Mont. Code § 85-7-2113
Whenever districts have been established in order to cooperate with the United States under the federal reclamation laws or under any act of congress which permits the performance by the United States of work in this state, for the purposes of construction of irrigation works, including drainage works, or for purchase, extension, operation, or maintenance of construction works or for the assumption as principal or guarantor of indebtedness to the United States on account of district lands, the determination of the irrigable area of the lands in the district may be made by the board of commissioners in the manner provided in 85-7-2107 through 85-7-2112 or by the United States at the option of the latter, and, if the United States determines the irrigable area, the proceeding for the apportionment and distribution of the costs of the proposed works or improvements provided for in 85-7-2114 through 85-7-2116 may not be had.
§ 85-7-2113, MCA