Task: sc_issuearea

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Justice White
delivered the opinion of the Court.
The problem of irrigating the arid lands of the Colorado River Basin has been confronted by the peoples of that region for 2,000 years and by Congress and this Court for many decades. Today we conclude another chapter in this original action brought to determine rights to the waters of the Colorado River. In earlier proceedings in this case, the United States, an intervenor in the principal action, acquired water rights for five Indian reservations that are dependent upon the river for their water. The United States, and the Tribes which ask to intervene in the action, now seek to have those water rights increased.
I
The Colorado River Compact of 1922 divided the waters of the Colorado River between the Upper- and Lower-Basin States, but fell short of apportioning the respective shares among the individual States. Nor did the Boulder Canyon Project Act of 1928, 45 Stat. 1057, as amended, 43 U. S. C. § 617 et seq. (1976 ed. and Supp. V) (Project Act), a vast federal effort to harness and put to use the waters of the lower Colorado River, expressly effect such an apportionment. The principal dispute that became increasingly pressing over the years concerned the respective shares of the Lower-Basin States, particularly the shares of California and Arizona.
This litigation began in 1952 when Arizona, to settle this dispute, invoked our original jurisdiction, U. S. Const., Art. Ill, §2, cl. 2, by filing a motion for leave to file a bill of complaint against California and seven public agencies of the State. Arizona sought to confirm its title to water in the Colorado River system and to limit California’s annual consumptive use of the river’s waters. Nevada intervened, praying for determination of its water rights; Utah and New Mexico were joined as defendants; and the United States intervened, seeking water rights on behalf of various federal establishments, including the reservations of five Indian Tribes — the Colorado River Indian Tribes, Fort Mojave Indian Tribe, Chemehuevi Indian Tribe, Cocopah Indian Tribe, and Fort Yuma (Quechan) Indian Tribe.
After lengthy proceedings, Special Master Simon Rifkind filed a report recommending a certain division of the Colorado River waters among California, Arizona, and Nevada. The parties’ respective exceptions to the Master’s report were extensively briefed and the case was twice argued. The Court for the most part agreed with the Special Master, 373 U. S. 546 (1963), and our views were carried forward in the decree found at 376 U. S. 340 (1964).
The long and rich story of the efforts on behalf of the States involved to arrive at a mutually satisfactory plan of apportionment is set forth in the Special Master’s report and the Court’s opinion and need not be repeated here. We agreed with the Special Master that the allocation of Colorado River water was to be governed by the standards set forth in the Project Act rather than by the principles of equitable apportionment which in the absence of statutory directive this Court has applied to disputes between States over entitlement to water from interstate streams. Nor was the local law of prior appropriation necessarily controlling. The Project Act itself was held to have created a comprehensive scheme for the apportionment among California, Nevada, and Arizona of the Lower Basin’s share of the mainstream waters of the Colorado River, leaving each State its tributaries. Congress had decided that a fair division of the first 7.5 million acre-feet of such mainstream waters would give 4.4 million acre-feet to California, 2.8 million acre-feet to Arizona, and 300,000 acre-feet to Nevada. Arizona and California would share equally in any surplus. 373 U. S., at 565.
Over strong objection, we also agreed with the Special Master that the United States had reserved water rights for the Indian reservations, effective as of the time of their creation. Id., at 598-600. See Winters v. United States, 207 U. S. 564 (1908). These water rights, having vested before the Project Act became effective on June 25, 1929, were ranked with other “present perfected rights,” and as such were entitled to priority under the Act. 373 U. S., at 600. Rejecting more restrictive standards for measuring the water rights intended to be reserved for the reservations, we agreed with the Master and the United States, speaking on behalf of the Tribes, that the “only feasible and fair way by which reserved water for the reservations can be measured is irrigable acreage.” Id., at 601. We further sustained the Master’s findings, arrived at after full, adversary proceedings, as to the various acreages of practicably irrigable land on the different reservations. Ibid. These findings were subsequently incorporated in our decree of March 9, 1964. Article 11(D) of our decree specified each reservation’s entitlement to diversions from the mainstream.
Not all aspects of the case were finally resolved in the 1964 decree. First, in the course of determining irrigable acreage on the reservations, the Master resolved a dispute between the United States and the States with respect to the boundaries of the Colorado River and Fort Mojave Indian Reservations, generally finding that the reservations were smaller than the United States claimed them to be. Although we based the water rights decreed to these two reservations on the irrigable acreage within the boundaries determined by the Special Master, we found that it had been “unnecessary” for the Special Master finally to have determined these boundaries and provided in Article 11(D) that the quantities of water provided for the Fort Mojave Indian Reservation and the Colorado River Indian Reservation “shall be subject to appropriate adjustment by agreement or decree of this Court in the event that the boundaries of the respective reservations are finally determined.” 376 U. S., at 345. See Part V, infra. Second, Article VI of the decree provided that the parties, within two years, should provide the Court with a list of the outstanding present perfected rights in the mainstream waters. Finally, in Article IX of the decree we retained jurisdiction over the case for the purpose of further modifications and orders that we deemed proper.
On January 9, 1979, we entered a supplemental decree identifying the present perfected rights to the use of the mainstream water in each State and their priority dates as agreed to by the parties. 439 U. S. 419. We also decreed that, in the event of shortage, the Secretary of the Interior shall, before providing for the satisfaction of these present perfected rights, first provide for the satisfaction in full of the Indian water rights set forth in the 1964 decree for the five reservations. We expressly noted that these quantities, fixed in paragraphs 1 through 5 of Article 11(D) of the 1964 decree “shall continue to be subject to appropriate adjustment by agreement or decree of this Court in the event that the boundaries of the respective reservations are finally determined.” 439 U. S., at 421. The 1979 decree thus resolved outstanding issues in the litigation. But before that decree was entered new questions arose: The five Indian Tribes, ultimately joined by the United States, made claims for additional water rights to reservation lands.
Because the United States had represented their interests, the Indian Tribes previously had no part in the litigation. In 1977, however, the Fort Mojave, Chemehuevi, and Quechan (Fort Yuma) Indian Tribes moved for leave to intervene as indispensable parties. By April 10, 1978, the Colorado River Indian Tribes and the Cocopah Indian Tribe had also filed petitions for intervention. Three of the Tribes sought intervention to oppose entry of the 1979 decree that was to set the priority order for water rights in the Colorado River. The Tribes also raised claims for additional water rights appurtenant to two types of land: (1) the so-called “omitted” lands — irrigable lands, within the recognized 1964 boundaries of the reservations, for which it was said that the United States failed to claim water rights in the earlier litigation; and (2) “boundary” lands — lands that were or should have been officially recognized as part of the reservations and that had assertedly been finally determined to lie within the reservations within the meaning of the 1964 decree.
Initially, both the state parties and the United States opposed intervention. Subsequently, the United States dropped its opposition to the Tribes’ intervention. Still later, on December 22, 1978, the United States joined the Indians in moving for a supplemental decree to grant additional water rights to the reservations. In our 1979 decree, we denied the motion of the Fort Mojave, Chemehuevi, and Que-chan Tribes to' intervene insofar as they sought to oppose entry of the supplemental decree. Other matters raised by their motion, as well as that of the United States’ and the other two Tribes, were not resolved. We appointed Senior Judge Elbert P. Tuttle Special Master and referred these motions to him. 439 U. S., at 436-437.
After conducting hearings, the Special Master issued a preliminary report on August 28, 1979, granting the Indian Tribes leave to intervene in subsequent hearings on the merits. In addition, the Special Master concluded that certain boundaries of the reservations had now been finally determined within the meaning of Article 11(D) of the 1964 decree, primarily because of administrative decisions taken by the Secretary of the Interior. These decisions purported considerably to enlarge the reservations affected and, with respect to the Colorado River and Mojave Reservations, were for the most part reassertions of the positions submitted by the United States to Special Master Rifkind, rejected by him, and left open by us to later final resolution. We refused to allow the States to file exceptions at that time, 444 U. S. 1009 (1980), and the Special Master held further hearings on the merits.
On' February 22, 1982, the Special Master issued his final report. The Special Master’s findings were almost entirely consistent with the position of the United States and the Indian Tribes. Rejecting the States’ strong objections to reopening the question of whether more practicable irrigable acreage actually existed than the United States claimed, Special Master Rifkind found, and our 1963 opinion and 1964 decree specified, the Special Master concluded that each of the Tribes was entitled to additional water rights based on land that he determined to be irrigable over and beyond that previously found. Furthermore, based on his earlier boundary determination, the Master determined that there was additional practicably irrigable acreage for which the Indians were entitled to further water rights. The States have filed exceptions to both of these determinations, as well as to various factual findings concerning the amount of practicably irrigable acreage.
Ill
The States have also refiled their exceptions to the Special Master’s preliminary findings allowing the Indian Tribes to intervene in the action. We consider this matter first.
We agree with the Special Master that the Indian Tribes’ motions to intervene should be granted. The States oppose the motions and insist that, without their consent, the Tribes’ participation violates the Eleventh Amendment. Assuming, arguendo, that a State may interpose its immunity to bar' a suit brought against it by an Indian tribe, United States v. Minnesota, 270 U. S. 181, 193-195 (1926), the States involved no longer may assert that immunity with respect to the subject matter of this action. Water right claims for the Tribes were brought by the United States. Nothing in the Eleventh Amendment “has ever been seriously supposed to prevent a State’s being sued by the United States.” United States v. Mississippi, 380 U. S. 128, 140 (1965). See, e. g., United States v. Texas, 143 U. S. 621, 646 (1892); United States v. California, 297 U. S. 175 (1936); United States v. California, 332 U. S. 19, 26-28 (1947). The Tribes do not seek to bring new claims or issues against the States, but only ask leave to participate in an adjudication of their vital water rights that was commenced by the United States. Therefore, our judicial power over the controversy is not enlarged by granting leave to intervene, and the States’ sovereign immunity protected by the Eleventh Amendment is not compromised. See, e. g., Maryland v. Louisiana, 451 U. S. 725, 745, n. 21 (1981).
The States also oppose intervention on grounds that the presence of the United States insures adequate representation of the Tribes’ interests. The States maintain that the prerequisites for intervention as of right set forth in Rule 24 of the Federal Rules of Civil Procedure are not satisfied. Aside from the fact that our own Rules make clear that the Federal Rules are only a guide to procedures in an original action, see this Court’s Rule 9.2; Utah v. United States, 394 U. S. 89, 95 (1969), it is obvious that the Indian Tribes, at a minimum, satisfy the standards for permissive intervention set forth in the Federal Rules. The Tribes’ interests in the water of the Colorado basin have been and will continue to be determined in this litigation since the United States’ action as their representative will bind the Tribes to any judgment. Heckman v. United States, 224 U. S. 413, 444-445 (1912). Moreover, the Indians are entitled “‘to take their place as independent qualified members of the modern body politic.’” Poafpybitty v. Shelly Oil Co., 390 U. S. 365, 369 (1968), quoting Board of County Comm’rs v. Seber, 318 U. S. 705, 715 (1943). Accordingly, the Indians’ participation in litigation critical to their welfare should not be discouraged. The States have failed to present any persuasive reason why their interests would be prejudiced or this litigation unduly delayed by the Tribes’ presence. The Tribes’ motions to intervene are sufficiently timely with respect to this phase of the litigation. Of course, permission to intervene does not carry with it the right to relitigate matters already determined in the case, unless those matters would otherwise be subject to reconsideration. The motions to intervene are granted.
l — i <1
We turn now to the first major question m the case: whether the determination of practicably irrigable acreage within recognized reservation boundaries should be reopened to consider claims for “omitted” lands for which water rights could have been sought in the litigation preceding the 1964 decree. The Special Master agreed with the United States and the Tribes that it is not too late in the day to modify the 1964 adjudication and decree, notwithstanding his own finding that “[t]he claim in the original case... embraced the totality of water rights for the Reservation lands.” Tuttle Report, at 31. We disagree with the Special Master and sustain the exceptions filed by the States and state agencies to his conclusion. In our opinion, the prior determination of Indian water rights in the 1964 decree precludes relitigation of the irrigable acreage issue.
Arizona v. California, unlike many other disputes over water rights that we have adjudicated, has been and continues to be governed mainly by statutory considerations. The primary isfeue in the case — the allocation of the waters of the Lower Colorado River Basin among the States — was resolved by the distribution of waters intended by Congress and written into the Project Act. The question of Indian water rights — an important but ancillary concern — was also decided by recourse to congressional policy rather than judicial equity. We held that the creation of the reservations by the Federal Government implied an allotment of water necessary to “make the reservation livable.” 373 U. S., at 599-600. See Winters v. United States, 207 U. S. 564 (1908); Cappaert v. United States, 426 U. S. 128, 141 (1976). We rejected the argument, urged by the States, that equitable apportionment should govern the question. We were “not convinced by Arizona’s argument that each reservation is so much like a State that its rights to water should be determined by the doctrine of equitable apportionment.” 373 U. S., at 597. “Moreover, even were we to treat an Indian reservation like a State, equitable apportionment would still not control, since, under our view, the Indian claims here are governed by the statutes and Executive Orders creating the reservations. ” Ibid.
We went on to reject Arizona’s further arguments that (1) the doctrine of Pollard’s Lessee v. Hagan, 3 How. 212 (1845), and Shively v. Bowlby, 152 U. S. 1 (1894), prevented the Federal Government from reserving waters for federally reserved lands, 373 U. S., at 597; (2) water rights could not be reserved by Executive Order, id., at 598; and (3) there was insufficient evidence that the United States intended to reserve water for the Tribes, id., at 598-600.
The standard for quantifying the reserved water rights was also hotly contested by the States, who argued that the Master adopted a much too liberal measure. Our decision to rely upon the amount of practicably irrigable acreage contained within the reservation constituted a rejection of Arizona’s proposal that the quantity-of water reserved should be measured by the Indians’ “reasonably foreseeable needs,” i. e., by the number of Indians. The practicably-irrigable-acreage standard was preferable because how many Indians there will be and what their future needs will be could “only be guessed,” id., at 601. By contrast, the irrigable-acreage standard allowed a present water allocation that would be appropriate for future water needs. Id., at 600-601. Therefore, with respect to the question of reserved rights for the reservations, and the measurement of those rights, the Indians, as represented by the United States, won what can be described only as a complete victory. A victory, it should be stressed, that was in part attributable to the Court’s interest in a fixed calculation of future water needs. Applying the irrigable-acreage standard, we found that the Master’s determination as to the amount of practicably irrigable acreage, an issue also subject to adversary proceedings, was reasonable. Our subsequent decree reflected this judgment. 376 U. S. 340 (1964).
The Tribes and the United States now claim that certain practicably irrigable acreage was “omitted” from those calculations. There is no question that if these claims were presented in a different proceeding, a court would be without power to reopen the matter due to the operation of res judicata. That would be true here were it not for Article IX of the 1964 decree which provides:
“Any of the parties may apply at the foot of this decree for its amendment or for farther relief. The Court retains jurisdiction of this suit for the purpose of any order, direction, or modification of the decree, or any supplementary decree, that may at any time be deemed proper in relation to the subject matter in controversy.”
We agree with the United States and the Tribes that this provision grants us power to correct certain errors, to determine reserved questions, and, if necessary, to make modifications in the decree. We differ in our understanding of the circumstances which make exercise of this power appropriate.
The Special Master believed that the decision whether to exercise that discretion should be governed by “law of the case” principles. Unlike the more precise requirements of res judicata, law of the case is an amorphous concept. As most commonly defined, the doctrine posits that when a court decides upon a rule of law, that decision should continue to govern the same issues in subsequent stages in the same case. See IB J. Moore & T. Currier, Moore’s Federal Practice ¶ 0.404 (1982) (hereinafter Moore). Law of the case directs a court’s discretion, it does not limit the tribunal’s power. Southern R. Co. v. Clift, 260 U. S. 316, 319 (1922); Messenger v. Anderson, 225 U. S. 436, 444 (1912). In that sense, the doctrine might appear applicable here. But law of the case doctrine was understandably crafted with the course of ordinary litigation in mind. Such litigation proceeds through preliminary stages, generally matures at trial, and produces a judgment, to which, after appeal, the binding finality of res judicata and collateral estoppel will attach. To extrapolate wholesale law of the case into the situation of our original jurisdiction, where jurisdiction to accommodate changed circumstances is often retained, would weaken to an intolerable extent the finality of our decrees in original actions, particularly in a case such as this turning on statutory rather than Court-fashioned equitable criteria.
For the following reasons, we hold that Article IX must be given a narrower reading and should be subject to the general principles of finality and repose, absent changed circumstances or unforeseen issues not previously litigated.
First, while the the technical rules of preclusion are not strictly applicable, the principles upon which these rules are founded should inform our decision. It is clear that res judicata and collateral estoppel do not apply if a party moves the rendering court in the same proceeding to correct or modify its judgment. IB Moore ¶ 0.407, pp. 931-935; R. Field, B. Kaplan, & K. Clermont, Materials on Civil Procedure 860 (4th ed. 1978). Nevertheless, a fundamental precept of common-law adjudication is that an issue once determined by a competent court is conclusive. Montana v. United States, 440 U. S. 147, 153 (1979); Federated Department Stores, Inc. v. Moitie, 452 U. S. 394, 398 (1981); Cromwell v. County of Sac, 94 U. S. 351, 352-353 (1877). “To preclude parties from contesting matters that they have had a full and fair opportunity to litigate protects their adversaries from the expense and vexation attending multiple lawsuits, conserves judicial resources, and fosters reliance on judicial action by minimizing the possibility of inconsistent decisions.” Montana v. United States, supra, at 153-154.
In no context is this more true than with respect to rights in real property. Abraham Lincoln once described with scorn those who sat in the basements of courthouses combing property records to upset established titles. Our reports are replete with reaffirmations that questions affecting titles to land, once decided, should no longer be considered open. Minnesota Co. v. National Co., 3 Wall. 332, 334 (1866); United States v. Title Ins. Co., 265 U. S. 472, 486 (1924). Certainty of rights is particularly important with respect to water rights in the Western United States. The development of that area of the United States would not have been possible without adequate water supplies in an otherwise water-scarce part of the country. Colorado River Water Conservation District v. United States, 424 U. S. 800, 804 (1976). The doctrine of prior appropriation, the prevailing law in the Western States, is itself largely a product of the compelling need for certainty in the holding and use of water rights.
Recalculating the amount of practicably irrigable acreage runs directly counter to the strong interest in finality in this case. A major purpose of this litigation, from its inception to the present day, has been to provide the necessary assurance to States of the Southwest and to various private interests, of the amount of water they can anticipate to receive from the Colorado River system. “In the arid parts of the West... claims to water for use on federal reservations inescapably vie with other public and private claims for the limited quantities to be found in the rivers and streams.” United States v. New Mexico, 438 U. S. 696, 699 (1978). If there is no surplus of water in the Colorado River, an increase in federal reserved water rights will require a “gallon-for-gallon reduction in the amount of water available for water-needy state and private appropriators.” Id., at 705. As Special Master Tuttle recognized, “[n]ot a great deal of evidence is really needed to convince anyone that western states would rely upon water adjudications.” Tuttle Report, at 46. Not only did the Metropolitan Water District in California and the Central Arizona Project predicate their plans on the basis of the 1964 allocations, but, due to the high priority of Indian water claims, an enlargement of the Tribes’ allocation cannot help but exacerbate potential water shortage problems for these projects and their States.
Article IX did not contemplate a departure from these fundamental principles so as to permit retrial of factual or legal issues that were fully and fairly litigated 20 years ago. The Article does not explicate the conditions under which changes in the decree are appropriate. Very little discussion surrounded the Article, which was included in Master Rifkind’s recommended decree as an agreed-upon provision. This in itself suggests that the Article was mainly a safety net added to retain jurisdiction and to ensure that we had not, by virtue of res judicata, precluded ourselves from adjusting the decree in light of unforeseeable changes in circumstances.
This reading is supported by the proceedings before Master Rifkind. The record demonstrates that it was the understanding of the parties and Master Rifkind’s intention that the calculation of practicably irrigable acreage be final. That was our understanding as well, and was reflected in his and our choice of the practicably-irrigable-acreage standard as a measure which would allow a fixed present determination of future needs for water. It is untenable that the parties, the Special Master, and this Court would have intended Article IX to undercut the prevailing understanding that the calculation of practicably irrigable acreage was to be final without so much as discussing the subject.
This interpretation of Article IX is consistent with our action in prior original cases. Our long history of resolving disputes over boundaries and water rights reveals a simple fact: This Court does not reopen an adjudication in an original action to reconsider whether initial factual determinations were correctly made. In two original cases in which provisions virtually identical to Article IX were included, subsequent modifications were made in reaction to changed circumstances. Wisconsin v. Illinois, 278 U. S. 367 (1929), 281 U. S. 179, decree entered, 281 U. S. 696 (1930), temporarily modified, 352 U. S. 945 (1956), 352 U. S. 983 (1957), superseded, 388 U. S. 426 (1967); New Jersey v. New York, 283 U. S. 336, decree entered, 283 U. S. 805 (1931), modified, 347 U. S. 995 (1954). The Court’s purpose in retaining jurisdiction in those cases can be gleaned from the respective reports of the Special Masters, which note the need for flexibility in light of changed conditions and questions which could not be disposed of at the time of an initial decree. This interpretation is also consistent with the role of a “court of equity to modify an injunction in adaptation to changed conditions.” Railway Employes v. Wright, 364 U. S. 642, 647 (1961); United States v. Swift & Co., 286 U. S. 106, 114 (1932).
We note that our cases with similar reservations of jurisdiction involved equitable apportionment where our latitude to correct inequitable allocations injustices is at its broadest. If even there our retention of jurisdiction was limited to the consideration of new issues and changed circumstances, rather than to permit the relitigation of factual determinations on which a decree has been based, a fortiori the reservation of jurisdiction in this case, not governed by equitable apportionment, is no broader.
We also fear that the urge to relitigate, once loosed, will not be easily cabined. The States have already indicated, if the issue were reopened, that the irrigable-acreage standard itself should be reconsidered in light of our decisions in United States v. New Mexico, 438 U. S. 696 (1978), and Washington v. Washington Commercial Passenger Fishing Vessel Assn., 443 U. S. 658 (1979), and we are not persuaded that a defensible line can be drawn between the reasons for reopening this litigation advanced by the Tribes and the United States on the one hand and the States on the other. It would be counter to the interests of all parties to this case to open what may become a Pandora’s Box, upsetting the certainty of all aspects of the decree. These considerations, combined with the practice in our original cases and the strong res judicata interests involved, lead us to conclude that the irrigable-acreage question should not be relitigated.
Because we have determined that the principles of res judi-cata advise against reopening the calculation of the amount of practicably irrigable acreage, and that Article IX does not demand that we do so, it is unnecessary to resolve the bitterly contested question of the extent to which the States have detrimentally relied on the 1964 decree. Detrimental reliance is certainly relevant in a balancing of the equities when determining whether changed circumstances justify modification of a decree. We believe that a certain manner of reliance has occurred, swpra, at 621, but even the absence of detrimental reliance cannot open an otherwise final determination of a fully litigated issue. Finality principles would become meaningless if an adversarially determined issue were final only if the equities were against revising it.
Similarly, it is hardly determinative that the changes requested by the United States and the Indian Tribes do not involve reallocations of as much water as was involved in the initial litigation. Aside from the fact that the requested increases of between 15 and 22 percent in the amount of irrigable acreage determined in the initial decree hardly constitute “relatively minor adjustments,” the magnitude of the adjustment requested is relevant only after it is established that the underlying legal issue is one which should be redetermined.
Finally, the absence of the Indian Tribes in the prior proceedings in this case does not dictate or authorize relitigation of their reserved rights. As a fiduciary, the United States had full authority to bring the Winters rights claims for the Indians and bind them in the litigation. Heckman v. United States, 224 U. S. 413 (1912). We find no merit in the Tribes’ contention that the United States’ representation of their interests was inadequate whether because of a claimed conflict of interests arising from the Government’s interest in securing water rights for other federal property, or otherwise. The United States often represents varied interests in litigation involving water rights, particularly given the large extent and variety of federal land holdings in the West. See, e. g., Colorado River Water Conservation District v. United States, 424 U. S., at 805. The Government’s representation of these varied interests does not deprive our decisions of finality. In this case, there is no demonstration that the United States, as a fiduciary, was involved in an actual conflict of interest. From the initiation of this case, the Government has taken seriously its responsibility to represent the Tribes’ interests, and we have no indication that the Government’s representation of the Tribes’ interests with respect to the amount of practicably irrigable acreage was legally inadequate. Recognition of Indian water rights would not diminish other federally reserved water rights. Under the Project Act, there was no basis for the Government to believe that Indian water rights and water needs for other federal property were in direct competition. Our 1963 opinion bore this out: perfected rights for the use of federal establishments were charged against the States’ apportionment, 373 U. S., at 601, and, in times of shortage, under the decree, the Secretary of the Interior retained broad power to ensure that perfected rights for the use of federal establishments are satisfied. Id., at 593-594; 376 U. S., at 343-344. Indeed, the substantial water allocations awarded the Tribes reflect the competency of the United States’ representation. We believe the issue of practicably irrigable acreage was fully and fairly litigated in 1963.
Accordingly, we sustain the States’ exceptions to this aspect of the Special Master’s report.
V
We now address the dispute over reservation boundaries, which first arose during the hearing before Special Master Rifkind.
A
In the course of the proof by the United States as to the extent of the irrigable acreage of the Colorado River and Fort Mojave Reservations, California disputed the location of the boundaries of these reservations. On the theory that failure to adjudicate these controversies would leave non-Indian users in doubt as to the water available for their use, and would leave the Secretary in doubt as to how to operate Hoover Dam and the mainstream works below, the former Master deemed it necessary to resolve the boundary disputes, see Rifkind Report, at 256-257, and he held several days of hearings on these matters. Tr. 19,992 et seq. California objected to these proceedings. The State felt it lacked authority to represent the private individuals who claimed title to land the United States contended was part of the reservations. Id., at 19,998-20,000. The Master nevertheless ruled on the boundary issues, for the most part in California’s favor — that is, the Master concluded that the reservations covered a smaller area than the United States claimed and that the irrigable acreage and reserved water rights should be determined on this basis.
California maintained its position before this Court that the Master should not have determined the disputed boundary of the Colorado River Reservation. California contended that it would be unfair to prejudice any of the parties in future litigation over land titles or political jurisdiction by approving findings on a tangential issue never pleaded by the United States. The State also observed that postponing determination of the boundary dispute would not materially affect the priority of the water right to which the disputed land was entitled, since both the Indians and the Palo Verde Irrigation District, in which California would place the disputed land, had high priorities. California did not specifically object to the Master’s resolution of the Fort Mojave boundary dispute, no doubt because, on the merits of this issue, the Master entirely agreed with the State’s position.
The United States responded that the Master acted properly by resolving the boundary disputes:
“The determination of the boundary of each Reservation is an essential prerequisite to the determination of the quantum of the water rights for that Reservation. There is no question of the Court’s jurisdiction to resolve boundary questions nor of the authority of California to act as parens patriae for its citizens in such matters.”
The United States did not file any exceptions to the boundary determinations of the Special Master.
We did not accept the Master’s resolution of the boundary disputes:
“We disagree with the Master’s decision to determine the disputed boundaries of the Colorado River Indian Reservation and the Fort Mohave Indian Reservation. We hold that it is unnecessary to resolve those disputes here. Should a dispute over title arise because of some future refusal by the Secretary to deliver water to either area, the dispute can be settled at that time.” 373 U. S., at 601.
The decree that we entered limited the water rights of the two reservations to those awarded by the Master, based on the irrigable acreage within the boundaries as he had found them, but with respect to the boundary disputes, as stipulated by the parties, Article 11(D)(5) of the decree provided:
“[T]he quantities [of water] fixed in [the paragraphs setting the water rights of the Colorado River and Fort Mojave Reservations] shall be subject to appropriate adjustment by agreement or decree of this Court in the event that the boundaries of the respective reservations are finally determined.” 376 U. S., at 345.
B
The disputes about the boundaries of the Colorado River and the Fort Mojave Reservations are still with us. And since the

Question: What is the issue area of the decision?
A. Criminal Procedure
B. Civil Rights
C. First Amendment
D. Due Process
E. Privacy
F. Attorneys
G. Unions
H. Economic Activity
I. Judicial Power
J. Federalism
K. Interstate Relations
L. Federal Taxation
M. Miscellaneous
N. Private Action
Answer:

Answer: B