Montana v. Wyoming
The Supreme Court adopted a Special Master's findings and entered a final judgment and decree resolving Montana's lawsuit against Wyoming over shared water from the Yellowstone River system.
The Court awarded Montana about $20,340 in damages plus interest and roughly $67,000 in costs for two specific violations of the interstate water-sharing agreement, while dismissing most of Montana's broader claims and laying out detailed rules for how the two states must share water from the Tongue River going forward.
How it got here: Montana filed an original bill of complaint directly with the Supreme Court; a Special Master investigated and reported findings and a proposed decree, which the Court adopted.
The Case in Depth
What happened
Montana sued Wyoming (with North Dakota also a party to the multistate compact) directly in the Supreme Court, which handles disputes between states. Montana claimed Wyoming's water users took more than their share from the Tongue River, part of the Yellowstone River system, violating a 1950 interstate agreement that protects older ('pre-1950') water rights in Montana from newer Wyoming diversions.
The question before the Court
Did Wyoming break the Yellowstone River Compact by cutting the water Montana was entitled to from the Tongue River, and what should Wyoming pay for it?
The Court's answer
Partly — the Court found that Wyoming did violate the Yellowstone River Compact on two specific occasions, in 2004 and 2006, by reducing the water reaching Montana below what the Compact required, and it ordered Wyoming to pay Montana roughly $20,340 in damages plus interest and about $67,000 in litigation costs. But the Court rejected the rest of Montana's claims, ruling that the Compact does not guarantee Montana a fixed amount of water or limit Wyoming to its historical pre-1950 usage.
Beyond resolving the money claims, the Court also adopted a detailed decree spelling out exactly how the two states must share Tongue River water going forward — including when Montana can demand more water, how Wyoming must respond, and how reservoirs must be run — to reduce the chance of future disputes over the same Compact language.
Curious how the Court got there? See the step-by-step legal reasoning →
Why it matters
Farmers and water users along the Tongue River in both states now operate under a clear, detailed rulebook for when Montana can demand more water ('calls'), how Wyoming must respond, and how the Tongue River Reservoir must be managed. This reduces the likelihood of future disputes by spelling out exactly how the decades-old Yellowstone River Compact works in practice.
What changes now
A Special Master appointed to investigate and report already held proceedings and issued a report with proposed findings and a proposed decree. The Court adopted that report, entered judgment, and issued a detailed decree. Wyoming must pay the awarded damages, interest, and costs within 90 days, into an account Montana designates for improvements to the Tongue River Reservoir. The Court retains jurisdiction to handle future disputes over the decree.
What this does not decide
The decree explicitly does not address or determine the water rights of the Northern Cheyenne Tribe or any other Indian tribe under the Yellowstone River Compact, leaving those questions for another proceeding.
How the Court got there
The legal reasoning, step by step
- The Court relied on the findings of a Special Master, a court-appointed investigator who examines the facts and recommends a resolution in disputes between states, rather than reweighing the evidence itself.
- The Court found that Wyoming's water users cut the volume of water reaching Montana at the state line by specific amounts in 2004 and 2006, in violation of the Compact provision protecting Montana's older, 'pre-1950' water rights from newer Wyoming diversions.
- Applying the Compact's protection for beneficial, non-wasteful pre-1950 uses, the Court concluded Montana was entitled to a fixed dollar award for those two violations, plus interest running from each violation and its litigation costs.
- The Court rejected Montana's broader claims for additional violations, concluding that the Compact does not guarantee Montana any fixed quantity of water and does not require Wyoming to limit its use to historical pre-1950 consumption levels.
- Because ongoing water-sharing disputes are likely, the Court set out detailed operating rules — covering when Montana may issue a 'call' for more water, how Wyoming must respond, and how reservoirs must be managed — to guide the states going forward rather than leaving the Compact's terms to future litigation.