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
