Page:Mississippi v. Tennessee (2021).pdf/11

8 it would.

First, we have applied equitable apportionment only when transboundary resources were at issue. See Virginia v. Maryland, 540 U. S. 56, 74, n. 9 (2003); Colorado v. New Mexico, 459 U. S., at 183. The Middle Claiborne Aquifer’s “multistate character” seems beyond dispute. See Sporhase v. ''Nebraska ex rel. Douglas'', 458 U. S. 941, 953 (1982). Mississippi concedes that the “geologic formation in which the groundwater is stored straddles two states.” Complaint ¶41. Indeed, a core premise of Mississippi’s suit is that Tennessee is pumping water that was once in Mississippi. The evidence shows that wells in Memphis and wells in northwest Mississippi are “pumping from the same aquifer.” Hearing Tr. 492; see Report of Special Master 20 (noting that the “scientific consensus holds that the Middle Claiborne Aquifer is a single hydrogeological unit” spanning multiple States).

Also pertinent is that the Middle Claiborne Aquifer contains water that flows naturally between the States. All of our equitable apportionment cases have concerned such water, Kansas v. Colorado, 206 U. S., at 98, or fish that live in it, ''Idaho ex rel. Evans'', 462 U. S., at 1024. Mississippi suggests the Middle Claiborne Aquifer is distinguishable from interstate rivers and streams because its natural flow is “extremely slow.” Exceptions Brief for Mississippi 8. But we have long applied equitable apportionment even to streams that run dry from time to time. See Kansas v. Colorado, 206 U. S., at 115. And although the transboundary flow here may be a mere “one or two inches per day,” Exceptions Brief for Mississippi 8, that amounts to over 35 million gallons of water per day, and over ten billion gallons per year, see Hearing Tr. 532–533. So the speed of the flow, at least in the context of this case, does not place the aquifer beyond equitable apportionment.

Finally, it is clear that actions in Tennessee “reach[] through the agency of natural laws” to affect the portion of