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, 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 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.