Mont. Code Ann. § 85-2-134

If the change of name of a watercourse is ordered, a copy of the judgment, duly certified by the clerk of the court, must be filed with the county clerk and recorder of the county in which the watercourse or other natural source of water supply is situated, and the clerk of the court shall, annually, in January, make a return to the office of the secretary of state of all changes of names made in the district court of the clerk's county under this section.