Decree 

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

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 ordered filed. The proposed judgment and decree are entered: 

JUDGMENT 

Judgment is awarded against the State of Wyoming and 

in favor of the State of Montana for violations of the Yellowstone River Compact resulting from Wyoming’s reduction 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 prejudgment 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 Montana. 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 Attorney 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. 
 

Per Curiam Decree 

2 MONTANA v. WYOMING 

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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 (Compact) protects pre-1950 appropriative rights to the beneficial 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 appropriative 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 Montana 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 appropriative 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 (including rules for reservoir accounting) govern the administration 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 Article 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 continues 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 Montana 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 initiate 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 
 

Per Curiam 

 

4 MONTANA v. WYOMING 

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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 physically 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 pre1950 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 Wyoming 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 appropriative 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 engineering 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 conditions, including, but not limited to, the needs of downstream 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. 
 

Per Curiam 

6 MONTANA v. WYOMING 

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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 Wyoming thereafter will annually inform the other State of 

any changes in these water rights, unless such information is publicly and readily available to the other State.

2. If requested, Montana and Wyoming also shall provide 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 particular, 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 Montana 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.