California Code, Streets and Highways Code - SHC § 33117

The authority may provide for the making of a covenant requiring the authority to provide for the establishment and maintenance of reserve funds, sinking funds, or other special funds in the city treasury or special trust accounts in a bank or trust company to insure payment, when due or payable, whether at maturity or upon redemption, of the principal of and interest on any bonds, including premiums, if any due, upon the redemption of any thereof, or to insure the application of the proceeds of such bonds to the purposes for which the same were issued, or for any other appropriate purpose.  Any money placed in any such reserve, sinking, or other special fund or trust account shall constitute a trust fund and shall be applied only to the purposes for which it was created.