S.D. Codified Laws § 46A-6-8

The board of directors of any irrigation district established and organized under the laws of South Dakota, if deemed advisable and in the best interests of the district, may enter into any contract with the United States supplementing or amending any original contract with the United States, if the original contract was entered into pursuant to the provisions of chapters 46A-4 to 46A-7, inclusive, and if the supplementary or amendatory contract does not increase the amount of principal indebtedness of the district to the United States as it exists at the date of the supplementary or amendatory contract.