delivered the opinion of the Court.
This case concerns overlapping federal and state regulation of a hydroelectric project located near a California stream. California seeks to ensure that the project’s operators maintain water flowing in the stream sufficient, in the State’s judgment, to protect the stream’s fish. The Federal Government claims the exclusive authority to set the minimum stream flows that the federally licensed powerplant must maintain. Each side argues that its position is consistent with the Federal Power Act, ch. 285, 41 Stat. 1063, as *494 amended, 16 U. S. C. §791a et seq. (1982 ed.), and, in particular, with §27 of that Act. We granted certiorari to resolve these competing claims.
I
The Rock Creek hydroelectric project lies near the confluence of the South Fork American River and one of the river’s tributaries, Rock Creek. Rock Creek runs through federally managed land located within California. The project draws water from Rock Creek to drive its generators and then releases the water near the confluence of the stream and river, slightly less than one mile from where it is drawn. The state and federal requirements at issue govern the “minimum flow rate” of water that must remain in the bypassed section of the stream and that thus remains unavailable to drive the generators.
In 1983, pursuant to the Federal Power Act (FPA or Act), the Federal Energy Regulatory Commission (FERC) issued a license authorizing the operation of the Rock Creek project.
Keating,
The licensee had also applied for state water permits, and in 1984 the State Water Resources Control Board (WRCB) issued a permit that conformed to FERC’s interim minimum flow requirements but reserved the right to set different permanent minimum flow rates. App. 65-67. When the WRCB in 1987 considered a draft order requiring permanent minimum flow rates of 60 cfs from March through June and 30 cfs during the remainder of the year, the licensee petitioned FERC for a declaration that FERC possessed exclusive jurisdiction to determine the project’s minimum flow requirements.
Rock Creek Limited Partnership,
In March 1987, FERC issued an order directing the licensee to comply with the minimum flow requirements of the federal permit. In that order, FERC concluded that the task of setting minimum flows rested within its exclusive jurisdiction.
Id.,
at 61,774. The Commission reasoned that setting minimum flow requirements was integral to its planning and licensing process under FPA § 10(a); giving effect to competing state requirements “would interfere with the Commission’s balancing of competing considerations in licensing” and would vest in States a veto power over federal
*496
projects inconsistent with the FPA, as interpreted in
First Iowa Hydro-Electric Cooperative
v.
FPC,
The Court of Appeals for the Ninth Circuit affirmed FERC’s order denying rehearing.
California ex rel. State Water Resources Board
v.
FERC,
II
In the Federal Power Act of 1935, 49 Stat. 863, Congress clearly intended a broad federal role in the development and licensing of hydroelectric power. That broad delegation of power to the predecessor of FERC, however, hardly determines the extent to which Congress intended to have the Federal Government exercise exclusive powers, or intended *497 to pre-empt concurrent state regulation of matters affecting federally licensed hydroelectric projects. The parties’ dispute regarding the latter issue turns principally on the meaning of § 27 of the FPA, which provides the clearest indication of how Congress intended to allocate the regulatory authority-of the States and the Federal Government. That section provides:
“Nothing contained in this chapter shall be construed as affecting or intending to affect or in any way to interfere with the laws of the respective States relating to the control, appropriation, use, or distribution of water used in irrigation or for municipal or other uses, or any vested right acquired therein.” 16 U. S. C. §821 (1982 ed.).
Were this a case of first impression, petitioner’s argument based on the statute’s language could be said to present a close question. As petitioner argues, California’s minimum stream flow requirement might plausibly be thought to “re-latte] to the control, appropriation, use, or distribution of water used . . . for. . . other uses,” namely the generation of power or the protection of fish. This interpretation would accord with the “presumption against finding pre-emption of state law in areas traditionally regulated by the States” and “ ‘with the assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.’”
California
v.
ARC America Corp.,
But the meaning of §27 and the pre-emptive effect of the FPA are not matters of first impression. Forty-four *498 years ago, this Court in First Iowa construed the section and provided the understanding of the FPA that has since guided the allocation of state and federal regulatory authority over hydroelectric projects. The Court interpreted § 27 as follows:
“The effect of § 27, in protecting state laws from supersedure, is limited to laws as to the control, appropriation, use or distribution of water in irrigation or for municipal or other uses of the same nature. It therefore has primary, if not exclusive, reference to such proprietary rights. The phrase ‘any vested right acquired therein’ further emphasizes the application of the section to property rights. There is nothing in the paragraph to suggest a broader scope unless it be the words ‘other uses.’ Those words, however, are confined to rights of the same nature as those relating to the use of water in irrigation or for municipal purposes.” First Iowa,328 U. S., at 175-176 (emphasis added).
The Court interpreted § 27’s reservation of limited powers to the States as part of the congressional scheme to divide state from federal jurisdiction over hydroelectric projects and, “in those fields where rights are not thus ‘saved’ to the States ... to let the supersedure of the state laws by federal legislation take its natural course.” Id., at 176.
We decline at this late date to revisit and disturb the understanding of §27 set forth in
First Iowa.
As petitioner prudently concedes, Tr. of Oral Arg. 7,
First Iowa’s
interpretation of § 27 does not encompass the California regulation at issue: California’s minimum stream flow requirements neither reflect nor establish “proprietary rights” or “rights of the same nature as those relating to the use of water in irrigation or for municipal purposes.”
First Iowa, supra,
at 176; see
Fullerton
v.
State Water Resources Control Board,
Petitioner asks this Court fundamentally to restructure a highly complex and long-enduring regulatory regime, implicating considerable reliance interests of licensees and other participants in the regulatory process. That departure would be inconsistent with the measured and considered change that marks appropriate adjudication of such statutory issues. See Square D Co., supra, at 424 (for statutory determinations, “‘it is more important that the applicable rule of law be settled than that it be settled right. . . . This is commonly true, even where the error is a matter of serious concern, provided correction can be had by legislation,’” quoting Burnet v. Coronado Oil & Gas Co., 285 U. S. 393, 406 (1932) (Brandeis, J., dissenting)).
Petitioner also argues that we should disregard
First Iowa’s
discussion of § 27 because it was merely dictum. It is true that our immediate concern in
First Iowa
was the interpretation of § 9(b) of the FPA, which governs submission to the federal licensing agency of evidence of compliance with state law.
1
The Court determined that § 9(b) did not require
*501
licensees to obtain a state permit or to demonstrate compliance with the state law prerequisites to obtaining such a permit.
First Iowa,
The Court’s related, but more general, rationale for its reading of § 9(b) in First Iowa also necessarily rested on its narrow construction of § 27. The Court framed the issue as whether the Act allowed the States to regulate through permit requirements such as Iowa’s “the very requirements of the project that Congress has placed in the discretion of the Federal Power Commission.” Id., at 165 (footnote citing FPA § 10(a) omitted). The Court rejected the possibility of concurrent jurisdiction and interpreted the FPA as mandating divided powers and “a dual system involving the close integration of these powers rather than a dual system of futile duplication of two authorities over the same subject matter.” Id., at 171; see id., at 174 (no “divided authority over any one subject”); id., at 181 (comprehensive federal role “leave[s] no room or need for conflicting state controls”). Section 9 reflected the operation of this exclusive federal authority. See id., at 167-169; id., at 168 (“Where the Federal Government supersedes the state government there is no suggestion that the two agencies both shall have final authority”). In accord with this view, the Court interpreted § 9(b) as requiring compliance only with state measures relevant to federal requirements rather than, as would exist under a system of concurrent jurisdiction, compliance with the state requirements necessary to secure the state permit. Id., at 167-169. Instead, only §27 preserved and defined the States’exclusive regulatory sphere. Id., at 175-178. That is, the Court rejected an interpretation of §9(b) that would have “saved” or accommodated the state permit system and its underlying requirements. To reach its interpretation of § 9(b), however, the Court had to interpret § 27 consistently with the limited state regulatory sphere and in a manner that did not, by “saving” the Iowa requirements, establish “divided authority over any one subject.” Id., at 174. Con *503 stricting § 27 to encompass only laws relating to proprietary rights, and thus leaving the permit requirements at issue to the federal sphere, accomplished that goal. The Court’s discussion immediately after its extended discussion of § 27 illustrates the relation between the sections. Before distinguishing §27’s role in saving state law from §9(b)’s role in the sphere of exclusive federal regulation, the Court concluded:
“[Section 27] is therefore thoroughly consistent with the integration rather than the duplication of federal and state jurisdictions under the Federal Power Act. It strengthens the argument that, in those fields where rights are not thus “saved” to the States, Congress is willing to let the supersedure of the state laws by federal legislation take its natural course.” Id., at 176.
The Court’s interpretation of § 9(b), of course, rested on that supersedure and required that the remaining field “saved” to the States by § 27 be limited correspondingly.
Petitioner also argues that our decision in
California
v.
United States,
Even if the two saving clauses were properly viewed in isolation from the remainder of their respective Acts and resulting regulatory schemes, significant differences exist between them. Section 8 of the Reclamation Act, after referring to state water laws relating to water used in irrigation and preserved by the Act, contains an explicit direction that “the Secretary of the Interior, in carrying out the provisions of this Act, shall proceed in conformity with such [state] laws.” 43 U. S. C. §383 (1982 ed.). This language has no counterpart in § 27 of the FPA and was crucial to the Court’s interpretation of §8. See
California
v.
United States, supra,
at 650, 664-665, 674-675. Although
California
v.
United States
and
First Iowa
accord different effect to laws relating
*505
to water uses, this difference stems in part from the different roles assumed by the federal actor in each case, as reflected in § 8’s explicit directive to the Secretary. The Secretary in executing a particular reclamation project is in a position analogous to a licensee under the FPA and need not comply with state laws conflicting with congressional directives respecting particular reclamation projects, see
Given these differences between the statutes and saving provisions, it should come as no surprise that
California
v.
United States
did not refer either to §27 or to
First Iowa.
Since the Court decided
California
v.
United States,
we have continued to cite
First Iowa
with approval. See,
e. g., Escondido Mut. Water Co.,
Finally, petitioner argues that §27’s legislative history requires us to abandon
First Iowa’s
interpretation of that section. Whatever the usefulness of legislative history for statutory interpretation in the usual case, that source provides petitioner with no aid. If a quite natural reading of the statutory language fails to displace an intervening decision providing a contrary interpretation, legislative history supporting that reading and by definition before the Court that
*506
has already construed the statute provides little additional reason to overturn the decision. Cf.
Patterson,
Adhering to
First Iowa’s
interpretation of §27, we conclude that the California requirements for minimum in-stream flows cannot be given effect and allowed to supplement the federal flow requirements. A state measure is “pre-empted to the extent it actually conflicts with federal law, that is, when it is impossible to comply with both state and federal law, or where the state law stands as an obstacle to the accomplishment of the full purposes and objectives of Congress.”
Silkwood
v.
Kerr-McGee Corp.,
For the foregoing reasons, the decision of the Court of Appeals for the Ninth Circuit is
Affirmed.
Notes
Section 9(b), 16 U. S. C. §802(a)(2) (formerly 16 U. S. C. §802(b) (1982 ed.)), provides:
“(a) Each applicant for a license under this chapter shall submit to the commission—
“(2) Satisfactory evidence that the applicant has complied with the requirements of the laws of the State or States within which the proposed *501 project is to be located with respect to bed and banks and to the appropriation, diversion, and use of water for power purposes and with respect to the right to engage in the business of developing, transmitting and distributing power, and in any other business necessary to effect the purposes of a license under this chapter.”
Section 8 of the Reclamation Act of 1902, 32 Stat. 390, now 43 U. S. C. §§ 372, 383 (1982 ed.), provided in part:
“[N]othing in this Act shall be construed as affecting or intended to affect or in any way interfere with the laws of any State or Territory relating to the control, appropriation, use, or distribution of water used in irrigation, or any vested right acquired thereunder, and the Secretary of the Interior, in carrying out the provisions of this Act, shall proceed in conformity with such laws, and nothing herein shall in any way affect any right of any State or of the Federal Government or of any landowner, appropriator, or user of water in, to, or from any interstate stream or the waters thereof . . .
