Document ID: ./input/supremecourt_opinions/opinions/21pdf/143orig_1qm1.pdf
Page Number: 10.0

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

7 

Opinion of the Court 

II 
A 
Equitable  apportionment  aims  to  produce  a  fair  alloca-
tion 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 consid-
ered whether equitable apportionment applies to interstate 
aquifers.  See  Exceptions  Brief  for  Mississippi  28.  Con-
fronted 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 ex-
tending the doctrine to anadromous fish).  We conclude that