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-
