Montana v. Wyoming
The Supreme Court adopted a Special Master's findings and entered judgment against Wyoming for cutting water flows to Montana on the Tongue River in 2004 and 2006, awarding Montana just over $20,000 in damages plus interest and about $67,000 in costs.
The Court also issued a detailed decree spelling out how the two states must share and manage Tongue River water going forward under their decades-old interstate water-sharing agreement, including new rules for when Montana can demand more water and how Wyoming must respond.
How it got here: The Supreme Court, exercising its original jurisdiction over disputes between states, appointed a Special Master to investigate; the Court now adopts the Special Master's report and enters judgment and a decree.
The Case in Depth
What happened
Montana sued Wyoming (and named North Dakota) claiming Wyoming violated the Yellowstone River Compact, a decades-old agreement dividing water among the three states, by reducing the water reaching Montana's side of the Tongue River in 2004 and 2006. Montana argued this harmed water users whose rights predated 1950 and were supposed to be protected under the Compact.
The question before the Court
After Wyoming cut the water flowing to Montana in the Tongue River, how much did Montana deserve in damages, and what rules should govern water sharing going forward?
Why it matters
Farmers, ranchers, and water managers in Montana and Wyoming now have a clear rulebook for sharing the Tongue River, including exactly when Montana can 'call' for more water, how much Wyoming must supply, and how much water Montana's reservoir can store each year — reducing the risk of future cross-border water fights.
What changes now
The decree is now in effect: Wyoming must pay Montana its damages, interest, and costs within 90 days, directed toward improvements to Montana's Tongue River Reservoir. The Court retains ongoing jurisdiction, meaning either state can come back and ask for the decree to be amended or enforced if disputes arise over how these rules are applied in practice.
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 Compact, leaving those questions for another day.
How the Court got there
The legal reasoning, step by step
- The Court treated the Yellowstone River Compact's Article V(A) as protecting only water rights that existed in Wyoming and Montana before 1950, meaning newer water users in Wyoming generally cannot be forced to cut back to benefit Montana.
- The Court found that this pre-1950 protection does not guarantee Montana any fixed amount of water, but instead protects those older water users from having their supply cut off by newer diversions or storage in Wyoming.
- Applying the doctrine of prior appropriation (the 'first in time, first in right' rule that governs water law in both states), the Court concluded pre-1950 rights are protected only when put to genuine, non-wasteful use.
- Because Wyoming's reduced releases in 2004 and 2006 interfered with protected pre-1950 uses in Montana without justification under the Compact, the Court found Wyoming liable for those two specific years.
- The Court then set out detailed rules — including a 'call' system requiring Montana to formally notify Wyoming when it needs more water — to govern how the two states must operate going forward, while rejecting Montana's broader claims for all other years and issues.