MISSISSIPPI v. TENNESSEE
Syllabus
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.
