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10 

MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

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 appor-
tionment 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 interpreta-
tion of an interstate compact.  We held that the compact did
not authorize the party States to “cross each other’s bound-
aries  to  access  a  shared  pool  of  water.”  Id.,  at  627.  Our 
decision turned on the language of the compact and back-
ground principles of contract law.  We did not consider eq-
uitable  apportionment,  because  the  affected  States  had 
taken it upon themselves to negotiate a compact that deter-
mined 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 wa-
ter 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  State-
ment  of  Stipulated  and  Contested  Facts  106.    When  Ten-
nessee  pumps  groundwater,  it  is  pumping  water  located 
within its own territory.  That some of the water was previ-
ously 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 ap-
portionment.  But  that  feature  alone  cannot  place  the  re-
source 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