Montana v. Wyoming, 131 S. Ct. 1765

Montana sues Wyoming alleging breach of Yellowstone River Compact Article V(A) by upstream pre-1950 users increasing water consumption via irrigation efficiency gains that reduce downstream return flows to Montana.
Special Master found Montana stated a claim for breaches related to upstream efficiency, but concluded efficiency improvements that do not change total diverted volume may be permissible if used on the same acreage for the same purpose.
The Yellowstone River Compact divides water into three tiers, with Article V(A) protecting pre-1950 rights existing as of January 1, 1950 and governed by the doctrine of appropriation.
Under the doctrine of appropriation, senior rights are allocated by priority; downstream Montana pre-1950 rights can be impacted when upstream Wyoming pre-1950 users consume more water.
Montana contends Article V(A) either incorporates background appropriation law limiting such efficiency gains or envisions a depletion-based beneficial-use scope that would protect Montana’s flow rates, while Wyoming argues the Compact preserves ordinary appropriation rights including efficiency gains.
The Court held that Wyoming pre-1950 users may improve irrigation efficiency (e.g., sprinkler systems) without expanding their claimed rights, thereby affirming the Special Master’s conclusion and overruling Montana’s exception.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether Article V(A) allows upstream pre-1950 users to increase consumption via efficiency gains that reduce downstream flow	Montana asserts Article V(A) protects downstream pre-1950 rights from any efficiency-driven depletion beyond 1950 flows.	Wyoming maintains background appropriation doctrine allows efficiency improvements within existing rights and does not enlarge downstream detriment.	Yes; Article V(A) permits efficiency increases within existing rights.
Whether the term 'beneficial use' in Article V(A) limits protected rights to net depletion rather than total diverted or whether it encompasses broader depletion-related uses	Montana argues depletion-based scope would cap downstream protection to 1950 depletion levels.	Wyoming argues the term is conventional and does not redefine pre-1950 scope to net depletion.	The term does not redefine the scope to net depletion; it preserves ordinary pre-1950 rights.
Whether Article V(A) should be read to freeze or evolve appropriation law over time	Montana implies a freeze at 1950 law; Wyoming implies evolution consistent with long-standing doctrine.	Wyoming contends the Compact incorporates laws governing appropriation without freezing them.	Article V(A) preserves the pre-1950 rights under ongoing appropriation doctrine; no freeze mandated.

Basey v. Gallagher, 20 Wall. 670 (U.S. 1875) (priority of appropriation; beneficiary use standard)
Hinderlider v. La Plata River & Cherry Creek Ditch Co., 304 U.S. 92 (U.S. 1938) (first in time, beneficial use; depletive rights)
Arizona v. California, 298 U.S. 588 (U.S. 1936) (extent of appropriative rights; seniority framework)
Quinn v. John Whitaker Ranch Co., 54 Wyo. 367, 92 P.2d 568 (Wyo. 1939) (place/purpose of use changes; no-injury rule scope)
Binning v. Miller, 55 Wyo. 451, 102 P.2d 54 (Wyo. 1940) (recapture/waste water on land rights; efficiency allowed)
Bower v. Big Horn Canal Assn., 77 Wyo. 80, 307 P.2d 593 (Wyo. 1957) (recapture of seepage; efficiency within original right)
Fuss v. Franks, 610 P.2d 17 (Wyo. 1980) (recapture/waste water on property; on-site use rights)
Rock Creek Ditch & Flume Co. v. Miller, 93 Mont. 248, 268, 17 P.2d 1074 (Mont. 1933) (recapture and return flows within rights on land)