*618
The Red River Compact, (or Compact),
*619 I
A
The Red River (or River) begins in the Llano Estacado Mesa on the border between New Mexico and Texas. From this broad plain, it first runs through the Texas Panhandle and then marks the border between Texas and Oklahoma. It continues in an easterly direction until it reaches the shared border with Arkansas. Once the River enters Arkansas, it turns southward and flows into Louisiana, where it empties into the Mississippi and Atchafalaya Rivers.
As an important geographic feature of this region, the Red River has lent its name to a valley, a Civil War campaign, and a famed college football rivalry between the Longhorns of Texas and the Sooners of Oklahoma. But college pride has not been the only source of controversy between Texas and Oklahoma regarding the Red River. The River has been the cause of numerous historical conflicts between the two States, leading to a mobilization of their militias at one time,
Oklahoma v. Texas,
Absent an agreement among the States, disputes over the allocation of water are subject to equitable apportionment by the courts,
Arizona v. California,
*620
Thus in 1955, to forestall future disputes over the River and its water, Congress authorized the States of Arkansas, Louisiana, Oklahoma, and Texas to negotiate a compact to apportion the water of the Red River basin among themselves. See Act of Aug. 11, 1955, Pub.L. 346,
*2126
One of the Compact's principal purposes was "[t]o provide an equitable apportionment among the Signatory States of the water of the Red River and its tributaries." § 1.01(b),
At issue in this case are rights under the Compact to water located in Oklahoma's portion of subbasin 5 of Reach II, which occupies "that portion of the Red River, together with its tributaries, from Denison Dam down to the Arkansas-Louisiana state boundary, excluding all tributaries included in the other four subbasins of Reach II." § 5.05(a),
The provisions of the Compact relating to Reach II were crafted to address this problem. To this end, Reach II was divided into five subbasins. The upstream subbasins, numbered 1 through 4, were drawn to end at "existing, authorized or proposed last downstream major damsites," see, e.g., § 5.01(a), id., at 22, on the tributaries leading to the Red River before reaching the main stem of the River. These dams allow the parties managing them to control water along the tributaries before it travels farther downstream and joins the flow of the main stem of the River. For the most part, the Compact granted control over the water in these subbasins to the States in which each subbasin is located. 2 The remaining subbasin, subbasin 5, instead requires that water be allowed to flow to Louisiana through the main stem of the River at certain minimum levels, assuring Louisiana an allocation of the River's waters and solving its flowthrough problem.
*622 The provision of the Compact central to the present dispute is § 5.05 (b)(1), which *2127 sets the following allocation during times of normal flow:
"(1) The Signatory States shall have equal rights to the use of runoff originating in subbasin 5 and undesignated water flowing into subbasin 5, so long as the flow of the Red River at the Arkansas-Louisiana state boundary is 3,000 cubic feet per second [hereinafter CFS] or more, provided no state is entitled to more than 25 percent of the water in excess of 3,000 [CFS]." 3 Id., at 25.
In these normal circumstances ( i.e., when flows at the Arkansas-Louisiana border are above 3,000 CFS), this provision and its interpretive comment make clear that "all states are free to use whatever amount of water they can put to beneficial use." Comment on Art. V, id ., at 30. But if the amount of water above 3,000 CFS cannot satisfy all such uses, then "each state will honor the other's right to 25% of the excess flow." Ibid . However, when the flow of the River diminishes at the Arkansas-Louisiana border, the upstream States must permit more water to reach Louisiana. 4
*623 Subbasin 5's allocation scheme allows upstream States to keep the water that they have stored, but also ensures that Louisiana will receive a steady supply of water from the Red River, with each upstream State contributing during times of low flow.
To ensure that its apportionments are honored, the Compact includes an accounting provision, but an accounting is not mandatory "until one or more affected states deem the accounting necessary." § 2.11, id., at 13; see Comment on Art. II, id., at 15-16. This is because the "extensive gaging and record keeping required" to carry out such an accounting would impose "a significant financial burden on the involved states." Id., at 16. Given these costs, the signatory States did "not envisio[n] that it w[ould] be undertaken as a routine matter." Ibid. Indeed, it appears that no State has ever asked for such an accounting in the Compact's history. See Brief for Respondents 45; Reply Brief 11-12.
While the Compact allocates water rights among its signatories, it also provides that it should not "be deemed to ... [i]nterfere with or impair the right or power of any Signatory State to regulate within its boundaries the appropriation, use, and control of water, or quality of water, not inconsistent with its obligations under this Compact." § 2.10,
In the years since the Red River Compact was ratified by Congress, the region's population has increased dramatically. In particular, the population of the Dallas-Fort Worth metropolitan area in north Texas has grown from roughly 5.1 million inhabitants in 2000 to almost 6.4 million in 2010, a jump of over 23 percent and among the largest in the United States during this period. See Dept. of Commerce, Census Bureau, P. Mackun & S. Wilson, Population Distribution and Change: 2000 to 2010 (Mar. 2011). This growth has strained regional water supplies, and north Texas' need for water has been exacerbated in recent years by a long and costly drought. See generally Galbraith, A Drought More Than Texas-Size, International Herald Tribune, Oct. 3, 2011, p. 4.
Against this backdrop, petitioner Tarrant, a Texas state agency responsible for providing water to north-central Texas (including the cities of Fort Worth, Arlington, and Mansfield), has endeavored to secure new sources of water for the area it serves. From 2000 to 2002, Tarrant, along with several other Texas water districts, offered to purchase water from Oklahoma and the Choctaw and Chickasaw Nations. See
Because Texas' need for water only continued to grow, Tarrant settled on a new course of action. In 2007, Tarrant sought a water resource permit from the Oklahoma Water Resources Board (OWRB), 5 respondents here, to take *625 310,000 acre feet 6 per year of surface water from the Kiamichi River, a tributary of the Red River located in Oklahoma. Tarrant proposed to divert the Kiamichi River, at a point located in subbasin 5 of Reach II, before it discharges into the Red River and, according to Tarrant, becomes too saline for potable use.
Tarrant knew, however, that Oklahoma would likely deny its permits because various state laws (collectively, the Oklahoma water statutes) effectively prevent out-of-state applicants from taking or diverting water from within Oklahoma's borders. These statutes include a requirement that the OWRB consider, when evaluating an application to take water out of State, whether that water "could feasibly be transported to alleviate water shortages in the State of Oklahoma." Okla. Stat., Tit. 82, § 105.12(A)(5) (West 2013). The statutes also require that no permit issued by the OWRB to use water outside of the State shall "[i]mpair the ability of the State of Oklahoma to meet its obligations under any interstate stream compact." § 105.12A(B)(1). A separate provision
*2129
creates a permitting review process that applies only to out-of-state water users. § 105.12(F). Oklahoma also requires legislative approval for out-of-state water-use permits, § 105.12A(D), and further provides that "[w]ater use within Oklahoma ... be developed to the maximum extent feasible for the benefit of Oklahoma so that out-of-state downstream users will not acquire vested rights therein to the detriment of the citizens of this state," § 1086.1(A)(3). Interpreting these laws, Oklahoma's attorney general has concluded that "we consider the proposition unrealistic that an out-of-state user is a proper permit applicant before the [OWRB]" because "[w]e can find no intention to create the possibility that such a valuable resource as water may become
*626
bound, without compensation, to use by an out-of-state user."
When Tarrant filed its permit application, it also filed suit against respondents in Federal District Court. As relevant here, Tarrant sought to enjoin enforcement of the Oklahoma water statutes by the OWRB. Tarrant argued that the statutes, and the interpretation of them adopted by Oklahoma's attorney general, were pre-empted by federal law and violated the Commerce Clause by discriminating against interstate commerce in water.
The District Court granted summary judgment for the OWRB on both of Tarrant's claims. See No. CIV-07-0045-HE,
We granted Tarrant's petition for a writ of certiorari, 568 U.S. ----,
II
A
Tarrant claims that under § 5.05(b)(1) of the Compact, it has the right to cross state lines and divert water from Oklahoma located in subbasin 5 of Reach II and that the Oklahoma water statutes interfere with its ability to exercise that right. Section 5.05(b)(1) provides:
"The Signatory States shall have equal rights to the use of runoff originating in subbasin 5 and undesignated water flowing into subbasin 5, so long as the flow of the Red River at the Arkansas-Louisiana state boundary is 3,000 [CFS] or more, provided no state is entitled to more than 25 percent of the water in excess of 3,000 [CFS].". 1 App. 25
*627 In Tarrant's view, this provision essentially creates a borderless common in which each of the four signatory States may cross each other's boundaries to access a shared pool of water. Tarrant reaches this interpretation in two steps. First, it observes that § 5.05(b)(1)'s "equal rights" language grants each State an equal entitlement to the waters of subbasin 5, subject to a 25 percent cap. Second, Tarrant argues § 5.05(b)(1)'s silence concerning state lines indicates that the Compact's drafters did not intend to allocate water according to state borders in this section. According to Tarrant, "the '25 percent' language [of § 5.05(b)(1) ] makes clear that, in exercising its 'equal rights' to the common pool of water, no State may take more than a one-quarter share, " Reply Brief 3, but any of the signatory States may "cross state lines to obtain [its]
*2130 shar[e] of Subbasin 5 waters," Brief for Petitioner 32.
The OWRB disputes this reading. In its view, the "equal rights" promised by § 5.05(b)(1) afford each State an equal opportunity to make use of the excess water within subbasin 5 of Reach II but only within each State's own borders. This is because the OWRB reads § 5.05(b)(1)'s silence differently from Tarrant. The OWRB interprets that provision's absence of language granting any cross-border rights to indicate that the Compact's drafters had no intention to create any such rights in the signatory States.
Unraveling the meaning of § 5.05(b)(1)'s silence with respect to state lines is the key to resolving whether the Compact pre-empts the Oklahoma water statutes. 8 If *628 § 5.05(b)(1)'s silence means that state borders are irrelevant to the allocation of water in subbasin 5 of Reach II, then the Oklahoma water laws at issue conflict with the cross-border rights created by federal law in the form of the Compact and must be pre-empted. But if § 5.05(b)(1)'s silence instead reflects a background understanding on the part of the Compact's drafters that state borders were to be respected within the Compact's allocation, then the Oklahoma statutes do not conflict with the Compact's allocation of water.
B
Interstate compacts are construed as contracts under the principles of contract law.
Texas v. New Mexico,
Tarrant argues that because other provisions of the Compact reference state borders, § 5.05(b)(1)'s silence with respect to state lines must mean that the Compact's drafters intended to permit cross-border diversions. For example, § 5.03(b), which governs subbasin 3 of Reach II, provides that
"[t]he States of Oklahoma and Arkansas shall have free and unrestricted use of the water of this subbasin within their respective states, subject, however, to the limitation that Oklahoma shall allow a quantity of water equal to ... 40 percent of the total runoff originating below the following existing, authorized or proposed last major downstream damsites in Oklahoma to flow into Arkansas."-24 (emphasis added). 1 App. 23
Section 6.03(b), which covers subbasin 3 of Reach III, similarly provides that "Texas and Louisiana
within their respective
*629
boundaries
shall each have the unrestricted use of the water of this subbasin subject to the following [conditions]."
But Tarrant's argument fails to account for other sections of the Compact that cut against its reading. For example, § 5.05(b)(3), which governs the waters of subbasin 5 in Reach II when flows are below 1,000 CFS, requires that during such periods, Arkansas, Texas, and Oklahoma allow water "
within their respective states
to flow into the Red River as required to maintain a 1,000 [CFS] flow at the Arkansas-Louisiana state boundary."
Applying Tarrant's understanding of § 5.05(b)(1)'s silence regarding state lines to other of the Compact's provisions would produce further anomalous results. Consider § 6.01(b). That provision states that "Texas is apportioned sixty (60) percent of the runoff of [subbasin 1 of Reach III] and shall have unrestricted use thereof; Arkansas is entitled to forty (40) percent of the runoff of this subbasin."
At the very least, the problems that arise from Tarrant's proposed reading suggest that § 5.05(b)(1)'s silence is ambiguous
*631
regarding cross-border rights under the Compact. We therefore turn to other interpretive tools to shed light on the intent of the Compact's drafters. See
Oklahoma v. New Mexico,
1
The background notion that a State does not easily cede its sovereignty has informed our interpretation of interstate compacts. We have long understood that as sovereign entities in our federal system, the States possess an "absolute right to all their navigable waters and the soils under them for their own common use."
Martin v. Lessee of Waddell,
Given these principles, when confronted with silence in compacts touching on the States' authority to control their waters, we have concluded that "[i]f any inference at all is to be drawn from [such] silence on the subject of regulatory authority, we think it is that each State was left to regulate the activities of her own citizens."
Virginia v. Maryland,
Tarrant asks us to infer from § 5.05(b)(1)'s silence regarding state borders that the signatory States have dispensed with the core state prerogative to *2133 control water within their own boundaries. 11 But as the above demonstrates, States rarely relinquish their sovereign powers, so when they do we would expect a clear indication of such devolution, not inscrutable silence. We think that the better understanding of § 5.05(b)(1)'s silence is that the parties drafted the Compact with this legal background in mind, and therefore did not intend to grant each other cross-border rights under the Compact.
In response, Tarrant contends that its interpretation would not intrude on any sovereign prerogative of Oklahoma because that State would retain its authority to regulate the *633 water within its borders. Because anyone seeking water from Oklahoma would still have to apply to the OWRB, receive a permit, and abide by its conditions, Tarrant argues that Oklahoma's sovereign authority remains untouched by its interpretation. But Tarrant cannot have it both ways. Adopting Tarrant's reading would necessarily entail assuming that Oklahoma and three other States silently surrendered substantial control over the water within their borders when they agreed to the Compact. Given the background principles we have described above, we find this unlikely to have been the intent of the Compact's signatories.
2
Looking to the customary practices employed in other interstate compacts also helps us to ascertain the intent of the parties to this Compact. See
Alabama v. North Carolina,
Tellingly, many of these compacts provide for the terms and mechanics of how
*2134
such cross-border relationships will operate, including who can assert such cross-border rights, see,
e.g.,
Kansas-Nebraska Big Blue River Compact, Art. VII(1),
Provisions like these are critical for managing the complexities that ensue from cross-border diversions. Consider the mechanics of a cross-border diversion or taking of water in this case. If Tarrant were correct, then applicants from Arkansas, Texas, and Louisiana could all apply to the OWRB for permits to take water from Oklahoma. The OWRB would then be obligated to determine the total amount of water in Oklahoma beyond the 25 percent cap created in § 5.05(b)(1), given that the Compact would only obligate Oklahoma to deliver water beyond its quarter share. This alone would be a herculean task because the Compact does not require ongoing monitoring or accounting, see Compact § 2.11,
*635 Put plainly, the end result would be a jurisdictional and administrative quagmire. The provisions in the other interstate water compacts resolve these complications. The absence of comparable provisions in the Red River Compact strongly suggests that cross-border rights were never intended to be part of the States' agreement.
Tarrant counters that not all interstate compacts that permit cross-border diversions have explicit language to this effect. On this front, Tarrant manages to identify one interstate compact that it contends permits cross-border diversions without express language to that effect, the Upper Niobrara River Compact, Pub.L. 91-52,
Tarrant also argues that § 2.05(d) of the Red River Compact, which provides that "[e]ach Signatory State shall have the right to" "[u]se the bed and banks of the Red River and its tributaries to convey stored water, imported or exported water, and water apportioned according to this Compact,"
*636
3
The parties' conduct under the Compact also undermines Tarrant's position. A "part[y's] course of performance under the Compact is highly significant" evidence of its understanding of the compact's terms.
Alabama v. North Carolina,
560 U.S., at ----,
In response, Tarrant maintains that there were "compelling business reasons" for it to purchase water. Reply Brief 17. We are unpersuaded. If Tarrant believed that it had a
*637
right to water located in Oklahoma, there would have been "compelling business reasons" to mention this right given that billions of dollars were at stake. See
4
The Compact creates no cross-border rights in Texas. Tarrant's remaining arguments do not persuade us otherwise.
First, Tarrant argues that its interpretation of the Compact is necessary to realize the "structure and purpose of Reach II." Brief for Petitioner 34-38. Tarrant contends that because the boundary of subbasin 5 is set by the location of the last existing, authorized, or proposed sites for a downstream dam before the Red River, see Compact §§ 5.01(a), 5.02(a), 5.03(b), 5.04(a),
*2136 This argument is founded on a shaky premise: It assumes that flows from these dammed-up tributaries are the sole source of water in subbasin 5. But § 5.05(b)(1) explains that "[s]ignatory States shall have equal rights to the use of runoff originating in subbasin 5," as well as "water flowing into subbasin 5," which would include flows from the main stem of the River itself. Id ., at 25. Thus, there are waters that are specific to subbasin 5 separate from those originating in *638 the tributaries covered by subbasins 1 through 4. Tarrant's account of the purposes of subbasin 5 does not explain how these waters were to be allocated.
Tarrant's second argument regarding the purposes of Reach II is that § 5.05(b)(1)'s 25 percent cap on each State's access to excess water in subbasin 5 should be read to imply that if a State cannot access sufficient water within its borders to meet its share under the cap, then it must be able to cross borders to reach that water. Were it otherwise, Tarrant explains, the 25 percent cap would have no purpose. To support this argument, Tarrant draws on a 1970 engineering report that it contends shows that only 16 percent of the freshwater flowing into subbasin 5 was located in Texas. Brief for Petitioner 9, n. 5. The OWRB challenges this percentage with its own calculations drawn from the report, and asserts that Texas had access to at least 29 percent of the excess water in subbasin 5 within its own borders. Brief for Respondents 26, 47-48, and n. 17.
Fortunately, we need not delve into calculations based on a decades-old engineering report to resolve this argument. As we have explained,
supra,
at 2126 - 2128, Texas does not have a minimum guarantee of 25 percent of the excess water in subbasin 5. If it believes that Oklahoma is using more than its 25 percent allotment and wishes to stop it from doing so, then it may call for an accounting under § 2.11 of the Compact and, depending on the results of that accounting, insist that Oklahoma desist from taking more than its provided share. See Compact § 2.11, and Comment on Art. II,
C
Under the Compact's terms, water located within Oklahoma's portion of subbasin 5 of Reach II remains under Oklahoma's *639 control. Accordingly, Tarrant's theory that Oklahoma's water statutes are pre-empted because they prevent Texas from exercising its rights under the Compact must fail for the reason that the Compact does not create any cross-border rights in signatory States.
III
Tarrant also challenges the constitutionality of the Oklahoma water statutes under a dormant Commerce Clause theory. Tarrant argues that the Oklahoma water statutes impermissibly " 'discriminat[e] against interstate commerce' for the 'forbidden purpose' of favoring local interests" by erecting barriers to the distribution of water left unallocated under the Compact. Brief for Petitioner 47-48 (quoting
Department of Revenue of Ky. v. Davis,
*2137 Brief for Petitioner 47. So, Tarrant continues, because Oklahoma's laws prevent this "unallocated water" from being distributed out of State, those laws violate the Commerce Clause.
Tarrant's assumption that that the Compact leaves some water "unallocated" is incorrect. The interpretive comment for Article V of the Compact makes clear that when the River's flow is above 3,000 CFS, "all states are free to use whatever amount of water they can put to beneficial use," subject to the requirement that "[i]f the states have competing uses and the amount of water available in excess of 3000 CFS cannot satisfy all such uses, each state will honor the other's right to 25% of the excess flow."
The Red River Compact does not pre-empt Oklahoma's water statutes because the Compact creates no cross-border rights in its signatories for these statutes to infringe. Nor do Oklahoma's laws run afoul of the Commerce Clause. We affirm the judgment of the Court of Appeals for the Tenth Circuit.
It is so ordered.
APPENDIX A
APPENDIX B
Notes
Interpretive comments were included in the Compact so that future readers "might be apprised of the intent of the Compact Negotiation Committee with regard to each Article of the Compact." Compact, Comment on Preamble,
Within subbasins 1, 2, and 4, water was fully apportioned to a single State. See Compact § 5.01(b),
The Compact defines "undesignated water" as "all water released from storage other than 'designated water.' " § 3.01( l ), id ., at 17. "[D]esignated water" means "water released from storage, paid for by non-Federal interests, for delivery to a specific point of use or diversion." § 3.01(k), ibid .
In such circumstances, the two relevant paragraphs provide:
"(2) Whenever the flow of the Red River at the Arkansas-Louisiana state boundary is less than 3,000 [CFS], but more than 1,000 [CFS], the States of Arkansas, Oklahoma, and Texas shall allow to flow into the Red River for delivery to the State of Louisiana a quantity of water equal to 40 percent of the total weekly runoff originating in subbasin 5 and 40 percent of undesignated water flowing into subbasin 5; provided, however, that this requirement shall not be interpreted to require any state to release stored water.
"(3) Whenever the flow of the Red River at the Arkansas-Louisiana state boundary falls below 1,000 [CFS], the States of Arkansas, Oklahoma, and Texas shall allow a quantity of water equal to all the weekly runoff originating in subbasin 5 and all undesignated water flowing in subbasin 5 within their respective states to flow into the Red River as required to maintain a 1,000 [CFS] flow at the Arkansas-Louisiana state boundary." § 5.05(b),
Under § 2.10 of the Compact each signatory State retains "the right or power ... to regulate within its boundaries the appropriation, use, and control of water."
An acre-foot is equivalent to the volume of one acre of surface area filled to a depth of one foot. Webster's Third New International Dictionary 19 (1966).
The parties have stipulated that OWRB will not take action on Tarrant's application until this litigation has concluded. Brief for Petitioner 16.
The Compact Clause of the Constitution provides that "[n]o State shall, without the Consent of Congress, ... enter into any Agreement or Compact with another State." Art. I, § 10, cl. 3. Accordingly, before a compact between two States can be given effect it must be approved by Congress. See
Virginia v. Maryland,
See Compact § 4.01(b),
There is, however, one interpretive tool that is inapplicable here: the presumption against pre-emption. The Court of Appeals repeatedly referenced and relied upon the presumption in its opinion. See
Of course, the power of States to control water within their borders may be subject to limits in certain circumstances. For example, those imposed by the Commerce Clause. See
Sporhase v. Nebraska ex rel. Douglas,
See also Amended Costilla Creek Compact, Art. III(2),
Moreover, even if Oklahoma utilized less than 25 percent of the excess subbasin 5 water within its territory and allowed the rest to flow down the River, that water would pass from Reach II into Reach V, see Compact § 2.12,
* * *