1 (Slip Opinion) OCTOBER TERM, 2021

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 pump ing of groundwater by the City of Memphis from the Middle Claiborne Aquifer, a valuable water re

source that lies beneath eight States. Mississippi argues that Tennes

see’s pumping—using wells Mississippi concedes are located entirely in Tennessee—siphons water away fr om Mississippi and amounts to a tortious taking of groundwater owne d by Mississippi. Mississippi ex

pressly disclaims any equitable a pportionment remedy, arguing that the “fundamental premise of this Co urt’s equitable apportionment ju

risprudence—that each of the opposi ng 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. Be

cause 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 rec

ommendation to dismiss; Tennessee objects to the recommendation to grant Mississippi leave to file an amended complaint.
Held : The waters of the Middle Claibo rne Aquifer are subject to the ju

dicial remedy of equitable apportionment; Mississippi’s complaint is dismissed without leave to amend. Pp. 7–12.

(a) The doctrine of equitable a pportionment aims to produce a fair allocation of a shared water resou rce 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 reason able 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 Ne

braska 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 Stat es during their lifetime.” Idaho ex rel. Evans v. Oregon , 462 U. S. 1017, 1018–1019, 1024. The Court has not before addressed whether equitable apportion

ment 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 con

cerned 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 Aq

uifer 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 fr om 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 apport ionment 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 appli

cation of equitable apportionment. The Court has “consistently de

nied” the proposition that a State ma y exercise exclusive ownership or control of interstate “waters fl owing within her boundaries.” Hinder

lider v. La Plata River & Cherry Creek Ditch Co. , 304 U. S. 92, 102. Although the Court’s past equitable apportionment cases have gener

ally concerned streams and rivers, no basis exists for a different result3 Cite as: 595 U. S. ____ (2021)

Syllabus

in the context of the Middle Claiborne Aquifer. To the contrary, Mis

sissippi’s ownership a pproach would allow an upstream State to com

pletely cut off flow to a downstream one, a result contrary to the Court’s equitable apportionment juri sprudence. 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 un

der the terms of an interstate comp act. The Court did not consider equitable apportionment, because the affected States had negotiated a compact that determined their respec tive 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 rele vant 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 eq uitable apportionment, the Court overrules Mississippi’s exceptions an d adopts the Special Master’s rec

ommendation to dismiss the bill of complaint. Pp. 9–11.

(c) Mississippi has neither sought leave to amend its complaint nor tendered a proposed complaint seek ing equitable apportionment. The Court does not address whether Mississippi should be granted such leave and sustains Tennessee’s object ion to the Special Master’s rec

ommendation to grant Mississippi leave to amend. Pp. 11–12. Exceptions overruled in part and sustained in part, and case dismissed.

R OBERTS , C. J., delivered the opinion for a unanimous Court._________________

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

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, Wash

ington, D. C. 20543, of any typographical or other form al 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]

C HIEF J USTICE R OBERTS 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 Aqui

fer. 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 Mis

sissippi 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 Mis

sissippi leave to file its complaint and appointed a Special Master to oversee proceedings. The Special Master has now2 MISSISSIPPI v. TENNESSEE

Opinion of the Court
issued his report, which recommends that this Court dis

miss 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 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 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 de

pressed water levels around th e 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; Hearing Tr. 176, 188, 435. The City of Memphis, through its public utility, the Mem

phis Light, Gas and Water Division (MLGW), pumps ap

proximately 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 located3 Cite as: 595 U. S. ____ (2021)

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 gro undwater within the Middle Claiborne Aquifer. Mississippi concedes that some water naturally flows from the part of the aquifer beneath Missis

sippi to the part beneath Tennessee. But only to the extent of some 30 to 60 feet per year. See Exceptions Brief for Mis

sissippi 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 nat

ural conditions.”). Mississippi contends that MLGW’s pumping has substantially hastened this existing flow, al

lowing Memphis to take billions of gallons of groundwater that otherwise would have re mained 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 gro undwater 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 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.” Un der that doctrine, this Court allocates rights to a disputed interstate water resource af

ter one State sues another under our original jurisdiction. See Kansas v. Colorado , 206 U. S. 46, 97–98 (1907). Tradi

tionally, equitable apportionment has been the exclusive ju

dicial remedy for interstate water disputes, unless a stat

ute, 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 rea

soned 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 oppose d to resting above ground,” was of “no analytical significance.” Ibid. Because deter

mining “Mississippi and Tennesse e’s relative rights to the Aquifer” brought the case within the equitable apportion

ment 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 fede ral 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 alterna

tive, it sought equitable appo rtionment of the aquifer, with a damages award for past diversions of groundwater. We5 Cite as: 595 U. S. ____ (2021)

Opinion of the Court declined to grant certiorari, 559 U. S. 904 (2010), and de

nied 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 com

plaint alleges that MLGW “has forcibly siphoned into Ten

nessee 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, nat

ural circumstances been drawn into Tennessee.” Id., ¶24. This “wrongful taking,” the State contends, “is evidenced bya substantial drop in pressure and corresponding drawdown of stored groundwater” in northwest Mississippi, and by a cone of depression extend ing 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 file 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.6 MISSISSIPPI v. TENNESSEE

Opinion of the Court Following motions practice, discovery, and a five-day evi

dentiary hearing, the Special Master issued a report recom

mending that we dismiss Mississ ippi’s complaint, but with leave to amend. He first determined that the Middle Claiborne Aquifer is an intersta te water resource. He found that the aquifer is a “single hy drogeological unit,” that Ten

nessee’s pumping affects groundwater beneath Mississippi, and that prior to such pumping, “groundwater flowed 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 aq

uifer is an interstate water resource, equitable apportion

ment is the appropriate remedy. He recognized that equi

table 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 Spe

cial 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 ar

gues that the Special Master erred in finding the water in the Middle Claiborne Aquifer subject to equitable appor

tionment. 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 as

sume 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 dis

miss Mississippi’s complaint and decline to grant leave to amend.7 Cite as: 595 U. S. ____ (2021)

Opinion of the Court

II A Equitable apportionment aims to produce a fair alloca

tion 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 in terstate 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 consid

ered whether equitable apportionment applies to interstate aquifers. See Exceptions Brief for Mississippi 28. Con

fronted 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 ex

tending the doctrine to anadromous fish). We conclude that8 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). 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 “scientific consensus holds that the Middle Claiborne Aquifer is a single hydrogeological unit” span

ning multiple States). Also pertinent is that the Middle Claiborne Aquifer con

tains water that flows naturally between the States. All of our equitable apportionment cases have concerned such wa

ter, Kansas v. Colorado , 206 U. S., at 98, or fish that live in it, Idaho ex rel. Evans , 462 U. S., at 1024. Mississippi sug

gests 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. Col

orado , 206 U. S., at 115. And although the transboundary flow here may be a mere “one or two inches per day,” Ex

ceptions 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 of9 Cite as: 595 U. S. ____ (2021)

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 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 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 inter

state 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 eq

uitable apportionment.

B Mississippi contends that it has sovereign ownership of all groundwater beneath its surface, so equitable apportion

ment ought not apply. We see things differently. It is cer

tainly 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 inters tate 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 inters tate “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 wa ter resource is shared be

tween several States, each one “has an interest which10 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 appor

tionment 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 interpreta

tion of an interstate compact. We held that the compact did not authorize the party States to “cross each other’s bound

aries to access a shared pool of water.” Id., at 627. Our decision turned on the language of the compact and back

ground principles of contract law. We did not consider eq

uitable apportionment, because the affected States had taken it upon themselves to negotiate a compact that deter

mined 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 wa

ter 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 Ten

nessee pumps groundwater, it is pumping water located within its own territory. That some of the water was previ

ously 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 ap

portionment. But that feature alone cannot place the re

source 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 adopt11 Cite as: 595 U. S. ____ (2021)

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—lit

erally—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 appor

tionment entirely. See Complaint ¶38 (“This case does not fall within the Court’s equitable apportionment jurispru

dence.”). The State instead sought relief under principles of tort law. And the proceedings below reflected this pos

ture, 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 proper

ties 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 Car

olina , 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 longstand

ing 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 con

vincing evidence some real and substantial injury or dam

age.” 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.

* * * Mississippi has failed to show that it is entitled to relief. We therefore overrule Mississippi’s exceptions to the Spe

cial Master’s report, sustain Tennessee’s, and dismiss the

case. It is so ordered.