All assessments collected for administrative, operative and maintenance purposes, all money collected and not otherwise provided for, and any transfers authorized by law from other funds made specifically to the fund, shall be placed by the county treasurer, ex officio treasurer of the district, in the expense fund, and it shall be the duty of the district board to make ample provision for the requirements of this fund by the levy of assessments or by the use of other revenues of the district.
RCW 86.09.535