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Page Number: 30.0

24 

SACKETT v. EPA 

Opinion of the Court 

States” is so “broad and unqualified” that, if viewed in iso-
lation,  it  would  extend  to  all  water  in  the  United  States. 
Brief for Respondents 32.  The EPA thus turns to the “sig-
nificant nexus” test in order to reduce the clash between its 
understanding of “the waters of the United States” and the 
term  defined  by  that  phrase,  i.e.,  “navigable  waters.”  As 
discussed, however, the meaning of “waters” is more limited 
than the EPA believes.  See supra, at 14.  And, in any event,
the CWA never mentions the “significant nexus” test, so the
EPA has no statutory basis to impose it.  See Rapanos, 547 
U. S., at 755–756 (plurality opinion). 

2 

Second,  the  EPA’s  interpretation  gives  rise  to  serious 
vagueness  concerns  in  light  of  the  CWA’s  criminal  penal-
ties.  Due process requires Congress to define penal statutes
“ ‘with sufficient definiteness that ordinary people can un-
derstand  what  conduct  is  prohibited’ ”  and  “ ‘in  a  manner 
that  does  not  encourage  arbitrary  and  discriminatory  en-
forcement.’ ”  McDonnell v. United States, 579 U. S. 550, 576 
(2016)  (quoting  Skilling  v.  United  States,  561  U. S.  358, 
402–403 (2010)).  Yet the meaning of “waters of the United 
States” under the EPA’s interpretation remains “hopelessly 
indeterminate.”  Sackett, 566 U. S., at 133 (ALITO, J., con-
curring); accord, Hawkes Co., 578 U. S., at 602 (opinion of 
Kennedy, J.).

The EPA contends that the only thing preventing it from 
interpreting “waters of the United States” to “conceivably 
cover  literally  every  body  of  water  in  the  country”  is  the 
significant-nexus test.  Tr. of Oral Arg. 70–71; accord, Brief 
for Respondents 32.  But the boundary between a “signifi-
cant” and an insignificant nexus is far from clear.  And to 
add to the uncertainty, the test introduces another vague
concept—“similarly  situated”  waters—and  then  assesses 
the  aggregate  effect  of  that  group  based  on  a  variety  of 
open-ended factors that evolve as scientific understandings