Document ID: ./input/supremecourt_opinions/opinions/21pdf/143orig_1qm1.pdf
Page Number: 14

Cite as:  595 U. S. ____ (2021) 

11 

Opinion of the Court 

the Special Master’s recommendation to dismiss the bill of 
complaint. 

III 
After recommending that this Court dismiss Mississippi’s 
suit,  the  Special  Master  went  on  to  recommend  that  we 
grant the State leave to file an amended complaint seeking 
equitable  apportionment.    We  decline  to  decide  whether 
Mississippi should be granted such leave, because the State 
has never sought it.  As Mississippi itself emphasizes—lit-
erally—it has “not yet requested equitable apportionment.” 
Reply Brief to Defendants’ Exception 7.

Nor can we assume Mississippi will do so.  Mississippi’s 
initial  pleadings  in  this  case  disavowed  equitable  appor-
tionment entirely.  See Complaint ¶38 (“This case does not
fall  within  the  Court’s  equitable  apportionment  jurispru-
dence.”).  The State instead sought relief under principles 
of tort law.  And the proceedings below reflected this pos-
ture, focusing on the physical properties of the aquifer and 
Tennessee’s  pumping.  An  equitable  apportionment  case
would require us to consider a broader range of evidence. 
Such evidence often includes not only the physical proper-
ties and flow of a water resource, but also existing uses, the 
availability of alternatives, practical effects, and the costs
and benefits to the States involved.  See Florida v. Georgia, 
585 U. S., at ___–___ (2018) (slip op., at 13–14); South Car-
olina, 558 U. S., at 271–272; Colorado v. New Mexico, 459 
U. S.,  at  183,  186–187.    A  just  equitable  apportionment 
might also require the joinder of additional parties, cf. Fed.
Rule Civ. Proc. 19(a), as Mississippi and Tennessee are not 
the only States that rely on the Middle Claiborne Aquifer 
for  groundwater,  see  Hearing  Tr.  660–662,  1038–1040; 
Joint Exh. J–71.  Finally, if Mississippi were to seek leave 
to amend, its complaint would be subject to our longstand-
ing rule that a “State seeking equitable apportionment un-