Page:Mississippi v. Tennessee (2021).pdf/13

10 should be respected by the other.” Wyoming v. Colorado, 259 U. S., at 466. Mississippi’s ownership approach would allow an upstream State to completely cut off flow to a downstream one, a result contrary to our equitable apportionment jurisprudence.

Mississippi argues that our decision in Tarrant Regional Water Dist. v. Herrmann, 569 U. S. 614 (2013), supports its position. We disagree. Tarrant concerned the interpretation of an interstate compact. We held that the compact did not authorize the party States to “cross each other’s boundaries to access a shared pool of water.” Id., at 627. Our decision turned on the language of the compact and background principles of contract law. We did not consider equitable apportionment, because the affected States had taken it upon themselves to negotiate a compact that determined their respective rights to the resource in question.

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 that all of Tennessee’s wells are drilled straight down and do not cross the Mississippi–Tennessee border. See Joint Statement of Stipulated and Contested Facts 106. When Tennessee pumps groundwater, it is pumping water located within its own territory. That some of the water was previously located in Mississippi is of no moment, just as it was not dispositive that the river at issue in Colorado v. New Mexico started in Colorado, 459 U. S., at 181, n. 8, or that certain fish at issue in ''Idaho ex rel. Evans'' hatched in Idaho, 462 U. S., at 1028, n. 12. The origin of an interstate water resource may be relevant to the terms of an equitable apportionment. But that feature alone cannot place the resource outside the doctrine itself.

We conclude that the waters contained in the Middle Claiborne Aquifer are subject to equitable apportionment. We therefore overrule Mississippi’s exceptions and adopt