There is created a trust fund outside the state treasury to be known as the "treasurer's trust fund." All nontreasury trust funds which are in the custody of the state treasurer on April 10, 1973, shall be placed in the treasurer's trust fund and be subject to the terms of this chapter. Funds of the state department of transportation shall be placed in the treasurer's trust fund only if mutually agreed to by the state treasurer and the department. In order to assure an orderly transition to a centralized management system, the state treasurer may place each of such trust funds in the treasurer's trust fund at such times as he or she deems advisable. Except for department of transportation trust funds, all such funds shall be incorporated in the treasurer's trust fund by June 30, 1975. Other funds in the custody of state officials or state agencies may, upon their request, be established as accounts in the treasurer's trust fund with the discretionary concurrence of the state treasurer. All income received from the treasurer's trust fund investments shall be deposited in the investment income account pursuant to RCW
43.79A.040.