Mississippi v. Tennessee
The Supreme Court unanimously ruled that Mississippi cannot sue Tennessee for tort damages over groundwater pumped from the Middle Claiborne Aquifer — an underground water source stretching beneath eight states — because disputes over shared interstate water resources must be resolved through the federal doctrine of equitable apportionment, a framework for fairly dividing water between states.
The decision extends equitable apportionment to underground aquifers for the first time and rejects the idea that a state can claim outright ownership of water that naturally flows across state lines.
How it got here: Mississippi filed a complaint directly in the Supreme Court under its original jurisdiction; the Court appointed a Special Master who held hearings and recommended dismissal with leave to amend; both states filed exceptions to that report.
The Case in Depth
What happened
The City of Memphis pumps roughly 120 million gallons of drinking water per day from the Middle Claiborne Aquifer, a vast underground water source beneath eight states, using more than 160 wells located entirely within Tennessee. Mississippi claimed this pumping created a miles-wide pressure depression extending into northern Mississippi, siphoning hundreds of billions of gallons of water away from beneath its soil. It sued Tennessee and Memphis for at least $615 million in damages, arguing it owned all groundwater beneath its surface.
The question before the Court
Can a state claim it owns all the groundwater beneath its soil and sue a neighboring state for damages when a city's pumping draws that water across the border from a shared underground aquifer?
The Court's answer
No. The Court ruled that when groundwater sits in an aquifer shared by multiple states, the federal doctrine of equitable apportionment — a framework the Court uses to fairly divide shared water between states, and historically the exclusive judicial remedy for interstate water disputes — applies. Mississippi cannot escape this framework by claiming it owns all groundwater beneath its surface. Because Mississippi's lawsuit sought only tort damages and expressly rejected equitable apportionment, the complaint had to be dismissed.
The Court also declined to give Mississippi permission to refile a new complaint seeking equitable apportionment, because Mississippi had never actually asked for that permission and had not submitted a proposed new complaint. If Mississippi wants to pursue equitable apportionment in the future, it must seek leave to do so and satisfy the Court's demanding standards — including proving real and substantial harm by clear and convincing evidence.
Curious how the Court got there? See the step-by-step legal reasoning →
Why it matters
States that share underground aquifers — including the seven other states drawing from the Middle Claiborne Aquifer — cannot assert a private ownership claim to the water and sue neighbors for damages. Any future dispute over who gets how much must go through the equitable apportionment process, which weighs all states' needs and uses, and may require joining all affected states as parties.
What changes now
Mississippi's lawsuit is dismissed and the case is closed. The Court declined to give Mississippi automatic permission to refile seeking equitable apportionment. Mississippi could still choose to start a new original-jurisdiction action requesting equitable apportionment, but would need to formally seek leave, present a proposed complaint, satisfy a demanding burden of proof, and potentially share proceedings with the six other states that also depend on the aquifer.
What this does not decide
The Court did not decide how the Middle Claiborne Aquifer's water should actually be divided among the states that rely on it. It also did not decide whether Mississippi would be entitled to any share, or whether it would succeed if it filed a new equitable apportionment case. The ruling addresses only the legal framework that governs the dispute, not its ultimate outcome.
How the Court got there
The legal reasoning, step by step
- The Court asked whether the doctrine of equitable apportionment — a federal common-law rule that is traditionally the exclusive judicial remedy for disputes between states over shared water resources, aimed at producing a fair division — should extend to an underground aquifer (a large, water-bearing underground rock formation) for the first time in the Court's history.
- Rather than lay down a sweeping rule, the Court borrowed an analogy-based 'sufficiently similar' test it had previously used to extend equitable apportionment to migratory Pacific salmon in Idaho ex rel. Evans v. Oregon (1983): does the Middle Claiborne Aquifer share the key features of resources where the doctrine already applies?
- Three features satisfied the test: the aquifer is transboundary, spanning eight states; water flows naturally between states through it — even at a slow one to two inches per day, that amounts to over ten billion gallons per year; and Tennessee's pumping has measurably caused water pressure to drop and storage to decline miles into northern Mississippi, a classic interstate effect the Court looks for.
- The Court rejected Mississippi's claim of sovereign ownership over all groundwater beneath its surface. The Court has long held that states cannot exercise exclusive ownership or control of interstate waters flowing within their borders, and no principled basis exists for treating underground water any differently from rivers and streams. Mississippi's ownership theory would let an upstream state cut off flow entirely to a downstream one — a result inconsistent with the whole point of equitable apportionment.
- Mississippi argued a 2013 decision — Tarrant Regional Water District v. Herrmann — supported its side, but the Court distinguished that case: Tarrant was about interpreting an interstate water-sharing compact, not equitable apportionment, and involved one state trying to physically cross another's border to take water. Here, Tennessee's wells stay entirely within Tennessee, so Tarrant's logic did not apply.
- Because equitable apportionment is the proper and exclusive remedy and Mississippi's complaint expressly disclaimed it, the complaint was dismissed. The Court also declined to grant Mississippi leave to refile seeking equitable apportionment, since Mississippi had never requested that permission or submitted a proposed amended complaint — and doing so would require a broader evidentiary record and potentially the joinder of all eight states drawing from the aquifer.
Doctrinal impact
Cases affected by this decision
Distinguishes Tarrant Regional Water Dist. v. Herrmann (569 U. S. 614)
That case was about an interstate water compact's text, not equitable apportionment, so it did not help Mississippi here.
Reaffirms Kansas v. Colorado (206 U. S. 46)
Reaffirmed as the foundational case pioneering equitable apportionment for shared interstate water resources.