Mississippi v. Tennessee

P R E L I M I N A R Y P R I N T
Volume 595 U. S. Part 1
Pages 15–29
OFFICIAL REPORTS
OF
T H E S U P R E M E C O U R T
November 22, 2021
REBECCA A. WOMELDORF
reporter of decisions
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N O T I C E : This preliminary print is subject to formal revision before
the bound volume is published. Users are requested to notify the Reporter
of Decisions, Supreme Court of the United States, Washington, D.C. 20543,
pio@supremecourt.gov, of any typographical or other formal errors.

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OCTOBER
TERM, 2021
15
Syllabus
MISSISSIPPI v. TENNESSEE et al.
on
exceptions to report of special master
No. 143, Orig. Argued October 4, 2021—Decided November 22, 2021
Mississippi brought an original action against Tennessee for damages and
other relief related to the pumping of groundwater by the City of Mem-
phis from the Middle Claiborne Aquifer, a valuable water resource that
lies beneath eight States. Mississippi argues that Tennessee's pump-
ing—using wells Mississippi concedes are located entirely in Tennes-
see—siphons water away from Mississippi and amounts to a tortious
taking of groundwater owned by Mississippi. Mississippi expressly dis-
claims any equitable apportionment remedy, arguing that the “funda-
mental premise of this Court's equitable apportionment jurisprudence—
that each of the opposing States has an equality of right to use the
waters at issue—does not apply to this dispute.” Complaint ¶49. The
Special Master appointed by the Court to assess Mississippi's claims
determined that the aquifer is an interstate water resource and that
equitable apportionment is the exclusive judicial remedy. Because Mis-
sissippi's complaint did not seek equitable apportionment, the Special
Master recommended that the Court dismiss the complaint but grant
Mississippi leave to amend. Mississippi challenges the recommendation
to dismiss; Tennessee objects to the recommendation to grant Missis-
sippi leave to fle an amended complaint.
Held: The waters of the Middle Claiborne Aquifer are subject to the judi-
cial remedy of equitable apportionment; Mississippi's complaint is dis-
missed without leave to amend. Pp. 23–29.
(a) The doctrine of equitable apportionment aims to produce a fair
allocation of a shared water resource between two or more States, see
Colorado v. New Mexico, 459 U. S. 176, 183, based on the principle that
States have an equal right to reasonable use of shared water resources.
Florida v. Georgia, 592 U. S. –––, –––. The Court has applied the doc-
trine to interstate rivers and streams, see South Carolina v. North
Carolina, 558 U. S. 256, to disputes over interstate river basins, see
Florida v. Georgia, 585 U. S. –––, –––, and in situations where the pump-
ing of groundwater has affected the fow of interstate surface waters,
see Nebraska v. Wyoming, 515 U. S. 1, 14. The Court has also applied
the doctrine to anadromous fsh that migrate between the Pacifc Ocean
and spawning grounds in the Columbia-Snake River system, “travel[ing]
through several States during their lifetime.” Idaho ex rel. Evans v.
Oregon, 462 U. S. 1017, 1018–1019, 1024.

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16 MISSISSIPPI
v. TENNESSEE
Syllabus
The Court has not before addressed whether equitable apportionment
appl
ies to interstate aquifers. Equitable apportionment of the Middle
Claiborne Aquifer is “suffciently similar” to past applications of the doc-
trine to warrant the same treatment, for several reasons. Id., at 1024.
First, the Court has applied equitable apportionment when transbound-
ary water resources were at issue. Here the Middle Claiborne Aqui-
fer's “multistate character” seems beyond dispute. Sporhase v. Ne-
braska ex rel. Do ugl as, 458 U. S. 941, 953. Second, the Middle
Claiborne Aquifer contains water that fows naturally between the
States, and the Court's equitable apportionment cases have all con-
cerned such water, Kansas v. Colorado, 206 U. S. 46, 98, or fsh that live
in it, Idaho ex rel. Evans, 462 U. S., at 1024. While Mississippi con-
tends the natural transboundary fow of the Middle Claiborne Aquifer
is slower than some streams and rivers, the Court has applied equitable
apportionment even to streams that run dry from time to time. See
Kansas, 206 U. S., at 115. The speed of the fow does not place the
aquifer beyond equitable apportionment. Finally, actions taken in Ten-
nessee to pump water from the aquifer clearly have effects on the por-
tion of the aquifer that underlies Mississippi. Tennessee's pumping has
contributed to a cone of depression that extends miles into northern
Mississippi, and Mississippi itself contends that this cone of depression
has reduced groundwater storage and pressure in northern Mississippi.
Such interstate effects are a hallmark of the Court's equitable appor-
tionment cases, see, e. g., Florida, 592 U. S., at –––. For all these rea-
sons, the Court holds that the judicial remedy of equitable apportion-
ment applies to the waters of the Middle Claiborne Aquifer. Pp. 23–26.
(b) The Court rejects Mississippi's contention that it has a sovereign
ownership right to all water beneath its surface that precludes applica-
tion of equitable apportionment. The Court has “consistently denied”
the proposition that a State may exercise exclusive ownership or control
of interstate “waters fowing within her boundaries.” Hinderlider v.
La Plata River & Cherry Creek Ditch Co., 304 U. S. 92, 102. Although
the Court's past equitable apportionment cases have generally con-
cerned streams and rivers, no basis exists for a different result in the
context of the Middle Claiborne Aquifer. To the contrary, Mississippi's
ownership approach would allow an upstream State to completely cut
off fow to a downstream one, a result contrary to the Court's equitable
apportionment jurisprudence. The Court's decision in Tarrant Re-
gional Water Dist. v. Herrmann, 569 U. S. 614, does not support Missis-
sippi's position. Tarrant concerned whether one State could cross
another's boundaries to access a shared water resource under the terms
of an interstate compact. The Court did not consider equitable appor-

Cite
as: 595 U. S. 15 (2021)
17
Syllabus
tionment, because the affected States had negotiated a compact that
deter
mined their respective rights to the resource. To the extent Tar-
rant stands for the broader proposition that one State may not physi-
cally enter another to take water in the absence of an express agree-
ment, that principle is not implicated here. The parties have stipulated
all of Tennessee's wells are drilled straight down and do not cross the
Mississippi-Tennessee border. While the origin of an interstate water
resource may be relevant to the terms of an equitable apportionment,
that feature alone cannot place the resource outside the doctrine itself.
Because the waters contained in the Middle Claiborne Aquifer are sub-
ject to equitable apportionment, the Court overrules Mississippi's ex-
ceptions and adopts the Special Master's recommendation to dismiss the
bill of complaint. Pp. 26–27.
(c) Mississippi has neither sought leave to amend its complaint nor
tendered a proposed complaint seeking equitable apportionment. The
Court does not address whether Mississippi should be granted such
leave and sustains Tennessee's objection to the Special Master's recom-
mendation to grant Mississippi leave to amend. Pp. 27–28.
Exceptions overruled in part and sustained in part, and case dismissed.
Roberts, C. J., delivered the opinion for a unanimous Court.
John V. Coghlan, Deputy Solicitor General of Mississippi,
argued the cause for plaintiff. With him on the briefs were
Lynn Fitch, Attorney General of Mississippi, C. Michael El-
lingburg, Ta'Shia S. Gordon, Larry D. Moffett, William J.
Harbison II, John W. “Don” Barrett, David M. McMullan,
Jr., George B. Ready, and Edward C. Taylor.
David C. Frederick argued the cause for defendant Ten-
nessee. With him on the briefs were Herbert H. Slatery III,
Attorney General of Tennessee, Andrée Sophia Blumstein,
Solicitor General, Barry Turner, Deputy Attorney General,
Sohnia W. Hong, David L. Bearman, Kristine L. Roberts,
and Cheryl W. Patterson. Mr. Bearman, Ms. Roberts, and
Ms. Patterson fled a brief for defendants City of Memphis,
Tennessee, et al.
Frederick Liu argued the cause for the United States as
amicus curiae in support of overruling plaintiff 's exceptions.
With him on the brief were Acting Solicitor General Prelo-
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18 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
gar, Acting Assistant Attorney General Williams, Deputy
So
licitor General Kneedler, and Judith E. Coleman.*
Chief Justice Roberts delivered the opinion of the
Court.
The City of Memphis sits on the banks of the Mississippi
River in the southwest corner of Tennessee. Arkansas
marks the City's western border, and Mississippi its south-
ern. Hundreds of feet beneath Memphis lies one of the
City's most valuable resources: the Middle Claiborne Aquifer.
Workers discovered the aquifer in 1886 while drilling a well
for the Bohlen-Huse Ice Company. Ever since, water
pumped from the aquifer has provided Memphis with an
abundant supply of clean, affordable drinking water.
The Middle Claiborne Aquifer underlies other States too,
including Mississippi. This case began in 2014 when Missis-
sippi invoked our original jurisdiction and sought leave to
fle a bill of complaint against Tennessee. Mississippi al-
leges that Tennessee's pumping has taken hundreds of bil-
lions of gallons of water that were once located beneath Mis-
sissippi. It seeks at least $615 million in damages, as well
as declaratory and injunctive relief. We granted Mississippi
leave to fle its complaint and appointed a Special Master to
oversee proceedings. The Special Master has now issued
his report, which recommends that this Court dismiss Missis-
*Briefs of amici curiae were fled for the State of Colorado et al. by
Philip J. Weiser, Attorney General of Colorado, Lain Leoniak, First As-
sistant Attorney General, and Preston V. Hartman, Katherine Duncan,
and Emily Halvorsen, Assistant Attorneys General, and by the Attorneys
General for their respective States as follows: Lawrence G. Wasden of
Idaho, Doug Peterson of Nebraska, Josh Stein of North Carolina, Wayne
Steneh jem of North Dakota, Ellen F. Rosenblum of Oregon, Jason R.
Ravnsborg of South Dakota, and Bridget Hill of Wyoming; for the Inter-
national Law Committee of the New York City Bar Association by Mat-
thew E. Draper, John B. Draper, and Corinne E. Atton; and for Law Pro-
fessors by Noah D. Hall, Jesse J. Richardson, Jr., Robert H. Abrams, and
Burke W. Griggs, all pro se.
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as: 595 U. S. 15 (2021)
19
Opinion of the Court
sippi's complaint with leave to amend. Both Mississippi and
T
ennessee have fled exceptions.
I
A
Layers of rock, clay, silt, sand, and gravel exist below the
Earth's surface. Groundwater percolates through the
spaces in and around these materials, sometimes forming un-
derground reservoirs of water known as aquifers. Some aq-
uifers are small, while others span tens of thousands of
square miles. The Middle Claiborne Aquifer is one of the
latter. It underlies portions of eight States in the Missis-
sippi River Basin: Alabama, Arkansas, Illinois, Kentucky,
Louisiana, Mississippi, Missouri, and Tennessee. See Re-
port of Special Master 16; Hearing Tr. 278–279. Many of
these States, including Mississippi and Tennessee, draw sig-
nifcant amounts of groundwater from the aquifer. Id., at
660–662, 1038–1040; Joint Exh. J–71.
To extract water from an aquifer, people drill wells.
Pumps then draw water to the surface, where it is processed
and piped to customers. Pumping does not just bring water
to the surface; it also lowers water pressure at the site of
the well. Water is naturally drawn to this area of lower
pressure. This, in turn, “causes a pattern of lower or de-
pressed water levels around the wells.” Report of Special
Master 13. Hydrogeologists call such areas “cones of de-
pression.” These cones of depression can be local—say, the
size of a backyard. Or they can be regional, stretching out
for many miles from a pumping site. See id., at 21–23; Hear-
ing Tr. 176, 188, 435.
The City of Memphis, through its public utility, the Mem-
phis Light, Gas and Water Division (MLGW), pumps approx-
imately 120 million gallons of groundwater from the Middle
Claiborne Aquifer each day. Id., at 186, 200; Pl. Exh. P–157.
It does so using more than 160 wells located in and around
Memphis. Joint Statement of Stipulated and Contested
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20 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
Facts 101. Some of these wells are located just a few miles
from
the Mississippi-Tennessee border, though all are drilled
straight down such that none crosses the physical border be-
tween the States. Id., at 101–102, 106. MLGW's pumping
contributes to a cone of depression that underlies both the
City of Memphis and DeSoto County, Mississippi. Report of
Special Master 21–23; Hearing Tr. 206, 435–436, 525.
Mississippi argues that MLGW's pumping has altered the
historic fow of groundwater within the Middle Claiborne
Aquifer. Mississippi concedes that some water naturally
fows from the part of the aquifer beneath Mississippi to the
part beneath Tennessee. But only to the extent of some 30
to 60 feet per year. See Exceptions Brief for Mississippi 8;
see also Report of Special Master 24 (“Mississippi does not
dispute the expert consensus that at least some quantity of
groundwater . . . crossed the border under natural condi-
tions.”). Mississippi contends that MLGW's pumping has
substantially hastened this existing fow, allowing Memphis
to take billions of gallons of groundwater that otherwise
would have remained under Mississippi for thousands of
years.
B
In 2005, prior to the present litigation, the Attorney Gen-
eral of Mississippi sued the City of Memphis and MLGW in
Federal District Court. The suit alleged that Memphis had
wrongfully appropriated groundwater belonging to Missis-
sippi through its pumping activities. Mississippi sought
hundreds of millions of dollars in damages.
The District Court dismissed the suit for failure to join
Tennessee, which it determined was an indispensable party.
Hood ex rel. Miss. v. Memphis, 533 F. Supp. 2d 646, 651 (ND
Miss. 2008). The Fifth Circuit then affrmed. Hood ex rel.
Miss. v. Memphis, 570 F. 3d 625 (2009).
Both decisions turned in large part on what is known as
“equitable apportionment.” Under that doctrine, this Court
allocates rights to a disputed interstate water resource after
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as: 595 U. S. 15 (2021)
21
Opinion of the Court
one State sues another under our original jurisdiction. See
Kan
sas v. Colorado, 206 U. S. 46, 97–98 (1907). Tradition-
ally, equitable apportionment has been the exclusive judicial
remedy for interstate water disputes, unless a statute, com-
pact, or prior apportionment controls. This Court has never
before held that an interstate aquifer is subject to equitable
apportionment, so Mississippi's suit implicated a question of
frst impression.
The Court of Appeals, affrming the District Court, held
that interstate aquifers are comparable to interstate rivers
and are thus subject to equitable apportionment. It rea-
soned that an aquifer “fows, if slowly.” Hood ex rel. Miss.,
570 F. 3d, at 630. And it said the fact that an aquifer is
“located underground, as opposed to resting above ground,”
was of “no analytical signifcance.” Ibid. Because deter-
mining “Mississippi and Tennessee's relative rights to the
Aquifer” brought the case within the equitable apportion-
ment doctrine, the Court of Appeals affrmed the District
Court's holding that Tennessee was an indispensable party.
Id., at 630–631; see also Fed. Rule Civ. Proc. 19(a). Joinder
of Tennessee in the lower federal courts was not possible,
however, because this Court has exclusive jurisdiction over
suits between States. See U. S. Const., Art. III, § 2; 28
U. S. C. § 1251(a). So the Fifth Circuit held that the District
Court had properly dismissed the suit. Hood ex rel. Miss.,
570 F. 3d, at 632–633; Fed. Rule Civ. Proc. 19(b).
Mississippi then petitioned for a writ of certiorari. It also
sought leave to fle a bill of complaint against Tennessee,
Memphis, and MLGW under our original jurisdiction. The
proposed complaint requested over $1 billion in damages for
the alleged taking of Mississippi's water. In the alternative,
it sought equitable apportionment of the aquifer, with a dam-
ages award for past diversions of groundwater. We declined
to grant certiorari, 559 U. S. 904 (2010), and denied without
prejudice Mississippi's request for leave to fle a bill of com-
plaint, 559 U. S. 901 (2010).
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22 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
C
In
2014, Mississippi again sought leave from this Court to
fle a bill of complaint against Tennessee, Memphis, and
MLGW. That is the basis of this suit. Mississippi's com-
plaint alleges that MLGW “has forcibly siphoned into Ten-
nessee hundreds of billions of gallons of high quality ground-
water owned by Mississippi.” Complaint ¶23. It says that
MLGW's “mechanical pumping ” is to blame and that the
“groundwater taken by Defendants from within Mississippi's
borders would have never under normal, natural circum-
stances been drawn into Tennessee.” Id., ¶24. This
“wrongful taking,” the State contends, “is evidenced by a
substantial drop in pressure and corresponding drawdown of
stored groundwater” in northwest Mississippi, and by a cone
of depression extending miles into its territory. Id., ¶25.
As a result, Mississippi says, it has to drill its own wells
deeper to access the aquifer, and use more electricity to
pump water to the surface. Id., ¶54(b).
Mississippi claims an absolute “ownership” right to all
groundwater beneath its surface—even after that water has
crossed its borders. See id., ¶¶8–12, 39. It argues that
Tennessee's pumping thus amounts to a tortious taking of
property, and it seeks at least $615 million in damages. See
id., ¶¶55–56. Mississippi expressly disclaims equitable ap-
portionment, arguing that the “fundamental premise of this
Court's equitable apportionment jurisprudence—that each of
the opposing States has an equality of right to use the waters
at issue—does not apply to this dispute.” Id., ¶49.
We granted Mississippi leave to fle its complaint and ap-
pointed Judge Eugene E. Siler, Jr., of the Sixth Circuit to
serve as Special Master. He has ably discharged his duties.
Following motions practice, discovery, and a fve-day eviden-
tiary hearing, the Special Master issued a report recom-
mending that we dismiss Mississippi's complaint, but with
leave to amend. He frst determined that the Middle Clai-
borne Aquifer is an interstate water resource. He found
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23
Opinion of the Court
that the aquifer is a “single hydrogeological unit,” that Ten-
nessee'
s pumping affects groundwater beneath Mississippi,
and that prior to such pumping, “groundwater fowed be-
tween Mississippi and Tennessee”—a fact Mississippi “does
not dispute.” Report of Special Master 15–24.
The Special Master then concluded that, because the aqui-
fer is an interstate water resource, equitable apportionment
is the appropriate remedy. He recognized that equitable ap-
portionment “stands alone as the federal common-law princi-
ple for disputes over interstate water,” id., at 31, and saw
“no compelling reason to chart a new path for groundwater
resources,” id., at 26. Because Mississippi's complaint did
not seek equitable apportionment, the Special Master recom-
mends that we dismiss it. But he also recommends that we
grant Mississippi leave to fle an amended complaint seeking
equitable apportionment, though the State has not yet
sought such leave.
Mississippi and Tennessee both fled exceptions to the Spe-
cial Master's report. Mississippi objects to the Special Mas-
ter's recommendation that we dismiss the suit. It argues
that the Special Master erred in fnding the water in the
Middle Claiborne Aquifer subject to equitable apportion-
ment. Tennessee objects only to the Special Master's rec-
ommendation that we grant Mississippi leave to amend its
complaint.
We “conduct an independent review of the record, and as-
sume the ultimate responsibility for deciding all matters.”
Kansas v. Nebraska, 574 U. S. 445, 453 (2015) (internal quo-
tation marks omitted). Based on that review, we dismiss
Mississippi's complaint and decline to grant leave to amend.
II
A
Equitable apportionment aims to produce a fair allocation
of a shared water resource between two or more States.
See Colorado v. New Mexico, 459 U. S. 176, 183 (1982). The
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24 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
doctrine's “guiding principle” is that States “have an equal
r
ight to make a reasonable use” of a shared water resource.
Florida v. Georgia, 592 U. S. –––, ––– (2021) (internal quota-
tion marks omitted).
We pioneered the doctrine in Kansas v. Colorado, 206
U. S. 46. Since then, we have often applied it to interstate
rivers and streams. See South Carolina v. North Carolina,
558 U. S. 256 (2010); Colorado v. New Mexico, 459 U. S. 176;
Nebraska v. Wyoming, 325 U. S. 589 (1945); Wyoming v. Col-
orado, 259 U. S. 419 (1922). We have also applied the doc-
trine to disputes over interstate river basins, see Florida v.
Georgia, 585 U. S. –––, ––– – ––– (2018), and in situations
where the pumping of groundwater has affected the fow of
interstate surface waters, see Nebraska v. Wyoming, 515
U. S. 1, 14 (1995). We have even applied the doctrine to
anadromous fsh—such as Chinook salmon and steelhead
trout—that migrate between the Pacifc Ocean and spawning
grounds in the Columbia-Snake River system, “travel[ing]
through several States during their lifetime.” See Idaho ex
rel. Evans v. Oregon, 462 U. S. 1017, 1018–1019, 1024 (1983).
Mississippi correctly observes that we have never consid-
ered whether equitable apportionment applies to interstate
aquifers. See Exceptions Brief for Mississippi 28. Con-
fronted as we are with this matter of frst impression, we
resist general propositions and focus our analysis on whether
equitable apportionment of the Middle Claiborne Aquifer
would be “suffciently similar” to past applications of the doc-
trine to warrant the same treatment. Idaho ex rel. Evans,
462 U. S., at 1024 (adopting such an approach in extending
the doctrine to anadromous fsh). We conclude that it
would.
First, we have applied equitable apportionment only when
transboundary resources were at issue. See Virginia v.
Maryland, 540 U. S. 56, 74, n. 9 (2003); Colorado v. New Mex-
ico, 459 U. S., at 183. The Middle Claiborne Aquifer's
“multistate character” seems beyond dispute. See Sporhase
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25
Opinion of the Court
v. Nebraska ex rel. Douglas, 458 U. S. 941, 953 (1982). Mis-
sissippi
concedes that the “geologic formation in which the
groundwater is stored straddles two states.” Complaint
¶41. Indeed, a core premise of Mississippi's suit is that Ten-
nessee is pumping water that was once in Mississippi. The
evidence shows that wells in Memphis and wells in north-
west Mississippi are “pumping from the same aquifer.”
Hearing Tr. 492; see Report of Special Master 20 (noting that
the “scientifc consensus holds that the Middle Claiborne Aq-
uifer is a single hydrogeological unit” spanning multiple
States).
Also pertinent is that the Middle Claiborne Aquifer con-
tains water that fows naturally between the States. All of
our equitable apportionment cases have concerned such
water, Kansas v. Colorado, 206 U. S., at 98, or fsh that live
in it, Idaho ex rel. Evans, 462 U. S., at 1024. Mississippi
suggests the Middle Claiborne Aquifer is distinguishable
from interstate rivers and streams because its natural fow
is “extremely slow.” Exceptions Brief for Mississippi 8.
But we have long applied equitable apportionment even to
streams that run dry from time to time. See Kansas v. Col-
orado, 206 U. S., at 115. And although the transboundary
fow here may be a mere “one or two inches per day,” Excep-
tions Brief for Mississippi 8, that amounts to over 35 million
gallons of water per day, and over ten billion gallons per
year, see Hearing Tr. 532–533. So the speed of the fow, at
least in the context of this case, does not place the aquifer
beyond equitable apportionment.
Finally, it is clear that actions in Tennessee “reach[ ]
through the agency of natural laws” to affect the portion of
the aquifer that underlies Mississippi. Kansas v. Colorado,
206 U. S., at 97. Tennessee's pumping has contributed to a
cone of depression that extends miles into northern Missis-
sippi. Hearing Tr. 484–485, 501–502, 926; see also Report of
Special Master 22–23. Mississippi itself contends that this
cone of depression has reduced groundwater storage and
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26 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
pressure in northern Mississippi. See Complaint ¶¶25, 54.
It
also alleges that Tennessee's pumping is “siphoning ” tens
of millions of gallons of groundwater each day from Missis-
sippi's portion of the aquifer. Id., ¶54. Such interstate ef-
fects are a hallmark of our equitable apportionment cases.
See, e. g., Florida v. Georgia, 592 U. S., at ––– – ––– (2021)
(examining the effects of Georgia's water use on Florida's
oyster fsheries and river ecosystem).
For these reasons, we hold that the waters of the Middle
Claiborne Aquifer are subject to the judicial remedy of equi-
table apportionment.
B
Mississippi contends that it has sovereign ownership of all
groundwater beneath its surface, so equitable apportionment
ought not apply. We see things differently. It is certainly
true that “each State has full jurisdiction over the lands
within its borders, including the beds of streams and other
waters.” Kansas v. Colorado, 206 U. S., at 93. But such
jurisdiction does not confer unfettered “ownership or con-
trol” of fowing interstate waters themselves. Wyoming v.
Colorado, 259 U. S., at 464. Thus, we have “consistently de-
nied” the proposition that a State may exercise exclusive
ownership or control of interstate “waters fowing within her
boundaries.” Hinderlider v. La Plata River & Cherry
Creek Ditch Co., 304 U. S. 92, 102 (1938). Although our past
cases have generally concerned streams and rivers, we see
no basis for a different result in the context of the Middle
Claiborne Aquifer. When a water resource is shared be-
tween several States, each one “has an interest which should
be respected by the other.” Wyoming v. Colorado, 259
U. S., at 466. Mississippi's ownership approach would allow
an upstream State to completely cut off fow to a downstream
one, a resu lt contrary to our equit able appor ti onment
jurisprudence.
Mississippi argues that our decision in Tarrant Regional
Water Dist. v. Herrmann, 569 U. S. 614 (2013), supports
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27
Opinion of the Court
its position. We disagree. Tarrant concerned the inter-
pret
ation of an interstate compact. We held that the
compact did not authorize the party States to “cross each
other's boundaries to access a shared pool of water.” Id., at
627. Our decision turned on the language of the compact
and background principles of contract law. We did not con-
sider equitable apportionment, because the affected States
had taken it upon themselves to negotiate a compact that
deter mi ned their respec tive r ights to the resource i n
question.
To the extent Tarrant stands for the broader proposition
that one State may not physically enter another to take
water in the absence of an express agreement, that principle
is not implicated here. The parties have stipulated that all
of Tennessee's wells are drilled straight down and do not
cross the Mississippi-Tennessee border. See Joint State-
ment of Stipulated and Contested Facts 106. When Tennes-
see pumps groundwater, it is pumping water located within
its own territory. That some of the water was previously
located in Mississippi is of no moment, just as it was not
dispositive that the river at issue in Colorado v. New Mexico
started in Colorado, 459 U. S., at 181, n. 8, or that certain
fsh at issue in Idaho ex rel. Evans hatched in Idaho, 462
U. S., at 1028, n. 12. The origin of an interstate water re-
source may be relevant to the terms of an equitable appor-
tionment. But that feature alone cannot place the resource
outside the doctrine itself.
We conclude that the waters contained in the Middle Clai-
borne Aquifer are subject to equitable apportionment. We
therefore overrule Mississippi's exceptions and adopt the
Special Master's recommendation to dismiss the bill of
complaint.
III
After recommending that this Court dismiss Mississippi's
suit, the Special Master went on to recommend that we grant
the State leave to fle an amended complaint seeking equita-
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28 MISSISSIPPI
v. TENNESSEE
Opinion of the Court
ble apportionment. We decline to decide whether Missis-
sippi
should be granted such leave, because the State has
never sought it. As Mississippi itself emphasi zes—
literally—it has “not yet requested equitable apportion-
ment.” Reply Brief to Defendants' Exception 7.
Nor can we assume Mississippi will do so. Mississippi's
initial pleadings in this case disavowed equitable apportion-
ment entirely. See Complaint ¶38 (“This case does not fall
within the Court's equitable apportionment jurisprudence.”).
The State instead sought relief under principles of tort law.
And the proceedings below refected this posture, focusing
on the physical properties of the aquifer and Tennessee's
pumping. An equitable apportionment case would require
us to consider a broader range of evidence. Such evidence
often includes not only the physical properties and fow of
a water resource, but also existing uses, the availability of
alternatives, practical effects, and the costs and benefts to
the States involved. See Florida v. Georgia, 585 U. S.,
at ––– – ––– (2018); South Carolina, 558 U. S., at 271–272;
Colorado v. New Mexico, 459 U. S., at 183, 186–187. A just
equitable apportionment might also require the joinder of
additional parties, cf. Fed. Rule Civ. Proc. 19(a), as Missis-
sippi and Tennessee are not the only States that rely on the
Middle Claiborne Aquifer for groundwater, see Hearing Tr.
660–662, 1038–1040; Joint Exh. J–71. Finally, if Mississippi
were to seek leave to amend, its complaint would be subject
to our longstanding rule that a “State seeking equitable
apportionment under our original jurisdiction must prove
by clear and convincing evidence some real and substan-
tial injury or damage.” Idaho ex rel. Evans, 462 U. S.,
at 1027.
As Mississippi has neither sought leave to amend nor ten-
dered a proposed complaint seeking equitable apportion-
ment, we have no occasion to determine how these and other
pertinent principles might apply.
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Cite
as: 595 U. S. 15 (2021)
29
Opinion of the Court
***
Mississippi
has failed to show that it is entitled to relief.
We therefore overrule Mississippi's exceptions to the Special
Master's report, sustain Tennessee's, and dismiss the case.
It is so ordered.
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Reporter’s Note
The attached opinion has been revised to refect the usual publication
and citation style of the United States Reports. The revised pagination
makes available the offcial United States Reports citation in advance of
publication. The syllabus has been prepared by the Reporter of Decisions
for the convenience of the reader and constitutes no part of the opinion of
the Court. A list of counsel who argued or fled briefs in this case, and
who were members of the bar of this Court at the time this case was
argued, has been inserted following the syllabus. Other revisions may
include adjustments to formatting, captions, citation form, and any errant
punctuation. The following additional edits were made:
None