[Expenses of district; charge for water; levy of

assessments.] For the purpose of defraying the expenses of the organization of the district, and the care, operation, management, repair and improvement of all canals, ditches, reservoirs and works, including salaries of officers and employees, or for payment of charges to the United States for the temporary rental of water, the board may either fix rates of tolls and charges and collect the same of all persons using said canal and water for irrigation, or other purposes, and in addition thereto may provide, in whole or in part, for the payment of such expenditures by levy of assessments therefor, as heretofore provided, or by both tolls and assessments; provided, that if any contract be made with the United States the charge for operation and maintenance of the district and for temporary rental of water may be fixed in accordance with the federal laws, notices, rules and regulations and the contract with the district.