with whom Justice O’Con-nor, Justice Scalia, and Justice Kennedy join, concurring in part and dissenting in part.
An interstate compact, though provided for in the Constitution, and ratified by Congress, is nonetheless essentially a contract between the signatory States. The Court’s opin*243ion overruling New Mexico’s objections to the Report of the Special Master varies the terms of a contract to which the States of Oklahoma, New Mexico, and Texas freely agreed. I do not believe it is within the Court’s power to do this, and I therefore dissent from Part III of the Court’s opinion, which restricts New Mexico’s use of waters that spill over Conchas Dam. I concur in Parts I, II, and IV of the Court’s opinion.
The Canadian River traverses three States. It originates in the high country of northern New Mexico, flowing southeast from there into the Texas Panhandle. New Mexico has erected two dams on the river, Conchas Dam and Ute Dam, which provide irrigation water for farming and municipal water for the city of Tucumcari, New Mexico. In Texas, the Sanford project diverts water to serve the municipal and industrial requirements of Texas cities throughout the Texas Panhandle region, from Amarillo to Lubbock. The river flows eastward from this project across the Texas Panhandle and into Oklahoma, and thence southeasterly throughout almost the entire State of Oklahoma until it joins the Arkansas River in the Eufala Reservoir a few miles west of Fort Smith, Arkansas.
In 1950, New Mexico, Texas, and Oklahoma convened to draft the Canadian River Compact (Compact), which apportioned the Canadian’s waters in a manner that they hoped would serve New Mexico’s and Texas’ already substantial needs while anticipating the future needs of those States and Oklahoma. Article IV of the Compact, which governs the allocation of water to New Mexico, provides as follows:
“(a) New Mexico shall have free and unrestricted use of all waters originating in the drainage basin of Canadian River above Conchas Dam.
“(b) New Mexico shall have free and unrestricted use of all waters originating in the drainage basin of Canadian River in New Mexico below Conchas Dam, provided that the amount of conservation storage in New Mexico available for impounding these waters which originate in *244the drainage basin of Canadian River below Conchas Dam shall be limited to an aggregate of two hundred thousand (200,000) acre-feet.” 66 Stat. 75.
I part company with the majority’s interpretation of this Article, based on my view that this provision means what it says. By its express terms, Article IV places no restrictions on New Mexico’s use of waters originating above Conchas Dam. It imposes only two restrictions on its use of the waters originating in the drainage basin of the Canadian River below Conchas Dam: First, New Mexico’s enjoyment of these lower-basin waters is restricted to waters located in New Mexico; second, New Mexico may allocate no more than 200,000 acre-feet of its total storage capacity for the conservation of these lower basin waters.
The Compact thus distinguishes between water “originating” in the lower basin and water “originating” at or above the upper basin. New Mexico enjoys free and unrestricted use of the latter. The ordinary understanding of what it means for waters to “originate” in a basin is that they “arise” or “com[e] into existence” in that location. See 10 Oxford English Dictionary 935-936 (2d ed. 1989). Thus, according to the plain meaning of Article IV(a), New Mexico may make unrestricted use of the Canadian River waters that arise above Conchas Dam. These waters may be stored, used, or diverted for use without limitation. Unlike the waters that enter the Canadian River below Conchas Dam, these waters may pass into the lower basin without being subject to the 200,000 acre-feet conservation storage restriction of Article IV(b).
Despite the clear import of the Compact’s terms, the Court concludes that the Compact cannot mean what it says, and instead fashions a different allocation than that which is literally described. The Court concludes that “the intent of the Compact drafters was to give New Mexico free and unrestricted use of waters originating in the Canadian River drainage basin above Conchas Dam only if the waters were *245‘stored, used or diverted for use at or above Conchas Dam.’” Ante, at 232 (quoting Report of Special Master 59) (emphasis in original). The emphasized terms do not appear anywhere in the Compact, and reflect not the intent of the parties, but instead the intent that the Court now imputes to them. Although the Compact grants New Mexico use of “all” waters originating above Conchas Dam, the Court reads this to mean “some”: specifically excluding water that eventually winds up below Conchas Dam. Ante, at 232-233. Accordingly, the Court holds that any water found in the river below Conchas, including spills and seepage from above Con-chas Dam, is not subject to free and unrestricted use — even though it clearly originated above Conchas Dam.
A compact is a contract among its parties. Texas v. New Mexico, 482 U. S. 124, 128 (1987). Congressional consent elevates an interstate compact into a law of the United States, yet it remains a contract which is subject to normal rules of enforcement and construction. Thus, “unless the compact to which Congress has consented is somehow unconstitutional, no court may order relief inconsistent with its express terms.” Texas v. New Mexico, 462 U. S. 554, 564 (1983). Accordingly, where the terms of the compact are unambiguous, this Court must give effect to the express mandate of the signatory States.
The Court asserts that we may rewrite the express terms of Article IV(a) because of its understanding of the practical consequences of faithfully applying that provision. Ante, at 230-232. The Court contends that, if taken at its word, the Compact would permit New Mexico to lay claim to any water originating above Conchas Dam, including tributaries that arise in Colorado. The Court further asserts that a literal interpretation would permit New Mexico to then chase this water down and continue to claim access to it as it passes down through Texas and Oklahoma. Based on its view that the Compact could not have been drafted to produce the implausible consequence that New Mexico could appropriate *246Colorado’s, Texas’, and Oklahoma’s waters, the Court abandons the literal text of the Compact and casts off in search of a new interpretation of the word “originating.” Ante, at 232.
The Court’s approach conjures up impractical consequences where none exist. The language of the Compact does not in any way support the notion that Colorado (a State that did not even participate in the Compact) might forfeit its waters to New Mexico. Colorado’s rights are not implicated by the Compact at all. Although a small portion of the Canadian River’s waters arise in Colorado, only New Mexico, Texas, and Oklahoma participated in the Compact and are parties to it. By its terms, the Compact allocates only those rights over the interstate waters of the Canadian River belonging to those three States. See Art. X. Thus, the Compact could not, and did not purport to, allocate Colorado’s portion of the Canadian River. Any dispute between Colorado and the signatory States to this Compact must be resolved outside the terms of the Compact, and there is no reason to construe this Compact as though it purported to deal with Colorado’s claims.
Similarly, Article V of the Compact dispels any concern that New Mexico’s rights under a literal reading of Article IV(a) extend to waters originating above Conchas Dam that have left the State. That provision gives Texas “free and unrestricted use of all waters of [the] Canadian River in Texas,” subject to certain storage limitations. The Compact gives New Mexico no rights to recapture errant water that reaches Texas because that water is then “in” Texas and therefore subject to Texas’ rights under the Compact. The majority’s failure to reconcile Article V with Article IV violates the ordinary rule of statutory and contract interpretation that all provisions of a Compact must be read together in a meaningful manner. See United States v. Utah, Nevada & California Stage Co., 199 U. S. 414, 423 (1905).
*247Had the Compact’s drafters intended to limit New Mexico’s free and unrestricted use of the Canadian River waters originating above Conchas Dam in the manner announced today, they would certainly have done so more directly. For example, they might have drafted Article IV(a) to provide that “the amount of conservation storage in New Mexico below Conchas Dam shall be limited to an aggregate of 200,000 acre-feet.” But they did not. Instead, they specifically agreed that only waters “which originate in the drainage basin of [the] Canadian River below Conchas Dam” were to be so restricted. The only reasonable conclusion to draw from this is that they intended the word “originating” to have some content.
The Court’s free-form exploration of the practical' consequences of the parties’ agreement, and its reliance on evidence outside of the Compact to introduce ambiguity into Compact terms, is both contrary to our precedents and unfair to the parties. When parties to a contract have expressed their intent on a matter in unambiguous terms, we should not substitute our will for their purpose. Texas v. New Mexico, supra, at 564. The parties made an agreement, and have acted in reliance upon the terms of that contract and settled principles of contract law. The contract law principles of all three States disallow recourse to evidence outside the record under these circumstances. In those jurisdictions, where the language of an agreement clearly expresses the intent of the parties, courts may not rely on extrinsic evidence to vary its terms. See, e. g., Mercury Investment Co. v. F. W. Woolworth Co., 706 P. 2d 523, 529 (Okla. 1985); Hobbs Trailers v. J. T. Arnett Grain Co., 560 S. W. 2d 85, 87 (Tex. 1977); Trujillo v. CS Cattle Co., 109 N. M. 705, 709-710, 790 P. 2d 502, 506-507 (1990). Even viewed as a federal statute, the Court’s treatment of the Compact’s plain language is improper. Congress gave its blessing to this Compact and, in doing so, codified the agreement as federal law. As we stated in Arizona v. California, 373 U. S. 546, 565-566 *248(1963): “Where Congress has . . . exercised its constitutional power over waters, courts have no power to substitute their own notions of an ‘equitable apportionment’ for the apportionment chosen by Congress.”
Even if I agreed with this case to look outside the Compact to determine the meaning of Article IV(a), I would not agree with its conclusion that the parties intended to include overflow waters from the upper basin of Conchas Dam within the term “waters which originate in the drainage basin of Canadian River below Con-chas Dam.” I do not find either piece of evidence relied upon by the Court to be supportive of that position, let alone persuasive.
The Court says that the Compact any limitation on the amount of storage of waters originating above Conchas Dam because they believed that those waters were already being fully used. Accordingly, the Court reasons, the negotiators assumed that any future development of the river’s waters in New Mexico would necessarily occur only below Conchas Dam, and that 200,000 acre-feet of storage rights would be more than sufficient to satisfy those development needs. Ante, at 236. The Court concludes that “these circumstances,” demonstrate that Texas and Oklahoma could not have intended to permit New Mexico to impound any more than the 200,000 acre-feet of water for conservation storage purposes below Conchas Dam. Ante, at 237.
As a preliminary matter, the record simply does not bear out the Court’s view. The only evidence that directly addresses the issue establishes that the 200,000 acre-feet limitation was derived solely from New Mexico’s perceived requirements for Canadian River waters originating in the lower basin. The “Hill memorandum,” authored by Raymond Hill, Chairman of the Engineering Advisory Committee, and approved by the Compact Commission at its final meeting on January 31, 1951, stated that the storage limita*249tion was directed only towards impoundment of “the flood flows of Ute Creek and other minor tributaries of Canadian River entering the stream below Conchas Dam and above any contemplated storage works on Canadian River in Texas.” Plaintiff’s Exh. 38, p. 3 (emphasis added). The storage limits thus appear to have been directed at waters entering New Mexico below Conchas Dam but before the river enters Texas. Indeed, a letter from New Mexico’s Commissioner, John Bliss, to Senator Anderson of New Mexico, written the day after the Compact was signed, expressly noted that the 200,000 acre-feet limitation did not apply to spills. Plaintiff’s Exh. 28. By contrast, there is no direct support whatsoever for the Court’s statement that the Compact’s 200,000 acre-feet limitation on lower basin waters was intended to apply to upper basin waters captured in the lower basin.
Even assuming that the Court’s view of the facts is correct, I do not see how these facts support its conclusion. The Court observes that at the time of the Compact, New Mexico had fully developed reliable supplies of water above Conchas Reservoir, and thus there would be no purpose in placing a limitation upon any increase in the amount of storage of those waters. The Engineering Advisory Committee determined that “above Conchas, the available water supply has all been put to use — any further development above Conchas would deplete the supply available for Tucumcari Project; thus future developments would emphasize the better utilization of existing supplies.” Plaintiff’s Exh. 109, p. 1. This assessment, on its face, refers to the usage of normal water flows and not to the specific issue raised in this case, overflows and spills. In asserting that further development of the upper basin would draw on Tucumcari project waters, the engineer advisers did not contemplate spill waters or return flows from Tucumcari. As the Special Master himself concluded: “The most that can be said about the Engineer Advisors’ treatment of Conchas spills is that they apparently did not *250project that they would recur with the frequency and magnitude that they subsequently have.” Report of Special Master 67 (emphasis added).
The Court also relies on the fact that a 1960 study by the Bureau of Reclamation included outflows from Conchas Dam in estimating water supply to Sanford Reservoir, Texas. See Plaintiff’s Exh. 102, pp. 64, 67, 70-71. This too has no bearing on the intent of the parties to this Compact, or the meaning of Article IV. The Bureau published the final draft of its report nearly a decade after the Compact was signed. The Bureau’s report simply acknowledges that in light of the massive spills over Conchas Dam that occurred in 1941 and 1942, it might be reasonable to assume that occasional spills might contribute to the Sanford project’s water supply. This conclusion does not favor one view or another about New Mexico’s right to capture some of the overflow from Conchas Dam, since it is clear that New Mexico was physically incapable of capturing all of the overflow from the massive floods that have occurred twice this century. The Bureau’s estimates merely reflect reality; they do not suggest that the Compact requires waters flowing from Conchas spills to serve the Sanford project alone.
Finally, putting aside the Court’s dismissive treatment of the Compact terms and the parties’ expectations, today’s decision makes little practical sense. The Court’s decision will not protect the rights of the downstream States, except to the extent that it will force New Mexico to behave inefficiently in using its water. Oklahoma and Texas do not dispute that New Mexico could, if it desired, enlarge the reservoir behind Conchas Dam to capture all of the Canadian River’s waters and dry up the river beds of the downstream States. Tr. of Oral Arg. 29, 33-34; ante, at 238. The Court also acknowledges that the Compact gives New Mexico the included right to capture additional waters at or above Con-chas and then divert them to downstream locations. See ante, at 242, n. 12. The Court’s construction, therefore, does *251not prevent New Mexico from capturing flood waters and diverting them to projects below Conchas Dam; it merely forces the State to take its rightful waters by means of costly, inefficient, and wasteful engineering.
The Canadian is an unpredictable river: For the first 36 years of the Compact it lay dormant before it boiled over Conchas Dam, spilling several hundred thousand acre-feet of water into the lower basin. The Compact allocated this water. New Mexico was entitled to keep as much as it wished in modest storage facilities to recapture its upper basin waters. All the rest would naturally flow down to Texas and Oklahoma. The Court today rewrites that simple allocation. While rivers such as the Canadian may be unpredictable, interpretation of contracts involving those rivers should not be. The Court frustrates settled expectations by rewriting the Compact to mean something other than what its language says. Accordingly, I dissent from Part III of the Court’s decision.