Montana v. Wyoming
The Supreme Court ruled that Wyoming's upstream farmers with water rights dating before 1950 could switch from flood irrigation to more efficient sprinkler systems, even though the change reduces the runoff that would otherwise flow downstream to Montana's own pre-1950 water users.
The decision resolves a dispute between the two states over the Yellowstone River Compact, holding that ordinary water law lets farmers improve irrigation efficiency without violating older users' downstream rights, as long as they don't divert more water or expand their land.
“They are simply different mechanisms for increasing the volume of water available to the crops without changing the amount of diversion.”
Explaining why switching to sprinkler irrigation doesn't count as expanding a water right.
How it got here: Montana sued Wyoming directly in the Supreme Court under its original jurisdiction; a Court-appointed Special Master recommended rejecting Montana's efficiency claim, and Montana asked the Court to overturn that recommendation.
The Case in Depth
What happened
Montana and Wyoming share the Yellowstone River under a 1950 interstate compact protecting water rights that existed as of January 1, 1950. Montana accused Wyoming of letting its pre-1950 farmers switch from flood irrigation to sprinklers, which soak up more water and send less runoff back into the river, leaving less for Montana's own pre-1950 water users downstream.
The question before the Court
Could Wyoming's upstream farmers switch to more efficient sprinkler irrigation, even though that left less water flowing downstream to Montana's older water users?
Why it matters
Farmers and ranchers across the water-scarce West often upgrade irrigation equipment to save water and money. This ruling confirms they can do so without triggering new legal liability to downstream water users, even if it means less water flows past their land — a rule with implications for water-sharing disputes throughout the region.
What changes now
This is a final resolution of Montana's specific exception regarding irrigation efficiency, though the broader lawsuit between the two states over the Yellowstone River Compact continues before the Special Master on other issues, including an intrastate remedy question the Court had earlier sent back for further review. The ruling establishes the legal framework for evaluating any remaining claims.
What this does not decide
The Court did not decide whether appropriation law should be frozen as it stood in 1949 or whether it evolves over time, leaving that question open. It also did not resolve Montana's other exception about whether an intrastate remedy is required before Wyoming's compliance is at issue.
Concurrences and dissents
Dissent — Justice Scalia
“For purposes of the Compact, Wyoming may lay claim only to its beneficial users’ net consumption of water, that is, the volume of water diverted from the river minus the volume that flows (or seeps) back into the river’s channel.”Scalia's competing reading of what the compact's 'beneficial use' definition actually limits.
Justice Scalia argued the majority ignored the compact's actual text, which defines 'beneficial use' in terms of water 'depleted' rather than 'diverted' — a deliberate word choice signaling the compact meant to cap Wyoming's users at their net water consumption, not the gross amount diverted. He would have ruled that increased efficiency reducing return flow breaches the compact, regardless of general Western water-law principles.
How the Court got there
The legal reasoning, step by step
- The Court applied the doctrine of appropriation, the traditional Western water-rights system where whoever first puts water to productive use gets priority over later users, and asked whether a switch to more efficient irrigation exceeds the scope of an existing water right or improperly expands it.
- Under the 'no-injury rule,' later water users can stop earlier users from expanding their rights in ways that cause new harm, but the Court found this rule in Montana and Wyoming case law has traditionally applied only to changes in where water is diverted or how and where it's used — not to efficiency improvements that keep the same diversion amount, acreage, and purpose.
- The Court also relied on the 'doctrine of recapture,' which lets a water user capture and reuse runoff and seepage still on their own property; because sprinklers simply reduce wastage rather than divert more water, the Court treated switching to sprinklers as within a farmer's existing right rather than a new claim on the river.
- The Court then rejected Montana's separate argument that the compact's phrase 'beneficial use,' defined as water 'depleted' by use, was meant to freeze the exact net amount of water consumed in 1950, finding instead that 'beneficial use' simply names a category of water use (like irrigation) rather than measuring a fixed volume.
- Concluding that neither general appropriation law nor the compact's specific wording limited Wyoming's pre-1950 users to their 1950 consumption levels, the Court found no legal basis for treating irrigation efficiency gains as a breach of the compact.