                 Cite as: 583 U. S. ____ (2018)           1

                           Decree
                          Judgment

SUPREME COURT OF THE UNITED STATES
 STATE OF MONTANA v. STATE OF WYOMING AND
          STATE OF NORTH DAKOTA
                  ON BILL OF COMPLAINT
           No. 137, Orig. Decided February 20, 2018

  The Report of the Special Master is received and or-
dered filed. The proposed judgment and decree are en-
tered. JUSTICE KAGAN took no part in the consideration or
decision of this case.
                        JUDGMENT
   Judgment is awarded against the State of Wyoming and
in favor of the State of Montana for violations of the Yel-
lowstone River Compact resulting from Wyoming’s reduc-
tion of the volume of water available in the Tongue River
at the Stateline between Wyoming and Montana by 1300
acre feet in 2004 and 56 acre feet in 2006. Judgment is
awarded in the amount of $20,340, together with pre-
judgment and post-judgment interest of seven percent
(7%) per annum from the year of each violation until paid.
Costs are awarded to Montana in the amount of
$67,270.87.
   Wyoming shall pay these damages, interest, and costs in
full not later than 90 days from the date of entry of this
Judgment. Wyoming shall make its payment into an
account specified by Montana to be used for improvements
to the Tongue River Reservoir or related facilities in Mon-
tana. Montana may distribute these funds to a state
agency or program, a political subdivision of the State, a
nonprofit corporation, association, and/or a charitable
organization at the sole discretion of the Montana Attor-
ney General in accordance with the laws of the State of
Montana, with the express condition that the funds be
used for improvements to the Tongue River Reservoir or
related facilities in Montana.
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   Except as herein provided, all claims in Montana’s Bill
of Complaint are denied and dismissed with prejudice.
                          DECREE
                   A. General Provisions
   1. Article V(A) of the Yellowstone River Compact (Com-
pact) protects pre-1950 appropriative rights to the benefi-
cial uses of water of the Yellowstone River System in
Montana from diversions and withdrawals of surface
water and groundwater in Wyoming, whether for direct
use or storage, that are not made pursuant to appropriat-
ive rights in Wyoming existing as of January 1, 1950.
   2. Article V of the Compact, including the protections of
Article V(A), applies to all surface waters tributary to the
Tongue and Powder Rivers (with the exception of the
explicit exclusions set out in Article V(E) of the Compact).
   3. Article V(A) of the Compact does not guarantee Mon-
tana a fixed quantity or flow of water, nor does it limit
Wyoming to the net volume of water actually consumed in
Wyoming prior to January 1, 1950.
   4. Article V(A) of the Compact protects pre-1950 appro-
priative rights only to the extent they are for “beneficial
uses,” as defined in Article II(H) of the Compact, and are
otherwise consistent with the doctrine of appropriation. In
particular, pre-1950 rights are not protected to the extent
they are wasteful under the doctrine of appropriation.
   5. Except as otherwise expressly provided in this Decree
or the Compact, the laws of Montana and Wyoming (in-
cluding rules for reservoir accounting) govern the admin-
istration and management of each State’s respective water
rights in the implementation of Article V(A) of the Compact.
                         B. Calls
   1. To protect pre-1950 appropriative rights under Ar-
ticle V(A) of the Compact, Montana must place a call.
Wyoming is not liable for flow or storage impacts that take
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place when a call is not in effect.
   2. Subject to paragraph B(3), Montana may place a call
on the Tongue River whenever (a) a pre-1950 direct flow
right in Montana is not receiving the water to which it is
entitled, or (b) Montana reasonably believes, based on
substantial evidence, that the Tongue River Reservoir
might not fill before the end of the water year.
   3. Montana cannot place a call under Article V(A) when
it can remedy shortages of pre-1950 appropriators in
Montana through purely intrastate means that do not
prejudice Montana’s other rights under the Compact.
   4. A call need not take any particular form, use any
specific language, or be delivered by or to any particular
official, but should be sufficient to place Wyoming on clear
notice that Montana needs additional water to satisfy its
pre-1950 appropriative rights.
   5. A call is effective upon receipt by Wyoming and con-
tinues in effect until Montana notifies Wyoming that
Montana is lifting the call.
   6. Montana shall promptly notify Wyoming that it is
lifting a call when (a) pre-1950 direct flow rights in Mon-
tana are receiving the water to which they are entitled,
and (b) Montana reasonably believes, based on substantial
evidence, that the Tongue River Reservoir will fill before
the end of the water year. Montana may place a new call
at a later date if the conditions of paragraph B(2) are
again met.
   7. Upon receiving a call, Wyoming shall promptly initi-
ate action to ensure, to the degree physically possible, that
only pre-1950 appropriators in Wyoming are diverting or
storing surface water and only to the degree permitted by
their appropriative rights and this Decree. Wyoming also
shall promptly initiate any action needed to ensure, to the
degree physically possible, that any groundwater
withdrawals under post-January 1, 1950 appropriative
rights are not interfering with the continued enjoyment of
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pre-1950 surface rights in Montana. Wyoming shall be
liable for diversions, storage, or withdrawals in violation
of Article V(A) of the Compact even if it was not physically
possible for Wyoming to prevent the diversions, storage, or
withdrawals during a call (including depletions caused by
groundwater withdrawals occurring before the call).
Where it is initially not physically possible to prevent the
storage of water in violation of Article V(A), Wyoming
shall deliver such water to Montana as soon as it is physi-
cally possible to do so after a request from Montana.
           C. Pre-1950 Appropriative Rights
  1. The Compact assigns the same seniority level to all
pre-1950 water users in Montana and Wyoming. Except
as otherwise provided in this Decree, the exercise of pre-
1950 appropriative rights in Wyoming does not violate the
Compact rights of pre-1950 appropriative rights in Montana.
  2. Article V(A) does not prohibit Montana or Wyoming
from allowing a pre-1950 appropriator to conserve water
through the adoption of improved irrigation techniques
and then use that water to irrigate the lands to which the
specific pre-1950 appropriative right attaches, even when
the increased consumption interferes with pre-1950 uses
in Montana. Article V(A) protects pre-1950 appropriators
in Montana from the use of such conserved water in Wyo-
ming on new lands or for new purposes. Such uses fall
within Article V(B) of the Compact and cannot interfere
with pre-1950 appropriative rights in Montana.
  3. Pre-1950 appropriators in Montana and Wyoming
may change their place of use, type of use, and point of
diversion pursuant to applicable state law, so long as any
such changes do not injure appropriators in the other
States as evaluated at the time of the change.
             D. Wyoming Storage Reservoirs
    1. Post-January 1, 1950 appropriators in Wyoming may
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not store water when Montana has issued a call, except as
provided in paragraph B(7) of this Decree. Post-January
1, 1950 appropriators in Wyoming may store water during
periods when a call is not in effect.
   2. Water stored under post-January 1, 1950 appropriat-
ive rights in Wyoming when a call is not in effect has been
legally stored under the Compact and can be subsequently
used at any time, including when pre-1950 appropriative
rights in Montana are unsatisfied. The Compact does not
require Wyoming to release such water to Montana in
response to a call.
               E. Tongue River Reservoir
   1. Article V(A) protects Montana’s right to store each
water year (October 1 to September 30) up to, but not
more than, 72,500 acre feet of water in the Tongue River
Reservoir, less carryover storage in excess of 6,571 acre
feet. If the Tongue River Reservoir begins the water year
on October 1 with over 6,571 acre feet of carryover water,
Article V(A) protects Montana’s right to fill the Tongue
River Reservoir to its current capacity of 79,071 acre feet.
   2. Montana must avoid wasting water in its operation of
the Tongue River Reservoir by not permitting outflows
during winter months that are not dictated by good engi-
neering practices. Any wasteful outflows reduce the
amount of water storage protected under Article V(A) for
that water year by an equal volume.
   3. The reasonable range for winter outflows from the
Tongue River Reservoir is 75 to 175 cubic feet per second.
The appropriate outflow at any particular point of time
varies within this range and depends on the specific condi-
tions, including, but not limited to, the needs of down-
stream appropriative water rights and risks such as ice
jams and flooding. Montana enjoys significant discretion
in setting the appropriate outflow within this range and in
other reservoir operations.
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              F. General Reservoir Rules
  1. Article V(A) of the Compact does not protect water
stored exclusively for non-depletive purposes, such as
hydroelectric generation and fish protection.
  2. Montana and Wyoming must operate and regulate
reservoirs on the Tongue River and its tributaries in a
fashion that is generally consistent with the appropriation
laws and rules that govern similar reservoirs elsewhere in
each respective State.
                G. Exchange of Information
   1. Within 30 days of the entry of this Decree, Montana
and Wyoming each shall provide the other State with a
list of its current surface water rights in the Tongue River
basin, including information on which rights are pre-1950
and which are post-January 1, 1950. Montana and Wyo-
ming thereafter will annually inform the other State of
any changes in these water rights, unless such infor-
mation is publicly and readily available to the other State.
   2. If requested, Montana and Wyoming also shall pro-
vide the other State annually with any data available in
the ordinary course of water administration that shows
the location and amount of groundwater pumping in the
Tongue River and Powder River basins, except where the
groundwater is used exclusively for domestic or stock
water uses as defined in Article II of the Compact.
   3. Montana and Wyoming shall exchange information,
as reasonable and appropriate, relevant to the effective
implementation of Article V(A) of the Compact. In partic-
ular, Wyoming in response to a call shall notify Montana
of the actions that it intends to take and has taken in
response to the call, and when requested, provide Mon-
tana with reasonable assurances and documentation of
these actions. In making a call, Montana in turn will
notify Wyoming of any intrastate actions it has taken to
remedy shortages of pre-1950 appropriators, and when
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requested, provide Wyoming with reasonable assurances
and documentation of these actions.
  4. The Yellowstone River Compact Commission remains
free to modify or supplement the terms of the provisions of
paragraph G of this Decree pursuant to its authority
under the Compact.
     H. Rights of the Northern Cheyenne Tribe
  Nothing in this Decree addresses or determines the
water rights of any Indian Tribe or Indian reservation or
the status of such rights under the Yellowstone River
Compact.
                I. Retention of Jurisdiction
  Any of the parties may apply at the foot of this Decree
for its amendment or for further relief. The Court retains
jurisdiction to entertain such further proceedings, enter
such orders, and issue such writs as it may from time to
time deem necessary or desirable to give proper force and
effect to this Decree.
