(Slip Opinion) OCTOBER TERM, 2021 1 

Syllabus 

NOTE: Where it is feasible, a syllabus (headnote) will be released, as is 

being done in connection with this case, at the time the opinion is issued. 

The syllabus constitutes no part of the opinion of the Court but has been 

prepared by the Reporter of Decisions for the convenience of the reader. 

See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337. 

SUPREME COURT OF THE UNITED STATES 

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 

Memphis from the Middle Claiborne Aquifer, a valuable water resource that lies beneath eight States. Mississippi argues that Tennessee’s pumping—using wells Mississippi concedes are located entirely 

in Tennessee—siphons water away from Mississippi and amounts to a

tortious taking of groundwater owned by Mississippi. Mississippi expressly disclaims any equitable apportionment remedy, 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.” 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 Mississippi’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 Mississippi leave to file an amended complaint. 

Held: The waters of the Middle Claiborne Aquifer are subject to the judicial remedy of equitable apportionment; Mississippi’s complaint is 

dismissed without leave to amend. Pp. 7–12. 

(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 doctrine

to interstate rivers and streams, see South Carolina v. North Carolina, 
 

 

 

 

 

 

 

MISSISSIPPI v. TENNESSEE 

Syllabus 

558 U. S. 256, to disputes over interstate river basins, see Florida v. 

Georgia, 585 U. S. __, __, and in situations where the pumping of 

groundwater has affected the flow of interstate surface waters, see Nebraska v. Wyoming, 515 U. S. 1, 14. The Court has also applied the 

doctrine to anadromous fish that migrate between the Pacific 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. 

The Court has not before addressed whether equitable apportionment applies to interstate aquifers. Equitable apportionment of the 

Middle Claiborne Aquifer is “sufficiently similar” to past applications

of the doctrine to warrant the same treatment, for several reasons. Id., 

at 1024. First, the Court has applied equitable apportionment when 

transboundary water resources were at issue. Here the Middle 

Claiborne Aquifer’s “multistate character” seems beyond dispute. 

Sporhase v. Nebraska ex rel. Douglas, 458 U. S. 941, 953. Second, the 

Middle Claiborne Aquifer contains water that flows naturally between 

the States, and the Court’s equitable apportionment cases have all concerned such water, Kansas v. Colorado, 206 U. S. 46, 98, or fish that 

live in it, Idaho ex rel. Evans, 462 U. S., at 1024. While Mississippi

contends the natural transboundary flow 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 flow does not 

place the aquifer beyond equitable apportionment. Finally, actions

taken in Tennessee to pump water from the aquifer clearly have effects 

on the portion 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 apportionment cases, see, e.g., Florida, 592 U. S., at 

__. For all these reasons, the Court holds that the judicial remedy of 

equitable apportionment applies to the waters of the Middle Claiborne 

Aquifer. Pp. 7–9. 

(b) The Court rejects Mississippi’s contention that it has a sovereign 

ownership right to all water beneath its surface that precludes application of equitable apportionment. The Court has “consistently denied” the proposition that a State may exercise exclusive ownership or 

control of interstate “waters flowing 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 concerned streams and rivers, no basis exists for a different result 
 

 

Cite as: 595 U. S. ____ (2021) 3 

Syllabus 

in the context of the Middle Claiborne Aquifer. To the contrary, Mississippi’s ownership approach would allow an upstream State to completely cut off flow to a downstream one, a result contrary to the 

Court’s equitable apportionment jurisprudence. The Court’s decision 

in Tarrant Regional Water Dist. v. Herrmann, 569 U. S. 614, does not 

support Mississippi’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 apportionment, because the affected States had negotiated a

compact that determined their respective rights to the resource. 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 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 subject to equitable apportionment, the Court 

overrules Mississippi’s exceptions and adopts the Special Master’s recommendation to dismiss the bill of complaint. Pp. 9–11.

(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 recommendation to grant Mississippi leave to amend. Pp. 11–12. 

Exceptions overruled in part and sustained in part, and case dismissed.

 ROBERTS, C. J., delivered the opinion for a unanimous Court. 
 

 

 

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Cite as: 595 U. S. ____ (2021) 1 

Opinion of the Court 

NOTICE: This opinion is subject to formal revision before publication in the 

preliminary print of the United States Reports. Readers are requested to 

notify the Reporter of Decisions, Supreme Court of the United States, Washington, D. C. 20543, of any typographical or other formal errors, in order that 

corrections may be made before the preliminary print goes to press. 

SUPREME COURT OF THE UNITED STATES 

No. 143, Orig. 

STATE OF MISSISSIPPI, PLAINTIFF v. TENNESSEE, 

CITY OF MEMPHIS, TENNESSEE, AND MEMPHIS 

LIGHT, GAS & WATER DIVISION 

ON EXCEPTIONS TO REPORT OF SPECIAL MASTER 

[November 22, 2021]

 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 southern. 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 Mississippi invoked our original jurisdiction and sought leave

to file a bill of complaint against Tennessee. Mississippi 

alleges that Tennessee’s pumping has taken hundreds of 

billions of gallons of water that were once located beneath 

Mississippi. It seeks at least $615 million in damages, as 

well as declaratory and injunctive relief. We granted Mississippi leave to file its complaint and appointed a Special 

Master to oversee proceedings. The Special Master has now 
 

2 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

issued his report, which recommends that this Court dismiss Mississippi’s complaint with leave to amend. Both 

Mississippi and Tennessee have filed 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 

underground reservoirs of water known as aquifers. Some 

aquifers 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 Mississippi River Basin: Alabama, Arkansas, Illinois, Kentucky,

Louisiana, Mississippi, Missouri, and Tennessee. See Report of Special Master 16; Hearing Tr. 278–279. Many of 

these States, including Mississippi and Tennessee, draw 

significant 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 depressed water levels around the wells.” Report of Special

Master 13. Hydrogeologists call such areas “cones of depression.” 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; 

Hearing Tr. 176, 188, 435. 

The City of Memphis, through its public utility, the Memphis Light, Gas and Water Division (MLGW), pumps approximately 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 
 

 

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Opinion of the Court 

in and around Memphis. Joint Statement of Stipulated and 

Contested 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 between 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 flow of groundwater within the Middle 

Claiborne Aquifer. Mississippi concedes that some water 

naturally flows 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 conditions.”). Mississippi contends that MLGW’s

pumping has substantially hastened this existing flow, 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 General of Mississippi sued the City of Memphis and MLGW in 

Federal District Court. The suit alleged that Memphis had 

wrongfully appropriated groundwater belonging to Mississippi 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 affirmed. 

Hood ex rel. Miss. v. Memphis, 570 F. 3d 625 (2009). 
 

 

4 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

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 one State sues another under our original jurisdiction. 

See Kansas v. Colorado, 206 U. S. 46, 97–98 (1907). Traditionally, equitable apportionment has been the exclusive judicial remedy for interstate water disputes, unless a statute, compact, 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 first impression.

The Court of Appeals, affirming the District Court, held

that interstate aquifers are comparable to interstate rivers 

and are thus subject to equitable apportionment. It reasoned that an aquifer “flows, 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 significance.” Ibid. Because determining “Mississippi and Tennessee’s relative rights to the 

Aquifer” brought the case within the equitable apportionment doctrine, the Court of Appeals affirmed 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 file 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 damages award for past diversions of groundwater. We 
 

Cite as: 595 U. S. ____ (2021) 5 

Opinion of the Court 

declined to grant certiorari, 559 U. S. 904 (2010), and denied without prejudice Mississippi’s request for leave to file

a bill of complaint, 559 U. S. 901 (2010). 

C 

In 2014, Mississippi again sought leave from this Court

to file a bill of complaint against Tennessee, Memphis, and 

MLGW. That is the basis of this suit. Mississippi’s complaint alleges that MLGW “has forcibly siphoned into Tennessee hundreds of billions of gallons of high quality 

groundwater 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 circumstances 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 apportionment, 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 file its complaint and appointed Judge Eugene E. Siler, Jr., of the Sixth Circuit to

serve as Special Master. He has ably discharged his duties. 
 

 

6 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

Following motions practice, discovery, and a five-day evidentiary hearing, the Special Master issued a report recommending that we dismiss Mississippi’s complaint, but with

leave to amend. He first determined that the Middle 

Claiborne Aquifer is an interstate water resource. He found 

that the aquifer is a “single hydrogeological unit,” that Tennessee’s pumping affects groundwater beneath Mississippi, 

and that prior to such pumping, “groundwater flowed between Mississippi and Tennessee”—a fact Mississippi “does 

not dispute.” Report of Special Master 15–24.

The Special Master then concluded that, because the aquifer is an interstate water resource, equitable apportionment is the appropriate remedy. He recognized that equitable apportionment “stands alone as the federal commonlaw principle 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 recommends that we dismiss it. But he also 

recommends that we grant Mississippi leave to file an

amended complaint seeking equitable apportionment, 

though the State has not yet sought such leave.

Mississippi and Tennessee both filed exceptions to the 

Special Master’s report. Mississippi objects to the Special

Master’s recommendation that we dismiss the suit. It argues that the Special Master erred in finding the water in

the Middle Claiborne Aquifer subject to equitable apportionment. Tennessee objects only to the Special Master’s 

recommendation that we grant Mississippi leave to amend

its complaint.

We “conduct an independent review of the record, and assume the ultimate responsibility for deciding all matters.” 

Kansas v. Nebraska, 574 U. S. 445, 453 (2015) (internal

quotation marks omitted). Based on that review, we dismiss Mississippi’s complaint and decline to grant leave to 

amend. 
 

 

Cite as: 595 U. S. ____ (2021) 7 

Opinion of the Court 

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 

doctrine’s “guiding principle” is that States “have an equal

right to make a reasonable use” of a shared water resource. 

Florida v. Georgia, 592 U. S. ___, ___ (2021) (slip op., at 4) 

(internal quotation 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. Colorado, 259 U. S. 419 (1922). We have also applied the

doctrine to disputes over interstate river basins, see Florida 

v. Georgia, 585 U. S. ___, ___–___ (2018) (slip op., at 2–3), 

and in situations where the pumping of groundwater has

affected the flow of interstate surface waters, see Nebraska 

v. Wyoming, 515 U. S. 1, 14 (1995). We have even applied 

the doctrine to anadromous fish—such as Chinook salmon 

and steelhead trout—that migrate between the Pacific 

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 considered whether equitable apportionment applies to interstate 

aquifers. See Exceptions Brief for Mississippi 28. Confronted as we are with this matter of first impression, we 

resist general propositions and focus our analysis on

whether equitable apportionment of the Middle Claiborne 

Aquifer would be “sufficiently similar” to past applications

of the doctrine to warrant the same treatment. Idaho ex rel. 

Evans, 462 U. S., at 1024 (adopting such an approach in extending the doctrine to anadromous fish). We conclude that 
 

8 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

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 

Mexico, 459 U. S., at 183. The Middle Claiborne Aquifer’s

“multistate character” seems beyond dispute. See Sporhase 

v. Nebraska ex rel. Douglas, 458 U. S. 941, 953 (1982). Mississippi 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 Tennessee is pumping water that was once in Mississippi. The 

evidence shows that wells in Memphis and wells in northwest Mississippi are “pumping from the same aquifer.” 

Hearing Tr. 492; see Report of Special Master 20 (noting 

that the “scientific consensus holds that the Middle 

Claiborne Aquifer is a single hydrogeological unit” spanning multiple States).

Also pertinent is that the Middle Claiborne Aquifer contains water that flows naturally between the States. All of 

our equitable apportionment cases have concerned such water, Kansas v. Colorado, 206 U. S., at 98, or fish 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 flow 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. Colorado, 206 U. S., at 115. And although the transboundary 

flow here may be a mere “one or two inches per day,” Exceptions 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 flow,

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 
 

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Opinion of the Court 

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 Mississippi. 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 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 

Mississippi’s portion of the aquifer. Id., ¶54. Such interstate effects are a hallmark of our equitable apportionment 

cases. See, e.g., Florida v. Georgia, 592 U. S., at ___–___ 

(2021) (slip op., at 2–3) (examining the effects of Georgia’s

water use on Florida’s oyster fisheries and river ecosystem).

For these reasons, we hold that the waters of the Middle 

Claiborne Aquifer are subject to the judicial remedy of equitable 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

control” of flowing interstate waters themselves. Wyoming 

v. Colorado, 259 U. S., at 464. Thus, we have “consistently 

denied” the proposition that a State may exercise exclusive 

ownership or control of interstate “waters flowing 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 between several States, each one “has an interest which 
 

10 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

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 flow to a 

downstream one, a result contrary to our equitable apportionment jurisprudence.

Mississippi argues that our decision in Tarrant Regional 

Water Dist. v. Herrmann, 569 U. S. 614 (2013), supports its 

position. We disagree. Tarrant concerned the interpretation 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 consider equitable apportionment, because the affected States had 

taken it upon themselves to negotiate a compact that determined their respective rights to the resource in 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 Statement of Stipulated and Contested Facts 106. When Tennessee 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 fish at issue in Idaho ex rel. Evans hatched in Idaho, 

462 U. S., at 1028, n. 12. The origin of an interstate water 

resource may be relevant to the terms of an equitable apportionment. But that feature alone cannot place the resource outside the doctrine itself. 

We conclude that the waters contained in the Middle 

Claiborne Aquifer are subject to equitable apportionment. 

We therefore overrule Mississippi’s exceptions and adopt 
 

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Opinion of the Court 

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 file an amended complaint seeking 

equitable apportionment. We decline to decide whether 

Mississippi should be granted such leave, because the State 

has never sought it. As Mississippi itself emphasizes—literally—it has “not yet requested equitable apportionment.” 

Reply Brief to Defendants’ Exception 7.

Nor can we assume Mississippi will do so. Mississippi’s 

initial pleadings in this case disavowed equitable apportionment 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 reflected 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 flow of a water resource, but also existing uses, the 

availability of alternatives, practical effects, and the costs

and benefits to the States involved. See Florida v. Georgia, 

585 U. S., at ___–___ (2018) (slip op., at 13–14); 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 Mississippi 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 un-
 

12 MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

der our original jurisdiction must prove by clear and convincing evidence some real and substantial injury or damage.” Idaho ex rel. Evans, 462 U. S., at 1027. 

As Mississippi has neither sought leave to amend nor tendered a proposed complaint seeking equitable apportionment, we have no occasion to determine how these and 

other pertinent principles might apply. 

* * * 

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.