A weekslong trial is underway to decide if Wyoming is liable to Montana and its farmers for taking too much water from the Yellowstone River basin.
The U.S. Supreme Court appointed Stanford Law School professor Barton Thompson, Jr. to oversee the trial that began Wednesday in a federal courtroom in Billings.
Montana alleges that the Tongue and Powder rivers are routinely depleted by upstream Wyoming farmers and oil and gas companies, violating a longstanding agreement.
Attorneys for the Big Sky state sued their southern neighbors in 2007.
Wyoming says Montana has offered no proof any of its farmers suffered harm.
The Supreme Court will have the final say. The two states have filed more than 1,000 exhibits leading up to the trial and plan to call dozens of witnesses.