The board shall cause an actuarial investigation and valuation of the system to be made immediately after the first year of operation and thereafter at not to exceed five-year intervals. The investigation shall be conducted under the supervision of an actuary and shall cover the mortality, service, and salary experience of the members and beneficiaries, and evaluate the assets and liabilities of the fund. Upon the basis of the determination, investigation, valuation, and recommendation of the actuary the board shall recommend to the board of supervisors such changes in the rate of interest, in the rates of contributions of members, and in county appropriations as it deems

necessary. No adjustment shall be included in the new rates for time prior to the effective date of the revision.