Montana v. Wyoming

Background

Montana sued Wyoming after some Wyoming farmers who had water rights before 1950 began using sprinkler systems that reduced wastewater returning to the Tongue and Powder Rivers. Montana said those efficiency gains deprived its downstream, pre-1950 water users of water guaranteed by Article V(A) of the Yellowstone River Compact. The Special Master found the complaint failed to state a claim about efficiency improvements, and the States largely accepted other findings; the Court reviewed Montana’s exception to that conclusion.

Reasoning

The Court examined how Western states apply the doctrine of appropriation, including the no-injury rule and the doctrine of recapture. It found that Montana and Wyoming law, cases, and scholars allow an original appropriator to reuse runoff on the same land and to change irrigation methods so long as diversion amounts and acreage remain the same. The Compact’s definition of “beneficial use” refers to a type of use that depletes water, not a numeric limit tied to 1950 consumption. So Article V(A) incorporates ordinary appropriation principles and does not bar efficiency improvements that reduce return flows.

Real world impact

Wyoming’s pre-1950 irrigators may install more efficient systems even if downstream Montana users receive less return flow. Montana’s pleading on this theory fails, and the Court overruled that exception to the Special Master’s report. Justice Kagan did not participate.

Dissents or concurrances

Justice Scalia dissented, arguing the Compact’s use of the word “depleted” shows the States intended to measure rights by net consumption, not mere diversion, and would limit upstream efficiency gains.