Mont. Code § 85-7-2119
Where a contract has been entered into or may be hereafter entered into between an irrigation district and the United States, the board of commissioners shall have the power to levy assessments against all of the land within the district for any and all of the purposes hereinbefore enumerated and, in addition thereto, the power to levy assessments against any or all of the lands in said district in compliance with such contract. Where irrigation works lie partly in the state of Montana and partly in an adjacent state, the board of commissioners may contract with the district or districts in the adjacent state for the mutual construction of works, operation and maintenance of works, drainage, and other matters and things pertaining to said works and shall have the power to levy assessments against any or all lands within the district necessary to carry out the provisions of such contract.
§ 85-7-2119, MCA