Mont. Code § 85-2-413
A person who owns or is in possession of lands susceptible of irrigation from any stream, the waters of which are so diminished by prior appropriations that a sufficient amount of water for the irrigation of the person's lands cannot be obtained from the natural flow of the stream, and who constructs a reservoir or purchases or leases water from a reservoir owned by the department or another or otherwise acquires an interest in the reservoir or in water stored in the reservoir, which is located so that because of natural or other obstacles the water impounded in the reservoir cannot be conducted to the lands that the person desires to irrigate, may, if the stored water can be discharged into the stream in a manner that it can be used beneficially by prior appropriators, divert the natural flow of the stream for the irrigation of the person's lands in lieu of an equal amount of stored water. However, an exchange of water under this section must be made without injury to prior appropriators.
§ 85-2-413, MCA