Mont. Code Ann. § 85-7-302

This part shall apply only when more than 15 owners of land, with water rights appurtenant thereto, shall have diverted water by means of a single intake from the source of supply and shall have provided a single canal for conveying such water to the branches and laterals of an established irrigation system, serving at least 1,000 acres of land contiguous in location or of reasonably compact area and in which the rights to the use of the water shall have been determined by decree of a court of competent jurisdiction.