Mont. Code § 85-2-412
If, by virtue of prior appropriation, a person has diverted all the water of any stream or to an extent that there may not be an amount sufficient left for those having a subsequent right to the waters of the stream and there is, at any time, a surplus of water so diverted, over and above what is actually and necessarily used by the prior appropriator, the prior appropriator must be required to turn and cause to flow back into the stream the surplus water. Upon failure to do so within 24 hours after demand being made upon the prior appropriator in writing, to the prior appropriator in person or at the prior appropriator's place of residence, by any person having a right to the use of the surplus water, the person diverting the water is liable to the person aggrieved for the damage resulting from the diversion in a sum determined by court.
§ 85-2-412, MCA