Justice O’Connor
delivered the opinion of the Court.
In this original action, the State of Colorado seeks an equitable apportionment of the waters of the Vermejo River, an interstate river fully appropriated by users in the State of New Mexico. A Special Master, appointed by this Court, initially recommended that Colorado be permitted a diversion of 4,000 acre-feet per year. Last Term, we remanded for additional factual findings on five specific issues. 459 U. S. 176 (1982). The case is before us again on New Mexico’s exceptions to these additional findings. We now conclude that Colorado has not demonstrated by clear and convincing evidence that a diversion should be permitted. Accordingly, we sustain New Mexico’s exceptions and dismiss the case.
I — I
The facts of this litigation were set forth in detail in our opinion last Term, see id., at 178-183, and we need recount them here only briefly. The Vermejo River is a small, non-navigable stream, originating in the snow belt of the Rocky Mountains. The river flows southeasterly into New Mexico for roughly 55 miles before feeding into the Canadian River. Though it begins in Colorado, the major portion of the Vermejo River is located in New Mexico. Its waters historically have been used exclusively by farm and industrial users in that State.
In 1975, however, a Colorado corporation, Colorado Fuel and Iron Steel Corp. (C. F. & I.), proposed to divert water from the Vermejo River for industrial and other uses in Colorado. As a consequence, several of the major New Mexico users sought and obtained an injunction against the proposed diversion. The State of Colorado, in turn, filed a motion for leave to file an original complaint with this Court, seeking an equitable apportionment of the Vermejo River’s waters. We granted Colorado its leave to file, 439 U. S. 975 (1978), and the Court of Appeals for the Tenth Circuit stayed C. F. & I.’s appeal pending our resolution of the equitable apportionment issue.
We then appointed a Special Master, 441 U. S. 902 (1979), the Honorable Ewing T. Kerr, Senior Judge of the United States District Court for the District of Wyoming, who held a lengthy trial at which both States presented extensive evidence. On the basis of this evidence, the Master recommended that Colorado be allowed to divert 4,000 acre-feet of water per year. His recommendation rested on two grounds: first, that New Mexico could compensate for some or all of the Colorado diversion through reasonable water conservation measures; and second, that the injury, if any, to New Mexico would be outweighed by the benefit to Colorado from the diversion.
New Mexico took exceptions, both legal and factual, to the Master’s recommendation. As to the Master’s view of the law of equitable apportionment, New Mexico contended that the Master erred in not focusing exclusively on the priority of uses along the Vermejo River. 459 U. S., at 181-182. The Court rejected that contention:
“We recognize that the equities supporting the protection of existing economies will usually be compelling. . . . Under some circumstances, however, the countervailing equities supporting a diversion for future use in one State may justify the detriment to existing users in another State. This may be the case, for example, where the State seeking a diversion demonstrates by clear and convincing evidence that the benefits of the diversion substantially outweigh the harm that might result. In the determination of whether the State proposing the diversion has carried this burden, an important consideration is whether the existing users could offset the diversion by reasonable conservation measures . . . .” Id., at 187-188 (footnote omitted).
In short, though the equities presumptively supported protection of the established senior uses, the Court concluded that other factors — such as waste, availability of reasonable conservation measures, and the balance of benefit and harm from diversion — could be considered in the apportionment calculus. Ibid.
New Mexico also took issue with the factual predicates of the Master’s recommendation. Specifically, it contended that Colorado had failed to prove by clear and convincing evidence that New Mexico currently uses more than its equitable share of the Vermejo River’s waters. On this matter, we found the Master’s report unclear and determined that a remand would be appropriate.
To help this Court assess whether Vermejo River water could reasonably be made available for diversion, the Master was instructed to make specific findings concerning:
“(1) the existing uses of water from the Vermejo River, and the extent to which present levels of use reflect current or historical water shortages or the failure of existing users to develop their uses diligently;
“(2) the available supply of water from the Vermejo River, accounting for factors such as variations in stream flow, the needs of current users for a continuous supply, the possibilities of equalizing and enhancing the water supply through water storage and conservation, and the availability of substitute sources of water to relieve the demand for water from the Vermejo River; [and]
“(3) the extent to which reasonable conservation measures in both States might eliminate waste and inefficiency in the use of water from the Vermejo River[.]” Id., at 189-190.
Then, to assist this Court in balancing the benefit and harm from diversion, the Master was asked to make findings concerning:
“(4) the precise nature of the proposed interim and ultimate use in Colorado of water from the Vermejo River, and the benefits that would result from a diversion to Colorado; [and]
“(5) the injury, if any, that New Mexico would likely suffer as a result of any such diversion, taking into account the extent to which reasonable conservation measures could offset the diversion.” Id., at 190 (footnote omitted).
Finally, the Court authorized the Master to consider any other relevant factors, to gather any additional evidence necessary to making the requested findings, and to offer another — although not necessarily different — recommendation. Id., at 190, and n. 14.
On remand, New Mexico filed a motion to submit new evidence. Colorado opposed the motion and attested that, unless the record were reopened, it did not intend to offer any additional evidence in support of its case. The Special Master denied New Mexico’s motion. Then, on the basis of the evidence previously received, he developed additional factual findings and reaffirmed his original recommendation.
I — I h — i
Last Term, because our initial inquiry turned on the factors relevant to determining a just apportionment, the Court explained in detail the law of equitable apportionment. This Term, because our inquiry turns on the evidentiary material Colorado has offered in support of its complaint, we find it necessary to explain the standard by which we judge proof in actions for equitable apportionment.
The function of any standard of proof is to “instruct the factfinder concerning the degree of confidence our society thinks he should have in the correctness of factual conclusions for a particular type of adjudication.” In re Winship, 397 U. S. 358, 370 (1970) (Harlan, J., concurring). By informing the factfinder in this manner, the standard of proof allocates the risk of erroneous judgment between the litigants and indicates the relative importance society attaches to the ultimate decision. See Addington v. Texas, 441 U. S. 418, 423-425 (1979).
Last Term, the Court made clear that Colorado’s proof would be judged by a clear-and-convincing-evidence standard. Colorado v. New Mexico, 459 U. S., at 187-188, and n. 13. In contrast to the ordinary civil case, which typically is judged by a “preponderance of the evidence” standard, we thought a diversion of interstate water should be allowed only if Colorado could place in the ultimate factfinder an abiding conviction that the truth of its factual contentions are “highly probable.” See C. McCormick, Law of Evidence § 320, p. 679 (1954). This would be true, of course, only if the material it offered instantly tilted the evidentiary scales in the affirmative when weighed against the evidence New Mexico offered in opposition. See generally McBaine, Burden of Proof: Degrees of Belief, 32 Calif. L. Rev. 242, 251-254 (1944).
Requiring Colorado to present clear and convincing evidence in support of its proposed diversion is necessary to appropriately balance the unique interests involved in water rights disputes between sovereigns. The standard reflects this Court’s long-held view that a proposed diverter should bear most, though not all, of the risks of erroneous decision: “The harm that may result from disrupting established uses is typically certain and immediate, whereas the potential benefits from a proposed diversion may be speculative and remote.” Colorado v. New Mexico, 459 U. S., at 187; see also id., at 182, n. 9. In addition, the clear-and-convincing-evidence standard accommodates society’s competing interests in increasing the stability of property rights and in putting resources to their most efficient uses: “[T]he rule of priority [will] not be strictly applied where it ‘would work more hardship’ on the junior user ‘than it would bestow benefits’ on the senior user . . . [,though] the equities supporting the protection of existing economies will usually be compelling.” Id., at 186-187 (quoting Nebraska v. Wyoming, 325 U. S. 589, 619 (1945)). In short, Colorado’s diversion should and will be allowed only if actual inefficiencies in present uses or future benefits from other uses are highly probable.
With these principles in mind, we turn to review the evidence the parties have submitted concerning the proposed diversion. As our opinion noted last Term, New Mexico has met its initial burden of showing “real or substantial injury” because “any diversion by Colorado, unless offset by New Mexico at its own expense, [would] necessarily reduce the amount of water available to New Mexico users.” 459 U. S., at 188, n. 13. Accordingly, the burden shifted on remand to Colorado to show, by clear and convincing evidence, that reasonable conservation measures could compensate for some or all of the proposed diversion and that the injury, if any, to New Mexico would be outweighed by the benefits to Colorado from the diversion. Though the Master’s findings on these issues deserve respect and a tacit presumption of correctness, the ultimate responsibility for deciding what are correct findings of fact remains with us. See Mississippi v. Arkansas, 415 U. S. 289, 291-292, 294 (1974); C. Wright, A. Miller, & E. Cooper, Federal Practice and Procedure § 4054, pp. 196-197 (1978). Upon our independent review of the record, we find that Colorado has failed to meet its burden.
A
To establish whether Colorado’s proposed diversion could be offset by eliminating New Mexico’s nonuse or inefficiency, we asked the Master to make specific findings concerning existing uses, supplies of water, and reasonable conservation measures available to the two States. After assessing the evidence both States offered about existing uses and available supplies, the Master concluded that “current levels of use primarily reflect failure on the part of existing users to fully develop and put to work available water.” Additional Factual Findings 28. Moreover, with respect to reasonable conservation measures available, the Master indicated his belief that more careful water administration in New Mexico would alleviate shortages from unregulated stockponds, fishponds, and water detention structures, prevent waste from blockage and clogging in canals, and ensure that users fully devote themselves to development of available resources. He further concluded that “the heart of New Mexico’s water problem is the Vermejo Conservancy District,” id., at 20, which he considered a failed “reclamation project [that had] never lived up to its expectations or even proved to be a successful project, . . . and [that] quite possibly should never have been built.” Id., at 8. Though the District was quite arguably in the “middle range in reclamation project efficiencies,” id., at 20, the Master was of the opinion “that [the District’s] inefficient water use should not be charged to Colorado.” Ibid. Furthermore, though Colorado had not submitted evidence or testimony of any conservation measures that C. F. & I. would take, the Master concluded that “it is not for the Master or for New Mexico to say that reasonable attempts to conserve water will not be implemented by Colorado.” Id., at 21.
We share the Master’s concern that New Mexico may be overstating the amount of harm its users would suffer from a diversion. Water use by appropriators along the Vermejo River has remained relatively stable for the past 30 years, and this historic use falls substantially below the decreed rights of those users. Unreliable supplies satisfactorily explain some of this difference, but New Mexico’s attempt to excuse three decades of nonuse in this way is, at the very least, suspect. Nevertheless, whatever the merit of New Mexico’s explanation, we cannot agree that Colorado has met its burden of identifying, by clear and convincing evidence, conservation efforts that would preserve any of the Vermejo River water supply.
For example, though Colorado alleged that New Mexico could improve its administration of stockponds, fishponds, and water detention structures, it did not actually point to specific measures New Mexico could take to conserve water. Thus, ultimately all the Master could conclude was that some unspecified “[Reduction and/or regulation . . . could not help but be an effort, however small, to conserve the water supply. ...” Id., at 18. Similarly, though Colorado asserted that more rigorous water administration could eliminate blocked diversion works and ensure more careful development of water supplies, it did not show how this would actually preserve existing supplies. Even if Colorado’s generalizations were true, they would prove only that some junior users are diverting water that senior appropriators ultimately could call; they would not prove that water is being wasted or used inefficiently by those actually diverting it. In short, the administrative improvements Colorado suggests are either too general to be meaningful or involve redistribution, as opposed to preservation, of water supplies.
Colorado’s attack on current water use in the Vermejo Conservancy District is inadequate for much the same reason. Our cases require only conservation measures that are “financially and physically feasible” and “within practicable limits.” See, e. g., Colorado v. New Mexico, 459 U. S., at 192; Wyoming v. Colorado, 259 U. S. 419, 484 (1922). New Mexico submitted substantial evidence that the District is in the middle of reclamation project efficiencies and that the District has taken considerable independent steps — including, the construction, at its own expense and on its own initiative, of a closed stockwater delivery system — to improve the efficiency of its future water use. Additional Factual Findings 20. The Master did not find to the contrary; indeed, he commended New Mexico for the substantial efforts it had taken. See ibid. Nevertheless, he accepted Colorado’s general assertion that the District was not as efficient as other reclamation projects and concluded that New Mexico’s inefficient use should not be charged to Colorado. But Colorado has not identified any “financially and physically feasible” means by which the District can further eliminate or reduce inefficiency and, contrary to the Master’s suggestion, we believe that the burden is on Colorado to do so. A State can carry its burden of proof in an equitable apportionment action only with specific evidence about how existing uses might be improved, or with clear evidence that a project is far less efficient than most other projects. Mere assertions about the relative efficiencies of competing projects will not do.
Finally, there is no evidence in the record that “Colorado has undertaken reasonable steps to minimize the amount of the diversion that will be required.” Colorado v. New Mexico, supra, at 186. Nine years have passed since C. F. & I. first proposed diverting water from the Vermejo River. Yet Colorado has presented no evidence concerning C. F. & I.’s inability to relieve its needs through substitute sources. Furthermore, there is no evidence that C. F. & I. has settled on a definite or even tentative construction design or plan, or that it has prepared an economic analysis of its proposed diversion. Indeed, C. F. & I. has not even conducted an operational study of the reservoir that Colorado contends will be built in conjunction with the proposed diversion. It may be impracticable to ask the State proposing a diversion to provide unerring proof of future uses and concomitant conservation measures that would be taken. But it would be irresponsible of us to apportion water to uses that have not been, at a minimum, carefully studied and objectively evaluated, not to mention decided upon. Financially and physically feasible conservation efforts include careful study of future, as well as prudent implementation of current, water uses. Colorado has been unwilling to take any concrete steps in this direction.
Society’s interest in minimizing erroneous decisions in equitable apportionment cases requires that hard facts, not suppositions or opinions, be the basis for interstate diversions. In contrast to Justice Stevens, we do not believe Colorado has produced sufficient facts to show, by clear and convincing evidence, that reasonable conservation efforts will mitigate sufficiently the injury that New Mexico successfully established last Term that it would suffer were a diversion allowed. No State can use its lax administration to establish its claim to water. But once a State successfully proves that a diversion will cause it injury, the burden shifts to the diverter to show that reasonable conservation measures exist. Colorado has not carried this burden.
B
We also asked the Master to help us balance the benefits and harms that might result from the proposed diversion. The Master found that Colorado’s proposed interim use is agricultural in nature and that more permanent applications might include use in coal mines, timbering, power generation, domestic needs, and other industrial operations. The Master admitted that “[t]his area of fact finding [was] one of the most difficult [both] because of the necessarily speculative nature of [the] benefits ...” and because of Colorado’s “natural reluctance to spend large amounts of time and money developing plans, operations, and cost schemes . . . .” Additional Factual Findings 23. Nevertheless, because the diverted water would, at a minimum, alleviate existing water shortages in Colorado, the Master concluded that the evidence showed considerable benefits would accrue from the diversion. Furthermore, the Master concluded that the injury, if any, to New Mexico would be insubstantial, if only becausi~reasonable conservation measures could, in his opinion, offset the entire impact of the diversion. Id., at 24-28.
Again, we find ourselves without adequate evidence to approve Colorado’s proposed diversion. Colorado has not committed itself to any long-term use for which future benefits can be studied and predicted. Nor has Colorado specified how long the interim agricultural use might or might not last. All Colorado has established is that a steel corporation wants to take water for some unidentified use in the future.
By contrast, New Mexico has attempted to identify the harms that would result from the proposed diversion. New Mexico commissioned some independent economists to study the economic effects, direct and indirect, that the diversion would have on persons in New Mexico. The study these economists produced was submitted at the original hearing, conducted prior to the remand, as evidence of the injury that would result from the reduction in water supplies. No doubt, this economic analysis involves prediction and forecast. But the analysis is surely no more speculative than the generalizations Colorado has offered as “evidence.” New Mexico, at the very least, has taken concrete steps toward addressing the query this Court posed last Term. Colorado has made no similar effort.
Colorado objects that speculation about the benefits of future uses is inevitable and that water will not be put to its best use if the expenditures necessary to development and operation must be made without assurance of future supplies. We agree, of course, that asking for absolute precision in forecasts about the benefits and harms of a diversion would be unrealistic. But we have not asked for such precision. We have only required that a State proposing a diversion conceive and implement some type of long-range planning and analysis of the diversion it proposes. Long-range planning and analysis will, we believe, reduce the uncertainties with which equitable apportionment judgments are made. If New Mexico can develop evidence to prove that its existing economy is efficiently using water, we see no reason why Colorado cannot take similar steps to prove that its future economy could do better.
In the nine years that have passed since C. F. & I. first requested a diversion, neither it nor Colorado has decided upon a permanent use for the diverted water. It therefore is no surprise that Colorado cannot conduct studies or make predictions about the benefits and harms of its proposed diversion. Under the clear-and-convincing-evidence standard, it is Colorado, and not New Mexico, that must bear the risk of error from the inadequacy of the information available.
C
As a final consideration, the Master pointed out that approximately three-fourths of the water in the Vermejo River system is produced in Colorado. He concluded, therefore, that “the equities are with Colorado, which requests only a portion of the water which it produces.” Additional Factual Findings 29. Last Term, the Court rejected the notion that the mere fact that the Vermejo River originates in Colorado automatically entitles Colorado to a share of the river’s waters. Colorado v. New Mexico, 459 U. S., at 181, n. 8. Both Colorado and New Mexico recognize the doctrine of prior appropriation, id., at 179, and appropriative, as opposed to riparian, rights depend on actual use, not land ownership. See id., at 179, n. 4. It follows, therefore, that the equitable apportionment of appropriated rights should turn on the benefits, harms, and efficiencies of competing uses, and that the source of the Vermejo River’s waters should be essentially irrelevant to the adjudication of these sovereigns’ competing claims. Id., at 181, n. 8. To the extent the Master continued to think the contrary, he was in error.
f> HH
We continue to believe that the flexible doctrine of equitable apportionment extends to a State’s claim to divert previously appropriated water for future uses. But the State seeking such a diversion bears the burden of proving, by clear and convincing evidence, the existence of certain relevant factors. The complainant must show, for example, the extent to which reasonable conservation measures can adequately compensate for the reduction in supply due to the diversion, and the extent to which the benefits from the diversion will outweigh the harms to existing users. This evidentiary burden cannot be met with generalizations about unidentified conservation measures and unstudied speculation about future uses. The Special Master struggled, as best he could, to balance the evidentiary requirement against the inherent limitations of proving a beneficial future use. However, we do not find enough evidence to sustain his findings. Until Colorado can generate sufficient evidence to show that circumstances have changed and that a diversion is appropriate, the equities compel the continued protection of the existing users of the Vermejo River’s waters.
Accordingly, we sustain the State of New Mexico’s exceptions to the Special Master’s Report and Additional Factual Findings, and dismiss the case.
It is so ordered.