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Page Number: 11.0

8 

MISSISSIPPI v. TENNESSEE 

Opinion of the Court 

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).  Mis-
sissippi 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 Ten-
nessee is pumping water that was once in Mississippi.  The 
evidence shows that wells in Memphis and wells in north-
west  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”  span-
ning multiple States).

Also pertinent is that the Middle Claiborne Aquifer con-
tains water that flows naturally between the States.  All of 
our equitable apportionment cases have concerned such wa-
ter, Kansas v. Colorado, 206 U. S., at 98, or fish that live in 
it, Idaho ex rel. Evans, 462 U. S., at 1024.  Mississippi sug-
gests 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. Col-
orado, 206 U. S., at 115.  And although the transboundary 
flow here may be a mere “one or two inches per day,” Ex-
ceptions  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