Mont. Code Ann. § 85-7-1703

If there is a vacancy on the board of commissioners from any cause, the vacancy must be filled until the next regular or special election by appointment by the board. The remaining commissioners constitute a quorum for the purpose of filling any vacancy. If a vacancy exists for every position on the board, the judge of the district court of the county in which the division or major portion of the division is situated shall make the appointments. The appointee must be an owner of irrigable land within the division of the district that the appointee represents and must be a resident of the county in which the division of the district or some portion of the division is situated. A commissioner appointed under this section holds office until a successor is elected and qualified.