Montana v. Wyoming
The Supreme Court, acting as the trial court in a dispute between two states, ruled that Wyoming violated the Yellowstone River Compact by reducing the Tongue River's flow into Montana in 2004 and 2006, and ordered Wyoming to pay Montana just over $87,000 in damages, interest, and costs.
The Court also adopted a detailed set of rules spelling out how Montana and Wyoming must handle water calls, reservoir storage, and pre-1950 water rights going forward, resolving a long-running fight over how an 80-year-old interstate water-sharing agreement actually works in practice.
How it got here: Montana filed an original bill of complaint directly with the Supreme Court; a Special Master took evidence and issued a report the Court is now adopting.
The Case in Depth
What happened
Montana sued Wyoming (with North Dakota also a party) at the Supreme Court, claiming Wyoming's water users diverted and stored too much water from the Tongue River, violating the 1950 Yellowstone River Compact and shortchanging Montana's older, "pre-1950" water rights, including water needed to fill Montana's Tongue River Reservoir.
The question before the Court
Did Wyoming break its water-sharing agreement with Montana by cutting the flow of the Tongue River, and how should the two states manage the river's water going forward?
Why it matters
Farmers, ranchers, and water managers who rely on the Tongue River in both states now have clear, enforceable rules for when Montana can demand more water from Wyoming and how Wyoming must respond. The decree also sets concrete limits on reservoir storage and reporting duties, reducing the chance of future disputes between the two states over the same river.
What changes now
Wyoming must pay the awarded damages, interest, and costs into a Montana-designated account within 90 days, to be used for improvements to the Tongue River Reservoir. The decree's detailed rules on calls, reservoirs, and information-sharing now govern the states' ongoing water administration, and the Court retains jurisdiction to handle future disputes or requests to amend the decree.
What this does not decide
The decree expressly 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 day.
How the Court got there
The legal reasoning, step by step
- The Court relied on the Special Master's factual findings and legal conclusions about how Article V(A) of the Yellowstone River Compact protects Montana's pre-1950 water rights from newer Wyoming diversions and storage.
- It held that this protection does not guarantee Montana any fixed amount of water, but instead requires Montana to affirmatively 'call' for water when its pre-1950 rights are not being satisfied, putting Wyoming on notice to cut back junior water use.
- Applying that framework to the record, the Court found Wyoming's reductions in Tongue River flow in 2004 and 2006 cut off water Montana was entitled to under its pre-1950 rights, causing quantifiable losses.
- Based on the evidence of those shortfalls, the Court calculated damages, interest, and Montana's litigation costs, and adopted specific rules for reservoir operation, storage limits, and information-sharing to prevent similar violations going forward.