## Facts

- Montana, invoking the Supreme Court’s original jurisdiction, sued Wyoming (and North Dakota) alleging breach of the Yellowstone River Compact.
- The Compact (ratified by Montana, Wyoming, and North Dakota and consented to by Congress) governs water use in the Yellowstone River System, whose tributaries originate in Wyoming and flow into Montana.
- Article V(A) provides that appropriative rights to beneficial uses existing as of January 1, 1950 “shall continue to be enjoyed” under the states’ laws applying the doctrine of prior appropriation.
- Pre-1950 irrigators historically used flood irrigation, which produced runoff and seepage returning to the river as return flows available to downstream users.
- Wyoming’s pre-1950 appropriators later adopted sprinkler irrigation, which did not increase the quantity diverted or the acreage irrigated but increased consumptive use and reduced return flows reaching Montana.
- Montana alleged that reduced return flows impaired Montana’s downstream pre-1950 appropriators and that Wyoming’s allowance of these efficiency improvements violated Article V(A).
- A Special Master recommended dismissing Montana’s “increased-efficiency” theory for failure to state a claim; Montana filed exceptions.

## Issues

1. Whether Article V(A) of the Yellowstone River Compact, by incorporating state prior-appropriation law, bars Wyoming’s pre-1950 appropriators from switching to more efficient irrigation methods that reduce return flows but do not increase diversions or irrigated acreage.
2. Whether Montana’s “increased-efficiency” allegation states a claim for breach of Article V(A).

## Decision

- The Court overruled Montana’s exception and agreed with the Special Master that the increased-efficiency allegation failed to state a claim.
- Article V(A) was construed to incorporate the ordinary doctrine of prior appropriation as applied in Wyoming and Montana, without freezing 1950-era irrigation practices or return-flow patterns.
- Under the relevant state-law appropriation doctrines, a senior appropriator may improve irrigation efficiency even if downstream users receive less return flow, so long as the senior does not enlarge the right by increasing diversion amounts or irrigated acreage.

## Legal Principles

- An interstate water compact that preserves existing rights “in accordance with” state prior-appropriation law generally adopts that doctrine’s ordinary rules unless the compact clearly provides otherwise.
- Under prior-appropriation principles in Wyoming and Montana, junior appropriators take the stream as they find it, and reliance on return flows does not, by itself, prevent a senior from changing irrigation methods within the scope of the decreed diversion and acreage.
- The “no-injury” limitation on changes to a water right focuses on enlargements of the right (e.g., increased diversions or expanded acreage), not mere improvements in irrigation technology that increase consumptive use of the same diverted water.

## Conclusion

The Court held that Article V(A) incorporates ordinary state prior-appropriation law, which allows Wyoming’s pre-1950 appropriators to adopt more efficient irrigation methods that reduce return flows, so long as they do not increase the quantity diverted or the acreage irrigated; therefore, Montana’s increased-efficiency claim was dismissed.


Montana v. Wyoming, 563 U.S. 368 (2011)