110
Opinion of the Court
standards and then comment more specifically on the Court of Appeals' approach.
As discussed above, an EPA regulation requires an NPDES permit to comply "with the applicable water quality requirements of all affected States." 40 CFR § 122.4(d) (1991). This regulation effectively incorporates into federal law those state-law standards the Agency reasonably determines to be "applicable." In such a situation, then, state water quality standards—promulgated by the States with substantial guidance from the EPA14 and approved by the Agency—are part of the federal law of water pollution control.
Two features of the body of law governing water pollution support this conclusion. First, as discussed more thoroughly above, we have long recognized that interstate water pollution is controlled by federal law. See supra, at 98-100. Recognizing that the system of federally approved state standards as applied in the interstate context constitutes federal law is wholly consistent with this principle. Second, treating state standards in interstate controversies as federal law accords with the Act's purpose of authorizing the EPA to create and manage a uniform system of interstate water pollution regulation.
Because we recognize that, at least insofar as they affect the issuance of a permit in another State, the Oklahoma standards have a federal character, the EPA's reasonable, consistently held interpretation of those standards is entitled to substantial deference. Cf. INS v. National Center for Immigrants' Rights, 502 U. S. 183, 189-190 (1991); Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837 (1984). In these cases, the Chief Judicial Officer ruled that the Oklahoma standards—which require that there be "no degradation" of the upper Illinois River—would
14 See supra, at 101. Oklahoma's water quality standards closely track the EPA's model standards in effect at that time. Compare § 3 of the Oklahoma standards with 40 CFR § 35.1550(e)(1) (1981).
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