band of Cahuilla
Indians and the United States, which declared that the United
States impliedly reserved appurtenant water sources,
including groundwater, when it created the Tribe’s
reservation in California’s arid Coachella Valley.

    The Tribe filed this action for declaratory and injunctive
relief against water agencies, and the parties stipulated to
divide the litigation into three phases. Phase I, at issue in this
interlocutory appeal, addressed whether the Tribe has a
reserved right to groundwater.

   Under the doctrine in Winters v. United States, 207 U.S.
564 (1908), federal reserved water rights are directly
applicable to Indian reservations.

     The panel held that the Winters doctrine does not
distinguish between surface water and groundwater. The
panel held that the United States, in establishing the Agua
Caliente reservation, impliedly reserved water. The panel
further held that because the United States intended to
reserve water when it established a home for the Agua
Caliente Band of Cahuilla Indians, the district court did not
err in determining that the government reserved appurtenant
water sources – including groundwater – when it created the

    **
       This summary constitutes no part of the opinion of the court. It
has been prepared by court staff for the convenience of the reader.
4               AGUA CALIENTE BAND V.
             COACHELLA VALLEY WATER DIST.

Tribe’s reservation in the Coachella Valley. The panel also
held that the creation of the Agua Caliente Reservation
carried with it an implied right to use water from the
Coachella Valley aquifer.

    The panel rejected the water agencies’ arguments
concerning the contours of the Tribe’s reserved water rights.
The panel held that state water rights are preempted by
federal reserved rights. The panel also held that the fact that
the Tribe did not historically access groundwater did not
destroy its right to groundwater now. Finally, the panel held
that the Tribe’s entitlement to state water did not affect the
analysis with respect to the creation of the Tribe’s federally
reserved water right.


                         COUNSEL

Steven Bane Abbott (argued), Gerald D. Shoaf, and Julianna
K. Tillquist, Redwine and Sherrill, Riverside, California, for
Defendants-Appellants Coachella Valley Water District, G.
Patrick O’Dowd, Ed Pack, John Powell Jr., Peter Nelson,
and Castulo R. Estrada.

Roderick E. Walston (argued), Arthur L. Littleworth,
Michael T. Riddell, and Steven G. Martin, Best Best &
Krieger LLP, Walnut Creek, California, for Defendants-
Appellants Desert Water Agency, Patricia G. Oygar,
Thomas Kieley III, James Cioffi, Craig A. Ewing, and
Joseph K. Stuart.

Catherine F. Munson (argued), Kilpatrick Townsend &
Stockton LLP, Washington, D.C.; Steven C. Moore and
Heather Whiteman Runs Him, Native American Rights
                AGUA CALIENTE BAND V.                        5
             COACHELLA VALLEY WATER DIST.

Fund, Boulder, Colorado; Mark H. Reeves, Kilpatrick
Townsend & Stockton LLP, Augusta, Georgia; Adam H.
Charnes, Kilpatrick Townsend & Stockton LLP, Dallas,
Texas; for Plaintiff-Appellee.

Elizabeth A. Peterson (argued), Yosef M. Negose, Daron T.
Carreiro, Patrick Barry, John L. Smeltzer, and William B.
Lazarus, Attorneys; John C. Cruden, Assistant Attorney
General; United States Department of Justice, Washington,
D.C.; Christopher Watson and Scott Bergstrom, Office of
the Solicitor, United States Department of the Interior,
Washington, D.C.; for Intervenor-Plaintiff-Appellee.


                         OPINION

TALLMAN, Circuit Judge:

       “When the well’s dry, we know the worth of
       water.” Benjamin Franklin (1706–1790),
       Poor Richard’s Almanac.

    The Coachella Valley Water District (“CVWD”) and the
Desert Water Agency (“DWA”) (collectively, the “water
agencies”) bring an interlocutory appeal of the district
court’s grant of partial summary judgment in favor of the
Agua Caliente Band of Cahuilla Indians (the “Tribe”) and
the United States. The judgment declares that the United
States impliedly reserved appurtenant water sources,
including groundwater, when it created the Tribe’s
reservation in California’s arid Coachella Valley. We agree.
In affirming, we recognize that there is no controlling federal
appellate authority addressing whether the reserved rights
doctrine applies to groundwater. However, because we
6              AGUA CALIENTE BAND V.
            COACHELLA VALLEY WATER DIST.

conclude that it does, we hold that the Tribe has a reserved
right to groundwater underlying its reservation as a result of
the purpose for which the reservation was established.

                              I

                              A

    The Agua Caliente Band of Cahuilla Indians has lived in
the Coachella Valley since before California entered
statehood in 1850. The bulk of the Agua Caliente
Reservation was formally established by two Presidential
Executive Orders issued in 1876 and 1877, and the United
States, pursuant to statute, now holds the remaining lands of
the reservation in trust for the Tribe. The reservation
consists of approximately 31,396 acres interspersed in a
checkerboard pattern amidst several cities within Riverside
County, including Palm Springs, Cathedral City, and
Rancho Mirage. See Agua Caliente Band of Mission Indians
v. Riverside County, 442 F.2d 1184, 1185 (9th Cir. 1971).

    The Executive Orders establishing the reservation are
short in length, but broad in purpose. In 1876, President
Ulysses S. Grant ordered certain lands “withdrawn from sale
and set apart as reservations for the permanent use and
occupancy of the Mission Indians in southern California.”
Exec. Order of May 15, 1876. Similarly, President
Rutherford B. Hayes’s 1877 Order set aside additional lands
for “Indian purposes.” Exec. Order of Sept. 29, 1877. These
orders followed on the heels of detailed government reports
from Indian agents, which identified the urgent need to
reserve land for Indian use in an attempt to encourage tribal
members to “build comfortable houses, improve their acres,
and surround themselves with home comforts.” Comm’r of
Indian Aff., Ann. Rep. 224 (1875). In short, the United
                  AGUA CALIENTE BAND V.                                 7
               COACHELLA VALLEY WATER DIST.

States sought to protect the Tribe and “secure the Mission
Indians permanent homes, with land and water enough.”
Comm’r of Indian Aff., Ann. Rep. 37 (1877).

    Establishing a sustainable home in the Coachella Valley
is no easy feat, however, as water in this arid southwestern
desert is scarce. Rainfall totals average three to six inches
per year, and the Whitewater River System—the valley’s
only real source of surface water—produces an average
annual supply of water that fluctuates between 4,000 and
9,000 acre-feet, most of which occurs in the winter months. 1
See CVWD, Engineer’s Report on Water Supply and
Replenishment Assessment at III-12 (2016–2017); CVWD,
Urban Water Management Plan at 3-2, 3-20 (2005). In other
words, surface water is virtually nonexistent in the valley for
the majority of the year. Therefore, almost all of the water




    1
      An acre-foot is the volume of water sufficient to cover one acre in
area at a depth of one foot. CVWD, 2010–2011 Annual Review at 2. It
is equivalent to 325,851 gallons. Id. It takes about four acre-feet of
water to irrigate one acre of land for a year in the Coachella Valley. See
U.S. Dep’t of Agric., A Review of Agricultural Water Use in the
Coachella Valley at 6 (2006). Therefore, at 9,000 acre-feet per year, the
river system provides enough water to irrigate around 2,250 acres. At
4,000 acre-feet per year, the system can only irrigate about 1,000 acres.
Considering that the Tribe is not the only user of the Whitewater River
System, and that its reservation alone accounts for 31,396 acres, even in
a peak year the river system provides very little water for irrigation or
for human consumption.
8                AGUA CALIENTE BAND V.
              COACHELLA VALLEY WATER DIST.

consumed in the region comes from the aquifer underlying
the valley—the Coachella Valley Groundwater Basin. 2

     The Coachella Valley Groundwater Basin supports 9
cities, 400,000 people, and 66,000 acres of farmland. See
CVWD-DWA, The State of the Coachella Valley Aquifer at
2. Given the demands on the basin’s supply, it is not
surprising that water levels in the aquifer have been
declining at a steady rate. Since the 1980s, the aquifer has
been in a state of overdraft, 3 which exists despite major
efforts to recharge the basin with water delivered from the
California Water Project and the Colorado River. In total,
groundwater pumping has resulted in an average annual
recharge deficit of 239,000 acre-feet, with cumulative
overdraft estimated at 5.5 million acre-feet as of 2010.

    The Tribe does not currently pump groundwater on its
reservation.    Rather, it purchases groundwater from
Appellant water agencies. The Tribe also receives surface
water from the Whitewater River System, particularly the
Andreas and Tahquitz Creeks that sometimes flow nearby.
The surface water received from this system is consistent
with a 1938 California Superior Court adjudication—the
Whitewater River Decree—which attempted to address
state-law water rights for users of the river system. Because
the United States held the lands in trust, it participated in the

    2
      The CVWD estimates that surface water accounts for less than five
percent of its water supply each year. See CVWD, Urban Water
Management Plan at 3-20 (2005).

    3
      Overdraft occurs when the amount of water extracted from the
underground basin exceeds its recharge rate. CVWD, 2010–2011
Annual Review at 2.
                   AGUA CALIENTE BAND V.                                   9
                COACHELLA VALLEY WATER DIST.

adjudication via a “Suggestion” on behalf of the Tribe and
the resulting state court order included a water allotment for
the Tribe’s benefit. 4 The amount of water reserved for the
Tribe from this adjudication, however, is minimal, providing
enough water to irrigate approximately 360 acres. Further,
most of this allotment is filled outside of the growing season
because the river system’s flow peaks between December
and March. Thus, groundwater supplied by the water
agencies remains the main source of water for all types of
consumption on the reservation throughout the year.

                                     B

    Given an ever-growing concern over diminishing
groundwater resources, the Agua Caliente Tribe filed this
action for declaratory and injunctive relief against the water
agencies in May 2013. The Tribe’s complaint requested a
declaration that it has a federally reserved right and an
aboriginal right to the groundwater underlying the
reservation. In June 2014, the district court granted the
United States’ motion to intervene as a plaintiff. The United
States also alleges that the Tribe has a reserved right to
groundwater.

    The parties stipulated to divide the litigation into three
phases. Phase I, at issue here, seeks to address whether the
Tribe has a reserved right and an aboriginal right to
groundwater. According to the parties’ stipulation, Phase II

    4
      In providing this “Suggestion,” the government maintained that it
was not “submitting the rights of the United States . . . to the jurisdiction
of the Department of Public Works of the State of California” and that
the court lacked “jurisdiction [to adjudicate] the water rights of the
United States.” The federal government continues to maintain this
position before us.
10                AGUA CALIENTE BAND V.
               COACHELLA VALLEY WATER DIST.

will address whether the Tribe beneficially owns the “pore
space” of the groundwater basin underlying the Agua
Caliente Reservation and whether a tribal right to
groundwater includes the right to receive water of a certain
quality. Finally, Phase III will attempt to quantify any
identified groundwater rights.

     In March 2015, the district court granted in part and
denied in part Plaintiffs’ and Defendants’ cross motions for
partial summary judgment with respect to Phase I of the
litigation. In its order, the district court held that the reserved
rights doctrine applies to groundwater and that the United
States reserved appurtenant groundwater when it established
the Tribe’s reservation. 5 The district court then certified its
order for interlocutory appeal pursuant to 28 U.S.C.
§ 1292(b), and we granted the water agencies’ petition for
permission to prosecute this appeal.

                                   II

    The district court’s grant of summary judgment is
reviewed de novo. Tohono O’odham Nation v. City of
Glendale, 804 F.3d 1292, 1297 (9th Cir. 2015); Lopez v.
Smith, 203 F.3d 1122, 1131 (9th Cir. 2000) (en banc).
Summary judgment is appropriate when there is no genuine
issue as to any material fact and the moving party is entitled
to judgment as a matter of law. Fed. R. Civ. P. 56(a);
Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 247–48

     5
       The district court also held that the Tribe does not have an
aboriginal right to the groundwater. An aboriginal right is a type of
property right that derives from territorial occupancy of land. See United
States ex rel. Chunie v. Ringrose, 788 F.2d 638, 641–42 (9th Cir. 1986).
However, the Tribe did not appeal this issue, and we do not review it
here.
                AGUA CALIENTE BAND V.                       11
             COACHELLA VALLEY WATER DIST.

(1986). A court shall grant summary judgment when, “under
the governing law, there can be but one reasonable
conclusion as to the verdict.” Anderson, 477 U.S. at 250.

                             III

    Due to the unusual trifurcation of this litigation, we are
concerned on appeal only with Phase I—whether the Tribe
has a federal reserved right to the groundwater underlying its
reservation. This question, however, is best analyzed in
three steps: whether the United States intended to reserve
water when it created the Tribe’s reservation; whether the
reserved rights doctrine encompasses groundwater; and,
finally, whether the Tribe’s correlative rights under state law
or the historic lack of drilling for groundwater on the
reservation, or the water the Tribe receives pursuant to the
Whitewater River Decree, impacts our answers to these
questions. We address each in turn.

                              A

    For over one hundred years, the Supreme Court has made
clear that when the United States “withdraws its land from
the public domain and reserves it for a federal purpose, the
Government, by implication, reserves appurtenant water
then unappropriated to the extent needed to accomplish the
purpose of the reservation.” Cappaert v. United States,
426 U.S. 128, 138 (1976) (citing U.S. Const. art. I, § 8; U.S.
Const. art. IV, § 3); see also Winters v. United States,
207 U.S. 564, 575–78 (1908); Colville Confederated Tribes
v. Walton, 647 F.2d 42, 46 (9th Cir. 1981).

    In what has become known as the Winters doctrine,
federal reserved water rights are directly applicable “to
Indian reservations and other federal enclaves,
12              AGUA CALIENTE BAND V.
             COACHELLA VALLEY WATER DIST.

encompassing water rights in navigable and nonnavigable
streams.” See Cappaert, 426 U.S. at 138. The creation of
these rights stems from the belief that the United States,
when establishing reservations, “intended to deal fairly with
the Indians by reserving for them the waters without which
their lands would have been useless.” Arizona v. California,
373 U.S. 546, 600 (1963); see also id. at 598–99 (“It is
impossible to believe that when Congress created the great
Colorado River Indian Reservation and when the Executive
Department of this Nation created the other reservations they
were unaware that most of the lands were of the desert
kind—hot, scorching sands—and that water from the river
would be essential to the life of the Indian people and to the
animals they hunted and the crops they raised.”).

    Despite the longstanding recognition that Indian
reservations, as well as other reserved lands, require access
to water, the Winters doctrine only applies in certain
situations: it only reserves water to the extent it is necessary
to accomplish the purpose of the reservation, and it only
reserves water if it is appurtenant to the withdrawn land.
Winters, 207 U.S. at 575–78; Cappaert, 426 U.S. at 138.
Once established, however, Winters rights “vest[] on the date
of the reservation and [are] superior to the rights of future
appropriators.” Cappaert, 426 U.S. at 138.

                               B

                               1

     Given the limitations in the Winters doctrine, we must
first decide whether the United States, in establishing the
Agua Caliente Reservation, impliedly reserved water. See
United States v. New Mexico, 438 U.S. 696, 701 (1978). We
conclude that it did. And although the parties and the district
                  AGUA CALIENTE BAND V.                              13
               COACHELLA VALLEY WATER DIST.

court focused on the application of the Winters doctrine to
groundwater specifically, their argument over the creation of
a federal reserved right—and, in particular, the relevance of
New Mexico to that question—depends on whether the Agua
Caliente Reservation carried with it a reserved right to water
generally. Whether the Tribe’s reserved right extends to the
groundwater underlying its reservation is a separate question
from whether the establishment of the reservation contained
an implicit right to use water.

    In New Mexico, the Supreme Court emphasized that,
under the reserved rights doctrine, the government reserves
only “that amount of water necessary to fulfill the purpose
of the reservation, no more.” Id. (quoting Cappaert,
426 U.S. at 141). “Where water is only valuable for a
secondary use of the reservation, . . . the United States
[must] acquire water in the same manner as any other public
or private appropriator.” Id. at 702. In other words, New
Mexico established a “primary-secondary use” distinction.
Water is impliedly reserved for primary purposes. It is not,
however, reserved for secondary purposes. 6

    The water agencies argue that New Mexico requires us—
when deciding if a reserved right exists at all—to determine
whether water is necessary to fulfill the primary purpose of
the Agua Caliente Reservation. If it is not, they argue, then
we are to conclude that Congress did not intend any water to
be impliedly reserved under a federal water right. Put

    6
       We have previously noted that New Mexico is “not directly
applicable to Winters doctrine rights on Indian reservations.” United
States v. Adair, 723 F.3d 1394, 1408 (9th Cir. 1983). However, it clearly
“establish[es] several useful guidelines.” Id. Thus, we consider its
application here.
14             AGUA CALIENTE BAND V.
            COACHELLA VALLEY WATER DIST.

differently, the water agencies argue that New Mexico stands
for the proposition that water is impliedly reserved only if
other sources of water then available cannot meet the
reservation’s water demands. According to the water
agencies, if other sources of water exist—and the lack of a
federal right would not entirely defeat the purpose of the
reservation—then Congress intended to defer to state water
law and require the United States to obtain water rights like
any other private user.

    New Mexico, however, is not so narrow. Congress does
not defer to state water law with respect to reserved rights.
Id. at 702, 715. Instead, Congress retains “its authority to
reserve unappropriated water . . . for use on appurtenant
lands withdrawn from the public domain for specific federal
purposes.” Id. at 698.

    The federal purpose for which land was reserved is the
driving force behind the reserved rights doctrine. “Each time
[the] Court has applied the ‘implied-reservation-of-water-
doctrine,’ it has carefully examined both the asserted water
right and the specific purposes for which the land was
reserved, and concluded that without the water the purposes
of the reservation would be entirely defeated.” Id. at 700.
But the question is not whether water stemming from a
federal right is necessary at some selected point in time to
maintain the reservation; the question is whether the purpose
underlying the reservation envisions water use.

    Winters itself established that the purpose of the
reservation is controlling. In Winters, the Supreme Court
addressed whether the federal government reserved water
for tribal usage at the Fort Belknap Indian Reservation,
which had been reserved by the United States “as and for a
permanent home” for several tribes. 207 U.S. at 565. The
                AGUA CALIENTE BAND V.                        15
             COACHELLA VALLEY WATER DIST.

Winters Court observed that the arid tribal reservation would
be “practically valueless,” and that a civilized community
“could not be established thereon,” without irrigation. Id. at
576. Thus, the Court held that, in creating the reservation,
the United States simultaneously reserved water “for a use
which would be necessarily continued through years.” Id. at
577. The reserved right turned on the purpose underlying
the formation of the Fort Belknap Reservation.

    Though it was decided seventy years after Winters, New
Mexico remains faithful to this construction. In analyzing
the reserved rights doctrine, the Court first sought to
determine Congress’ intent in creating the Gila National
Forest. New Mexico, 438 U.S. at 698. After reviewing the
congressional act that established the forest, the Court
determined that Congress intended only two purposes—“to
conserve the water flows, and to furnish a continuous supply
of timber for the people.” Id. at 707 (citation omitted). It
did not, however, reserve the forest lands for aesthetic,
environmental, recreational, or wildlife-preservation
purposes. Id. at 708. Thus, the Court deemed the latter uses
“secondary,” for which the reserved right did not attach, and
held that only “to fulfill the very purposes for which a federal
reservation was created . . . [did] the United States intend[]
to reserve the necessary water.” Id. at 702.

    As such, New Mexico’s primary-secondary use
distinction did not alter the test envisioned by Winters.
Rather, it added an important inquiry related to the question
of how much water is reserved. It also answered that
question by holding that water is reserved only for primary
purposes, those directly associated with the reservation of
land. It did not, however, eliminate the threshold issue—that
16               AGUA CALIENTE BAND V.
              COACHELLA VALLEY WATER DIST.

a reserved right exists if the purposes underlying a
reservation envision access to water.

                                 2

    Because New Mexico holds that water is reserved if the
primary purpose of the reservation envisions water use, we
now determine the primary purpose of the Tribe’s
reservation and whether that purpose contemplates water
use. To do so, we consider “the document and circumstances
surrounding [the reservation’s] creation, and the history of
the Indians for whom it was created.” Walton, 647 F.2d at
47.

    The Executive Orders establishing the Tribe’s
reservation declared that the land was to be set aside for “the
permanent use and occupancy of the Mission Indians” or,
more generally, for “Indian purposes.” 7 See supra Part I.
While imprecise, such a purpose is not indecipherable. Our
precedent recognizes that “[t]he specific purposes of an
Indian reservation . . . [are] often unarticulated. The
general purpose, to provide a home for the Indians, is a broad
one and must be liberally construed.” Walton, 647 F.2d at
47 (emphasis added). Moreover, “[m]ost of the land in these
reservations is and always has been arid,” and it is
impossible to believe that the United States was unaware
“that water . . . would be essential to the life of the Indian
people.” Arizona, 373 U.S. at 598–99.




    7
      Additionally, government reports preceding the Executive Orders
recognized the need to secure the Tribe “permanent homes, with land
and water enough.” See Comm’r of Indian Aff., Ann. Rep. 37 (1877).
                  AGUA CALIENTE BAND V.                              17
               COACHELLA VALLEY WATER DIST.

    The situation facing the Agua Caliente Tribe is no
different. Water is inherently tied to the Tribe’s ability to
live permanently on the reservation. Without water, the
underlying purpose—to establish a home and support an
agrarian society—would be entirely defeated.              Put
differently, the primary purpose underlying the
establishment of the reservation was to create a home for the
Tribe, and water was necessarily implicated in that purpose.
Thus, we hold that the United States implicitly reserved a
right to water when it created the Agua Caliente Reservation.

                                   C

    While we conclude that the federal government
envisioned water use when it established the Tribe’s
reservation, that does not end our inquiry. We must now
determine whether the Winters doctrine, and the Tribe’s
reserved water right, extends to the groundwater underlying
the reservation. And while we are unable to find controlling
federal appellate authority explicitly holding that the Winters
doctrine applies to groundwater, 8 we now expressly hold
that it does.

    Apart from the requirement that the primary purpose of
the reservation must intend water use, the other main
limitation of the reserved rights doctrine is that the

    8
      We previously held that the Winters doctrine applies “not only [to]
surface water, but also to underground water.” United States v.
Cappaert, 508 F.2d 313, 317 (9th Cir. 1974), aff’d on other grounds,
Cappaert, 426 U.S. at 142. But on appeal, the Supreme Court did not
reach this question. See Cappaert, 426 U.S. at 142. In that case, the
peculiarities of the hydrological forms led the Court to conclude as a
question of fact that the reserved water in a cavern pool was surface
water, not groundwater. Id.
18                AGUA CALIENTE BAND V.
               COACHELLA VALLEY WATER DIST.

unappropriated water must be “appurtenant” to the
reservation. See Cappaert, 426 U.S. at 138. Appurtenance,
however, simply limits the reserved right to those waters
which are attached to the reservation. It does not limit the
right to surface water only. Cappaert itself hinted that
impliedly reserved waters may include appurtenant
groundwater when it held that “the United States can protect
its water from subsequent diversion, whether the diversion
is of surface or groundwater.” Id. at 143. If the United States
can protect against groundwater diversions, it follows that
the government can protect the groundwater itself. 9

    Further, many locations throughout the western United
States rely on groundwater as their only viable water source.
See, e.g., In re Gen. Adjudication of All Rights to Use Water
in Gila River Sys. & Source, 989 P.2d 739, 746 (Ariz. 1999)
(en banc) (“The reservations considered in [Winters and
Arizona] depended for their water on perennial streams. But
some reservations lack perennial streams and depend for
present and future survival substantially or entirely upon
pumping of underground water. We find it no more
thinkable in the latter circumstance than in the former that

     9
       Although the district court found that the groundwater contained
in the Coachella Valley aquifer “does not ‘add to, contribute to or
support’ any surface stream from which the Tribe diverts water,” that
does not mean that the hydrological cycle in the Coachella Valley has
been severed. See U.S. Geological Surv., Ground Water and Surface
Water: A Single Resource, U.S.G.S. Circular 1139 at 9–10 (1998)
(recognizing a connection between surface and groundwater even where
the water table falls below the stream bed). Further, we note that surface
water is used here to replenish groundwater sources. As such, the district
court may wish to hear expert opinion on the interconnectedness of the
waters in the valley in the later phases of this litigation. Proper factual
findings on this issue will allow the district court to fashion appropriate
relief during the quantification phase.
                AGUA CALIENTE BAND V.                           19
             COACHELLA VALLEY WATER DIST.

the United States reserved land for habitation without
reserving the water necessary to sustain life.”). More
importantly, such reliance exists here, as surface water in the
Coachella Valley is minimal or entirely lacking for most of
the year. Thus, survival is conditioned on access to water—
and a reservation without an adequate source of surface
water must be able to access groundwater.

    The Winters doctrine was developed in part to provide
sustainable land for Indian tribes whose reservations were
established in the arid parts of the country. And in many
cases, those reservations lacked access to, or were unable to
effectively capture, a regular supply of surface water. Given
these realities, we can discern no reason to cabin the Winters
doctrine to appurtenant surface water. As such, we hold that
the Winters doctrine encompasses both surface water and
groundwater appurtenant to reserved land. 10 The creation of
the Agua Caliente Reservation therefore carried with it an
implied right to use water from the Coachella Valley aquifer.

                                D

    The final issue we must address is the contours of the
Tribe’s reserved right, including its relation to state water
law and the Tribe’s existing water rights.

    A “reserved right in unappropriated water . . . vests on
the date of the reservation and is superior to the rights of
future appropriators.” Cappaert, 426 U.S. at 138. Further,
reserved rights are not analyzed “in terms of a balancing
    10
       The parties do not dispute appurtenance, nor could they. The
Coachella Valley Groundwater Basin clearly underlies the Tribe’s
reservation. See generally CVWD, Engineer’s Report on Water Supply
and Replenishment Assessment (2016–2017).
20              AGUA CALIENTE BAND V.
             COACHELLA VALLEY WATER DIST.

test.” Id. Rather, they are federal water rights that preempt
conflicting state law. See Walton, 647 F.2d at 51–53; see
also New Mexico, 438 U.S. at 715 (“[T]he ‘reserved rights
doctrine’ . . . is an exception to Congress’ explicit deference
to state water law in other areas.”). Finally, the rights are not
lost through non-use. See Walton, 647 F.2d at 51. Instead,
they are flexible and can change over time. See id. at 47–48;
United States v. Ahtanum Irrigation Dist., 236 F.2d 321, 326
(9th Cir. 1956).

    Despite the federal primacy of reserved water rights, the
water agencies argue that because (1) the Tribe has a
correlative right to groundwater under California law and
(2) the Tribe has not drilled for groundwater on its
reservation, and (3) because the Tribe is entitled to surface
water from the Whitewater River Decree, the Tribe does not
need a federal reserved right to prevent the purpose of the
reservation from being entirely defeated. Put differently, the
water agencies argue that, because the Tribe is already
receiving water pursuant to California’s correlative rights
doctrine and the Whitewater River Decree, a federal
reserved right is unnecessary.

    However, the water agencies’ arguments fail for three
reasons. First, state water rights are preempted by federal
reserved rights. See Walton, 647 F.2d at 51; see also
Ahtanum Irrigation Dist., 236 F.2d at 329 (“Rights reserved
by treaties such as this are not subject to appropriation under
state law, nor has the state power to dispose of them.”).
Second, the fact that the Tribe did not historically access
groundwater does not destroy its right to groundwater now.
See Walton, 647 F.2d at 51. And third, the New Mexico
inquiry does not ask if water is currently needed to sustain
the reservation; it asks whether water was envisioned as
                AGUA CALIENTE BAND V.                        21
             COACHELLA VALLEY WATER DIST.

necessary for the reservation’s purpose at the time the
reservation was created. See supra Part III.B. Thus, state
water entitlements do not affect our analysis with respect to
the creation of the Tribe’s federally reserved water right.

                              IV

    In sum, the Winters doctrine does not distinguish
between surface water and groundwater. Rather, its limits
derive only from the government’s intent in withdrawing
land for a public purpose and the location of the water in
relation to the reservation created. As such, because the
United States intended to reserve water when it established
a home for the Agua Caliente Band of Cahuilla Indians, we
hold that the district court did not err in determining that the
government reserved appurtenant water sources—including
groundwater—when it created the Tribe’s reservation in the
Coachella Valley.

    Finally, we recognize that the district court’s failure to
conduct a thorough New Mexico analysis with respect to
whether the Tribe needs access to groundwater was largely
a function of the parties’ decision to trifurcate this case. We
also understand that a full analysis specifying the scope of
the water reserved under New Mexico will be considered in
the subsequent phases of this litigation.

    Presumably, however, the water agencies will continue
to argue in these later phases that the Winters doctrine is
dependent upon the Tribe’s demonstrated need—that is,
need above and beyond what the Tribe is already receiving
under state-law entitlements or could receive through a
paramount surface water right. And while we express no
opinion on how much water falls within the scope of the
Tribe’s federal groundwater right, there can be no question
22              AGUA CALIENTE BAND V.
             COACHELLA VALLEY WATER DIST.

that water in some amount was necessarily reserved to
support the reservation created. Thus, to guide the district
court in its later analysis, we hold that the creation of the
Agua Caliente Reservation carried with it an implied right to
use water from the Coachella Valley aquifer.

     Each party shall bear its own costs.

     AFFIRMED.
