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NOTICE: This opinion is subject to formal revision before publication in the
preliminary print of the United States Reports.  Readers are requested to
notify the Reporter of Decisions, Supreme Court of the United States, Wash-
ington, D.C. 20543, of any typographical or other formal errors, in order that
corrections may be made before the preliminary print goes to press.
SUPREME COURT OF THE UNITED STATES
--------
No. 105, Orig.
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STATE OF KANSAS, PLAINTIFF v. STATE OF
COLORADO
on bill of complaint
[May 15, 1995]

  Chief Justice Rehnquist delivered the opinion of the
Court.
  This original action involves a dispute among Kansas,
Colorado, and the United States over alleged violations
of the Arkansas River Compact.  The Special Master has
filed a report (Report) detailing his findings and recom-
mendations concerning the liability phase of the trial.
Both Kansas and Colorado have filed exceptions to those
findings and recommendations.  We agree with the
Special Master's disposition of the liability issues.
Accordingly, we overrule the parties' exceptions.

                            I
  The Continental Divide in the United States begins at
the Canadian border in the mountains of northwestern
Montana.  From there, it angles southeast through
Montana and Wyoming until it enters Colorado.  It then
runs roughly due south through Colorado, following first
the crest of the Front Range of the Rocky Mountains,
and then shifting slightly west to follow the crest of the
Sawatch Range.  The Arkansas River rises on the east
side of the Continental Divide, between Climax and
Leadville, Colorado.  Thence it flows south and east
through Colorado, Kansas, Oklahoma, and Arkansas,
emptying into the Mississippi River, which in turn flows
into the Gulf of Mexico.  As if to prove that the ridge
which separates them is indeed the Continental Divide,
a short distance away from the source of the Arkansas,
the Colorado River rises and thence flows southwest
through Colorado, Utah, and Arizona, and finally
empties into the Gulf of Baja California.
  The Arkansas River flows at a steep gradient from its
source south to Canon City, Colorado, whence it turns
east and enters the Royal Gorge.  As it flows through
the Royal Gorge, the Arkansas River is at some points
half a mile below the summit of the bordering cliffs.
The Arkansas River thence descends gradually through
the high plains of eastern Colorado and western Kansas;
its elevation at the Colorado-Kansas border is 3,350 feet.
It then makes its great bend northward through Kansas,
and from there flows southeasterly through northeastern
Oklahoma and across Arkansas.  The Arkansas River
covers about 1,450 miles from its source in the Colorado
Rockies to the point in southeastern Arkansas where it
flows into the Mississippi River.  It is the fourth longest
river in the United States, and it drains in an area of
185,000 square miles.
  The first Europeans to see the Arkansas River were
members of the expedition of Francisco Coronado, in the
course of their search for the fabled Southern Cities of
Cibola.  In 1541, they crossed the Arkansas River near
what is now the Colorado-Kansas border.  One year
later, those in the expedition of Hernando DeSoto would
see the Arkansas River 1,000 miles downstream at its
mouth.  The western borders of the Louisiana Purchase,
acquired from France in 1803, included within them
most, if not all, of the Arkansas River drainage basin.
Zebulon Pike, in his expedition of 1805-1806, in the
course of which he sighted the mountain peak named
after him, traveled up the Arkansas River.  John C.
Freemont traversed the River in the other direction in
his expedition of 1843-1844.
  Today, as a result of the Kerr McClellan Project, the
Arkansas River is navigable for ocean going vessels all
the way from its mouth to Tulsa, Oklahoma.  The
Arkansas River is unique in that the pronunciation of
its name changes from State to State.  In Colorado,
Oklahoma, and Arkansas, it is pronounced as is the
name of the State of Arkansas, but in Kansas, it is
pronounced Ar-KAN-sas.
  The reach of the Arkansas River system at issue here
is a fertile agricultural region that extends from Pueblo,
Colorado, to Garden City, Kansas.  This region has been
developed in Colorado by 23 major canal companies and
in Kansas by 6 canal companies, which divert the
surface flows of the Arkansas River and distribute them
to individual farmers.  Report 35-38.  Also relevant to
this dispute, the United States has constructed three
large water storage projects in the Arkansas River basin.
Id., at 43-48.  The John Martin Reservoir, located on
the Arkansas River about 60 miles west of the Kansas
border, was authorized by Congress in 1936, 49 Stat.
1570, and was completed in 1948.  It is the largest of
the federal reservoirs, and initially it had a storage
capacity of about 700,000 acre-feet.  Report 45.  The
Pueblo Reservoir, located on the Arkansas River about
150 miles west of the Kansas border, was authorized by
Congress in 1962, and was substantially completed in
1975.  Id., at 44.  In 1977, the storage capacity of the
Pueblo Reservoir was estimated to be about 357,000
acre-feet.  Ibid.  Finally, the Trinidad Reservoir, located
on the Purgatoire River (a major tributary of the
Arkansas River) was approved by Congress in 1958, and
was completed in 1977.  Id., at 43.  The total capacity
of the Trinidad Reservoir is about 114,000 acre-feet.
Ibid.
  Twice before in this century, the States of Kansas and
Colorado have litigated in this Court regarding their
respective rights to the waters of the Arkansas River.
See Kansas v. Colorado, 206 U. S. 46 (1907); Colorado
v. Kansas, 320 U. S. 383 (1943).  In the first suit, the
Court denied Kansas' request to enjoin diversions of the
Arkansas River by Colorado because the depletions
alleged by Kansas were insufficient to warrant injunctive
relief.  Kansas v. Colorado, supra, at 114-117.  In the
second suit, Colorado sought to enjoin lower court
litigation brought against Colorado water users, while
Kansas sought an equitable apportionment of the
Arkansas River.  Colorado v. Kansas, supra, at 388-389.
The Court granted Colorado an injunction, but concluded
that Kansas was not entitled to an equitable apportion-
ment.  320 U. S., at 400.  The Court suggested that the
States resolve their differences by negotiation and
agreement, pursuant to the Compact Clause of the
Constitution.  Id., at 392.  See U. S. Const., Art. I, 10,
cl. 3.
  In 1949, after three years of negotiations, Kansas and
Colorado approved, and Congress ratified, the Arkansas
River Compact (Compact).  See 63 Stat. 145; see also
Report 5-6; App. to Report 1-17 (reprinting text of
Compact).  Article VIII of the Compact creates the
Arkansas River Compact Administration (Administration)
and vests it with the power and responsibility for
administering the Compact.  Id., at 11-15.  The Admin-
istration is composed of a nonvoting presiding officer
designated by the President of the United States, and
three voting representatives from each State.  Each
State has one vote, and every decision, authorization, or
other action by the Administration requires a unanimous
vote.  Id., at 12-13 (Article VIII-D).
  The Compact's primary purposes are to -[s]ettle
existing disputes and remove causes of future contro-
versy . . . concerning the waters of the Arkansas River-
and to -[e]quitably divide and apportion- the waters of
the Arkansas River, -as well as the benefits arising from
the construction, operation and maintenance by the
United States of John Martin Reservoir.-  Id., at 1-2
(Articles I-A, I-B).  Article IV-D, the provision of the
Compact most relevant to this dispute, states:
      -This Compact is not intended to impede or
prevent future beneficial development of the Arkan-
sas River basin in Colorado and Kansas by Federal
or State agencies, by private enterprise, or by
combinations thereof, which may involve construction
of dams, reservoir, and other works for the purposes
of water utilization and control, as well as the
improved or prolonged functioning of existing works:
Provided, that the waters of the Arkansas River . . .
shall not be materially depleted in usable quantity
or availability for use to the water users in Colorado
and Kansas under this Compact by such future
development or construction.- Id., at 5 (emphasis
added).
  In 1983, Kansas conducted an independent investiga-
tion of possible violations of the Compact arising from
the impact of increases in post-Compact well pumping in
Colorado and the operation of two of the federal reser-
voirs.  Report 9-10.  In December 1985, Kansas brought
this original action against the State of Colorado to
resolve disputes arising under the Arkansas River
Compact.  Id., at 10.  The Court granted Kansas leave
to file its complaint, Kansas v. Colorado, 475 U. S. 1079
(1986), and appointed Judge Wade H. McCree, Jr., to
serve as Special Master, Kansas v. Colorado, 478 U. S.
1018 (1986).  Upon Judge McCree's death, the Court
appointed Arthur L. Littleworth as Special Master.
Kansas v. Colorado, 484 U. S. 910 (1987).
  Kansas advanced three principle claims, each involving
an alleged Compact violation.  See Report 58.  First,
Kansas alleged that increases in groundwater well
pumping in Colorado in the years following adoption of
the Compact have caused a significant decline in the
Arkansas River's surface flow in violation of Article
IV-D of the Compact.  Second, Kansas claimed that
Colorado's Winter Water Storage Program (WWSP)-a
program whereby the Bureau of Reclamation of the
Department of the Interior (Bureau of Reclamation) and
Colorado use excess capacity at the Pueblo Reservoir to
store a portion of the winter flow of the Arkansas
River-violates the Compact.  Third, Kansas claimed
that Colorado's failure to abide by the Trinidad Reser-
voir Operating Principles (Operating Principles) consti-
tuted a violation of the Compact.  Ibid.
  The Special Master bifurcated the trial into a liability
phase and a remedy phase.  At the conclusion of the
liability phase, the Special Master filed his Report,
outlining his findings and recommendations.  In his
Report, the Special Master recommended, among other
things, that the Court: (1) find that post-Compact well
pumping in Colorado has -materially depleted- the
-usable- flow at the Colorado-Kansas border (stateline)
in violation of Article IV-D of the Compact, Report 336;
(2) find that -Kansas has failed to prove that operation
of the [WWSP] program has violated the [C]ompact,-
ibid.; and (3) -dismiss the Kansas claim arising from the
operation of the Trinidad Reservoir,- ibid.
  Both Kansas and Colorado have filed exceptions to the
Special Master's Report.  Kansas excepts to the Special
Master's rejection of its (1) Trinidad Reservoir claim, see
id., at 373-433; (2) WWSP claim, see id., at 306-335;
and (3) preferred method for determining the usability
of depletions of stateline flows, see id., at 291-305.
Colorado excepts to the Special Master's determination
that: (1) Kansas was not guilty of inexcusable delay in
making its post-Compact well-pumping claim and that
Colorado was not prejudiced by this delay, see id., at
147-170; (2) pre-Compact wells in Colorado are limited
to pumping the highest amount pumped in the years
during which the Compact was negotiated and that the
highest amount of such pumping was 15,000 acre-feet
per year, see id., at 182-200; (3) increases in usable
stateline flows resulting from the operating plan for the
John Martin Reservoir adopted by the Arkansas River
Compact Administration in 1980 (1980 Operating Plan)
were -separately bargained for- and, therefore, should
not offset depletions caused by post-Compact well
pumping in Colorado, see id., at 171-181; and (4)
Kansas need only meet the -preponderance of the
evidence- standard to prove a breach of Article IV-D of
the Compact, see id., at 65-70.
  We turn to the parties' exceptions.

                           II

                            A
  In 1958, Congress authorized construction of the Trini-
dad Project, a dam and a reservoir system on the
Purgatoire River slightly upstream from the city of
Trinidad, Colorado.  See id., at 382-388.  Recognizing
that Article IV-D of the Compact prohibited any devel-
opment of the Arkansas River basin that resulted in a
material depletion of usable river flow, the Bureau of
Reclamation conducted studies regarding the future
operation of the Trinidad Project.  Id., at 388-390.  The
Bureau of Reclamation established Operating Principles
whereby the Trinidad Project could be administered
-without adverse effect on downstream water users and
the inflow to John Martin Reservoir.-  Id., at 390
(internal quotation marks omitted).  The Governor of
Kansas reviewed the Bureau of Reclamation's proposed
Operating Principles and indicated that if five additional
conditions were accepted, then -Kansas would be in a
position to approve the amended Operating Principles
and to support completion of the project.-  Id., at
392-393.  In June 1967, the Administration approved
the Operating Principles as well as Kansas' five addi-
tional conditions.  Id., at 395.
  In 1979, Colorado began storage of water at the Trini-
dad Reservoir.  Id., at 396.  Kansas immediately
complained that the Operating Principles were being
violated.  Id., at 397.  In 1988, at the request of the
Administration, the Bureau of Reclamation conducted a
study of the Trinidad Reservoir.  It concluded that two
storage practices at the Trinidad Reservoir constituted
a -`departure from the intent of the operating princi-
ples.'-  Ibid.
  At trial, Kansas argued that the Operating Principles
were binding on the State of Colorado and that any
departure from them constituted a violation of the
Compact -regardless of injury.-  Id., at 408 (internal
quotation marks omitted).  Kansas, however, -offered no
evidence, apart from the Bureau studies, to show that
the actual operation of the Trinidad project caused it to
receive less water than under historical, without-project
conditions.-  Id., at 412.  Instead, Kansas sought to
quantify depletions by -comparing the flows into John
Martin Reservoir `as they would have occurred under the
Operating Principles with the flows that occurred under
actual operations.'-  Id., at 409.  The Special Master
concluded that in order to prove a violation of the
Compact, Kansas was required to demonstrate that -the
Trinidad operations caused a material depletion within
the meaning of Article IV-D.-  Id., at 431.  The Special
Master recommends that we dismiss Kansas' Trinidad
claim because -Kansas has not established, and did not
attempt to establish, such injury.-  Ibid.
  Kansas argues that -[d]eparture from the Operating
Principles is ipso facto a violation of the Compact, and
it [is] entirely sufficient, for purposes of quantifying the
effect of the violation, to compare the actual operation
with simulated operation as it should have been under
the Operating Principles.-  Kansas' Exceptions to Special
Master's Report 12.  But, it must be recalled, this is an
original action to enforce the terms of the Compact.
Article IV-D provides that the Compact is not intended
to prevent future beneficial development of the Arkansas
River basin-including dams and reservoirs-provided
that the river flow shall not be materially depleted.  The
Compact thus permits the development of projects such
as Pueblo Reservoir so long as their operation does not
result in a material depletion of usable flow to Kansas
users.  For Kansas to prevail in its contention, it would
have to show that the Operating Principles had the
effect of amending the Compact by granting either party
the right to sue the other for violation of the Operating
Principles even though the violation resulted in no
material depletion of usable flow at stateline.  Although
the Administration is empowered to -[p]rescribe proce-
dures for the administration of th[e] Compact,- App. to
Report 11 (Article VIII-B(2)), it must do so -consistent
with the provisions of th[e] Compact,- ibid. (Article
VIII-B(1)) (emphasis added); see also Report 416 (-[T]he
Compact Administration was not delegated power to
change the Compact-).  The theory advocated by Kansas
is inconsistent with Article IV-D, which allows for the
development and operation of dams and reservoirs so
long as there is no resultant material depletion of usable
flows at stateline.
  Thus Kansas, in order to establish a Compact violation
based upon failure to obey the Operating Principles, was
required to demonstrate that this failure resulted in a
material depletion under Article IV-D.  Kansas -has not
established, and did not attempt to establish such
injury.-  Id., at 431.  We overrule Kansas' exception to
the Special Master's dismissal of its Trinidad Reservoir
claim.

                            B
  In 1964, the Bureau of Reclamation and Colorado
began planning a program to use excess capacity at
Pueblo Reservoir in order to store a portion of the
winter-time flow of the Arkansas River for beneficial use
at other times.  Under the WWSP, winter-time
flow-much of which was used previously to flood
uncultivated cropland-is instead stored at the Pueblo
Reservoir.  Kansas contends that the Special Master
erred in finding that it had failed to prove that the
WWSP had -materially depleted- usable stateline flows.
We disagree.
  In his Report, the Special Master concluded:
-Kansas has not proved that the WWSP has caused
material Stateline depletions.  Kansas' case has not
been helped by its own contradictions in quantifying
impacts to usable flow-ranging during this trial
from 255,000 acre-feet initially, to 44,000 to 40,000;
nor by the fact that depletions are essentially
eliminated if accretions are taken into account.-
Report 335.
The Special Master examined the computer models
submitted by Kansas and Colorado and determined that
-the depletions shown by the Kansas model are well
within the model's range of error.-  Id., at 334-335.  As
a result, -[o]ne [could not] be sure whether impact or
error [was] being shown.-  Id., at 335.
  We believe that the Special Master gave Kansas every
reasonable opportunity to meet its burden of proving its
WWSP claim.  Kansas, however, failed to prove that
operation of the WWSP program resulted in material
depletions of usable flows in violation of Article IV-D.
See ibid.  Therefore, we overrule Kansas' exception to
the Special Master's conclusion that Kansas had failed
to prove its WWSP claim.

                            C
  Article IV-D of the Compact permits future develop-
ment and construction along the Arkansas River Basin
provided that it does not materially deplete stateline
flows -in usable quantity or availability.-  App. to Report
5 (Article IV-D) (emphasis added).  In order to establish
a violation of Article IV-D, Kansas was required to
establish that development in Colorado resulted in
material depletions of -usable- river flow.  The Compact
does not define the term -usable.- Cf. Colorado v.
Kansas, 320 U. S., at 396-397 (-The critical matter is
the amount of divertible flow at times when water is
most needed for irrigation.  Calculations of average
annual flow, which include flood flows, are, therefore,
not helpful in ascertaining the dependable supply of
water usable for irrigation-).  At trial, Kansas presented
three methods for determining depletions of -usable-
flow.
  Kansas' first expert, Timothy J. Durbin, analyzed flow
data for the period between 1951 and 1985 by plotting
actual river diversions in Kansas against actual stateline
flows.  Report 293-294.  Using these data, Durbin
developed criteria to determine what river flows were
usable.  Durbin concluded that during the summer
months, April through October, (1) 78% of the stateline
flows were diverted; (2) flows greater than 40,000 acre-
feet per month were not usable; and (3) flows greater
than 140,000 acre-feet for the whole period were not
usable.  Id., at 293.  With respect to the winter months,
November through March, Durbin concluded that (1)
24% of the winter flow was diverted; (2) flows greater
than 7,500 acre-feet per month were not usable; and (3)
flows greater than 40,000 acre-feet for the whole period
were not usable.  Id., at 293-294.
  After Colorado isolated errors in Durbin's analysis,
Kansas presented a replacement case.  Kansas' second
group of experts, led by Stephen P. Larson, adopted the
same methodology but revised certain exhibits and made
minor corrections in data.  As a result, Larson modified
Durbin's coefficients, using 72% for the summer months
and 25% for the winter months.  Id., at 295.
  Later, well after trial had begun, Kansas enlisted the
aid of Brent Spronk, who proposed yet another method
to quantify depletions of -usable- stateline flow.  Id., at
300-305.  Spronk attempted to determine the -percent-
age of days in each month when flows were being fully
used in Kansas.-  Id., at 301.  Instead of seasonal
averages, the Spronk approach yielded coefficients that
varied from month to month.  Spronk then multiplied
these monthly coefficients by the estimated depletions in
flow predicted by Kansas' hydrological model.  Id., at
301-302.
  The Special Master concluded that -the Durbin ap-
proach, using Larson's coefficients, is the best of the
several methods presented for determining usable flow-
and that it provided -a reasonable way in which to
determine depletions of usable flow.-  Id., at 305.  We
agree.  Each of the three methods that Kansas proposed
for calculating usable depletions required two steps: (1)
a calculation of total depletions using the Kansas
hydrological model, and (2) an application of -usability-
criteria.  See Brief for United States in Response to
Exeptions of Kansas and Colorado 30.  Each of the three
methods proposed by Kansas was dependent on the
Kansas hydrological model to estimate total depletions.
The Spronk method required the Kansas hydrological
model to predict accurately depletions for each and every
month.  Report 303.  But as Durbin, Kansas' first
expert, testified, Kansas' hydrological model was only a
-`good predictor' when `looking at long periods of time.'-
Id., at 303, n. 130 (quoting Durbin's testimony).  Thus,
the Spronk method required the Kansas hydrologic
model to do something it was not designed to do, i.e.,
predict accurately depletions on a monthly basis.  Id., at
303 (-The Spronk analysis assumes that the H-I model
can accurately predict changes of Stateline flow on a
monthly basis-).  Because the Spronk method for
determining -usable- river flows was less compatible
with Kansas' hydrological model than the other methods
proposed, we conclude that the Special Master properly
rejected the Spronk method in favor of the Durbin
approach, as modified by the Larson coefficients.

                           III

                            A
  The Special Master concluded that Kansas was not
guilty of inexcusable delay in making its well-pumping
claim, and that Colorado had not been prejudiced by
Kansas' failure to press its claim earlier.  Id., at 170.
Colorado has excepted to this determination.  Colorado
argues that the equitable doctrine of laches should bar
Kansas' claim for relief.  See Colorado's Exceptions to
Special Master's Report (Colorado's Exceptions) 24-64.
We overrule Colorado's exception.
  The defense of laches -requires proof of (1) lack of
diligence by the party against whom the defense is
asserted, and (2) prejudice to the party asserting the
defense.-  Costello v. United States, 365 U. S. 265, 282
(1961); see also Black's Law Dictionary 875 (6th ed.
1990) (-`Doctrine of laches,' is based upon maxim that
equity aids the vigilant and not those who slumber on
their rights.  It is defined as neglect to assert a right or
claim which, taken together with lapse of time and other
circumstances causing prejudice to the adverse party,
operates as bar in court of equity-).  This Court has yet
to decide whether the doctrine of laches applies in a
case involving the enforcement of an interstate compact.
Cf. Illinois v. Kentucky, 500 U. S. 380, 388 (1991) (in
the context of an interstate boundary dispute, -the
laches defense is generally inapplicable against a State-);
Block v. North Dakota, 461 U. S. 273, 294 (1983)
(O'Connor, J., dissenting) (-The common law has long
accepted the principle `nullum tempus occurrit
regi'-neither laches nor statutes of limitations will bar
the sovereign-); Colorado v. Kansas, 320 U. S., at 394
(in the context of a suit seeking an equitable apportion-
ment of river flows, facts demonstrating a delay in filing
a complaint -might well preclude the award of the relief
[requested].  But, in any event, they gravely add to the
burden [the plaintiff] would otherwise bear-).  We need
not, however, foreclose the applicability of laches in such
cases, because we conclude that Colorado has failed to
prove an element necessary to the recognition of that
defense.  See Costello, supra, at 282.
  Colorado argues that Kansas knew or should have
known by 1956, or at the latest, before 1968, that both
the number of post-Compact wells and the amount of
post-Compact pumping in Colorado had increased
substantially.  Colorado's Exceptions 37, 39.  Colorado
argues that by 1956 Kansas had sufficient information
about increased well pumping in Colorado and its
potential impact on usable stateline flows to call for an
investigation to determine if a Compact violation existed.
Id., at 46.
  The Special Master concluded that prior to 1984,
Kansas had made no formal complaint to the Adminis-
tration regarding post-Compact well pumping in Colo-
rado.  Report 155-156.  Nevertheless, the Special Master
concluded that Colorado's evidence did not -deal with the
issue of impact on usable flow at the Stateline,- id., at
161, and did -not demonstrate that [the Kansas officials]
were aware of the number of wells, the extent of
Colorado's pumping, or the impact or even potential
impact of pumping on usable Stateline flows,- id., at
164.  The Special Master explained the difficulty of
assessing the impact of increases in post-Compact well
pumping on usable stateline flows because of changing
conditions during the 1970's and early 1980's:
-The 1970s were generally dry years and some
reduction in flow was to have been expected.  Pueblo
Dam came on line in 1976 and began to regulate
native flows.  Transmountain imports increased,
which to some extent provided an offset to pumping.
The 1980 Operating Plan was placed into effect,
which Colorado alleges offset the impacts of in-
creased pumping downstream from John Martin
Reservoir.  The Winter Water Storage Program was
instituted.  Moreover, there was no quantitative or
specific entitlement against which depletions to
usable flow could be judged.  Nor were there any
agreed upon criteria for establishing what flows
were usable.-  Id., at 162-163.
As late as 1985, Colorado officials refused to permit an
investigation by the Administration of well development
in Colorado because they claimed that the evidence
produced by Kansas did not -`suggest that well develop-
ment in Colorado has had an impact on usable stateline
flows.'- Id., at 163 (quoting memorandum of Mr. J.
William McDonald, chief of the Colorado delegation to
the Administration).  In light of the vague and conflict-
ing evidence available to Kansas, we conclude that
Colorado has failed to demonstrate lack of diligence, i.e.,
inexcusable delay, on the part of Kansas.
  Accordingly, we overrule Colorado's exception to the
Special Master's conclusion that the defense of laches
should not bar Kansas' well-pumping claim.

                            B
  The Compact prohibits -future beneficial development
of the Arkansas River basin- that -materially deplete[s]-
the usable flows of the Arkansas River.  App. to Report
5 (Article IV-D) (emphasis added).  Because some wells
in Colorado were in existence prior to the Compact, both
parties agree that a certain amount of post-Compact well
pumping is allowable under the Compact.  Report 182.
Kansas and Colorado, however, dispute the extent of this
allowance.  The Special Master determined that the
-highest annual amount shown to have been pumped
during the negotiations, namely 15,000 acre-feet, should
be allowed under the [C]ompact.-  Id., at 200.  Colorado
makes both a legal and a factual challenge to this
determination.  Colorado's Exceptions 66-73, 73-84.
  Colorado argues as a legal matter that the Compact
does not limit the pumping by pre-Compact wells to the
highest amount actually pumped in pre-Compact years;
rather, Colorado claims that the limit on its pre-Compact
pumping is the maximum amount that Colorado law
permitted or the maximum amount of pumping possible
using wells existing prior to the Compact.  Id., at 69-70.
In support of its position, Colorado argues that the
Special Master failed to consider the subsequent practice
of the parties, i.e., Kansas' failure to object to replace-
ment of centrifugal pumps with turbine pumps or
increased pumping by pre-Compact wells, and that
Article VI-A(2) of the Compact supports its position.
  We conclude that the clear language of Article IV-D
refutes Colorado's legal challenge.  Article IV-D permits
-future beneficial development of the Arkansas River
basin . . . which may involve construction of dams,
reservoir, and other works for the purposes of water
utilization and control, as well as the improved or
prolonged functioning of existing works: Provided, that
the waters of the Arkansas River . . . shall not be
depleted in usable quantity or availability . . . .-  App.
to Report 5 (emphasis added).  Regardless of subsequent
practice by the parties, improved and increased pumping
by existing wells clearly falls within Article IV-D's
prohibition against -improved or prolonged functioning
of existing works,- if such action results in -materia[l]
deplet[ions] in usable- river flows.  Ibid.; see Texas v.
New Mexico, 462 U. S. 554, 564 (1983) (-[U]nless the
compact to which Congress has consented is somehow
unconstitutional, no court may order relief inconsistent
with its express terms-).  Article VI-A(2) of the Com-
pact, which begins with the phrase, -Except as otherwise
provided,- App. to Report 10, must be read in conjunc-
tion with and as limited by Article IV-D.  We agree
with the Special Master that -new wells, the replace-
ment of centrifugal with turbine pumps, and increased
pumping from [pre-Compact] wells all come within
[Article IV-D].-  Report 194.
  Second, Colorado argues as a factual matter that the
Special Master unreasonably relied upon faulty reports
by the United States Geological Survey (USGS) and the
Colorado Legislature to conclude that the greatest
amount of annual pre-Compact pumping in Colorado was
15,000 acre-feet.  Colorado's Exceptions 73-74.  The
Special Master concluded:
      -There is no precise answer to the amount of [pre-
Compact] pumping. . . .  That amount must simply
remain as an estimate of water use that affected the
general allocation of water between the states when
the [C]ompact was being negotiated.  Two responsi-
ble reports, one published by the USGS and one
prepared by the Colorado legislature, reached similar
conclusions as to the amounts of Colorado pumping
during the 1940s. . . .  They have since been used
by the Colorado State Engineer.  I have relied on
these reports and recommend that the highest
annual amount shown to have been pumped during
the negotiations, namely 15,000 acre-feet, should be
allowed under the [C]ompact.-  Report 199-200.
Although the ultimate responsibility for deciding what
are correct findings of fact remains with the Court,
Colorado v. New Mexico, 467 U. S. 310, 317 (1984), in
this instance, we are in full agreement with the Special
Master.  Accordingly, we overrule Colorado's exception.

                            C
  In April 1980, the Administration adopted a resolution
concerning the method for operating John Martin
Reservoir (1980 Operating Plan).  Report 47.  The 1980
Operating Plan divides the water conserved in John
Martin Reservoir into separate accounts.  Kansas is
allocated 40% of the conservation storage, with the
remaining 60% being divided in specified percentages
among the nine canal companies in Colorado Water
District 67.  Id., at 173.  The Special Master concluded
that the 1980 Operating Plan for the John Martin
Reservoir was -separately bargained for- and therefore
should not offset depletions caused by post-Compact well
pumping in Colorado.  Id., at 180-181.  Colorado takes
exception to this ruling.
  Colorado argues that increases in usable stateline
flows resulting from the 1980 Operating Plan should
offset depletions to usable stateline flows.  Colorado's
Exceptions 85.  Colorado maintains that the Administra-
tion adopted the 1980 Operating Plan -for more efficient
utilization of water under its control because of changes
in the regime of the Arkansas River,- id., at 91, -includ-
ing [post-Compact] well pumping in Colorado and
Kansas,- ibid.; see also App. to Report 107 (Resolution
Concerning an Operating Plan for John Martin Reser-
voir) (-WHEREAS, the Arkansas River Compact Admin-
istration . . .  recognizes that, because of changes in the
regime of the Arkansas River, the present operation of
the conservation features of John Martin Reservoir does
not result in the most efficient utilization possible of the
water under its control-).  We disagree.
  As Colorado acknowledges, the resolution adopting the
1980 Operating Plan -does not state that [post-Compact]
well pumping in Colorado or Kansas was a cause of
changes in the regime of the Arkansas River.-
Colorado's Exceptions 88.  In fact, Colorado argues in a
separate part of its brief that -Kansas had made no
complaint about well pumping in Colorado to the
Compact Administration . . . before 1984.-  Id., at 32.
The 1980 Operating Plan expressly reserves the parties'
rights under the Compact, stating that -[a]doption of
this resolution does not prejudice the ability of Kansas
or of any Colorado ditch to object or to otherwise
represent its interest in present or future cases or
controversies before the Administration or in a court of
competent jurisdiction.-  App. to Report 116.  The
Special Master concluded:
      -The 1980 Operating Plan provided benefits to
both Kansas and Colorado which were separately
bargained for.  There is no evidence to support the
claim that benefits to Kansas were in settlement of
its well claims.  Colorado received ample consider-
ation under the agreement for the 1980 plan without
a waiver of Kansas' well claims.  The benefits
received by Kansas under the plan should not be
offset against compact violations, and should not be
a bar to any of the Kansas claims in this case.-
Report 180-181.
We agree with the Special Master's resolution of
Colorado's claim.  Accordingly, we overrule Colorado's
exception.

                            D
  Finally, Colorado argues that Kansas is required to
prove its well-pumping claim by clear and convincing
evidence.  Colorado's Exceptions 91.  The Special Master,
relying upon Nebraska v. Wyoming, 507 U. S. ___ (1993),
concluded that the proper burden of proof for enforcing
an interstate compact is the preponderance of the
evidence standard.  Report 70.  The Special Master
noted that the Nebraska Court had drawn a distinction
between actions seeking to -modify- a judicial decree and
actions seeking to -enforce- a judicial decree.  See
Nebraska, supra, at ___ (slip op., at 6).  (-[W]e find
merit in [the] contention that, to the extent that Nebras-
ka seeks modification of the decree rather than enforce-
ment, a higher standard of proof applies-).  The Special
Master concluded that an action seeking to enforce an
interstate compact stood on the same footing as an
action enforcing a judicial decree, and therefore was
subject to the -preponderance of the evidence- standard.
Report 70.
  We need not, however, resolve this issue.  The Special
Master concluded that -regardless of which burden of
proof applies- he had -no difficulty in concluding that
[post-Compact] pumping in Colorado ha[d] caused
material depletions of the usable Stateline flows of the
Arkansas River, in violation of the Arkansas River
Compact.-  Id., at 263.  We agree with this determina-
tion, and thus overrule Colorado's exception.

                           IV
  For these reasons, we overrule the exceptions filed by
the States of Kansas and Colorado.  We remand the
case to the Special Master for determination of the
unresolved issues in a manner not inconsistent with this
opinion.
It is so ordered.