Page:Mississippi v. Tennessee (2021).pdf/10

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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 (1982). The doctrine’s “guiding principle” is that States “have an equal right to make a reasonable use” of a shared water resource. Florida v. Georgia, 592 U. S. ___, ___ (2021) (slip op., at 4) (internal quotation marks omitted).

We pioneered the doctrine in Kansas v. Colorado, 206 U. S. 46. Since then, we have often applied it to interstate rivers and streams. See South Carolina v. North Carolina, 558 U. S. 256 (2010); Colorado v. New Mexico, 459 U. S. 176; Nebraska v. Wyoming, 325 U. S. 589 (1945); Wyoming v. Colorado, 259 U. S. 419 (1922). We have also applied the doctrine to disputes over interstate river basins, see Florida v. Georgia, 585 U. S. ___, ___–___ (2018) (slip op., at 2–3), 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 (1995). We have even applied the doctrine to anadromous fish—such as Chinook salmon and steelhead trout—that migrate between the Pacific Ocean and spawning grounds in the Columbia–Snake River system, “travel[ing] through several States during their lifetime.” See ''Idaho ex rel. Evans v. Oregon'', 462 U. S. 1017, 1018–1019, 1024 (1983).

Mississippi correctly observes that we have never considered whether equitable apportionment applies to interstate aquifers. See Exceptions Brief for Mississippi 28. Confronted as we are with this matter of first impression, we resist general propositions and focus our analysis on whether equitable apportionment of the Middle Claiborne Aquifer would be “sufficiently similar” to past applications of the doctrine to warrant the same treatment. ''Idaho ex rel. Evans'', 462 U. S., at 1024 (adopting such an approach in extending the doctrine to anadromous fish). We conclude that