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On May 2, the Supreme Court issued an opinion in Montana v. Wyoming, No. 137, Orig., a case brought under the Court's original jurisdiction over disputes between states, holding that the Yellowstone River Compact allows pre-1950 water appropriators in Wyoming to increase their net water consumption by improving the efficiency of their irrigation systems, and Montana's claim to the contrary should be dismissed.
In 1951, after nearly twenty years of negotiation, Montana, Wyoming, and North Dakota ratified the Yellowstone River Compact, and Congress consented to it. The compact governs the allocation of the Yellowstone River system among the three states. In pertinent part, Article V(A) of the compact provides "[a]ppropriative rights to the beneficial uses of the water of the Yellowstone River System existing in each signatory State as of January 1, 1950, shall continue to be enjoyed in accordance with the laws governing the acquisition and use of water under the doctrine of appropriation."
In 2008, Montana filed a bill of complaint against Wyoming for breach of the compact. Among other things, Montana alleged that Wyoming had increased its water consumption on existing agricultural acreage by switching from flood to sprinkler irrigation. Wyoming filed a motion to dismiss the complaint, which the Court referred to a Special Master. The Special Master recommended that the motion to dismiss be denied because at least some of Montana's allegations stated a claim for relief. But the Special Master agreed with Wyoming that Montana's allegations regarding "efficiency improvements by pre-1950 appropriators in Wyoming" do not state a claim for relief. Montana filed an exception to that recommendation.
The Court overruled Montana's exception and adopted the Special Master's recommendation. The question before the Court was whether Article V(A) of the compact allows Wyoming's pre-1950 water users – diverting the same quantity of water for the same irrigation purpose and acreage as before 1950 – to increase their consumption of water by improving their irrigation systems and reducing runoff, even if it reduces the flow of water to Montana's pre-1950 users. The Court first considered the doctrine of appropriation, primarily under the common law of Montana and Wyoming, and concluded that the scope of the original appropriative right includes a change to sprinkler irrigation so long as no additional water is diverted from the stream and the conserved water is used on the same acreage for the same agricultural purposes as before. That conclusion was supported by the states' courts' interpretations of the no-injury rule and the doctrine of recapture, as well as by the writings of water law scholars. The Court also considered and rejected Montana's argument that the compact's definition of "beneficial use" restricts the scope of pre-1950 appropriative rights to the net volume of water that was actually being consumed in 1950. The language of Article V(A) provided no support for Montana's argument, and the Court noted that if the parties had intended such a meaning, the Compact could have provided for such an interpretation with much more clarity.
Justice Thomas delivered the opinion of the Court, in which Chief Justice Roberts and Justices Kennedy, Ginsburg, Breyer, Alito, and Sotomayor joined. Justice Scalia filed a dissenting opinion. Justice Kagan took no part in the consideration or decision of the case.
