Cal. Gov. Code § 54519

The local agency shall hold in trust the revenues and other funds pledged to the payment of the principal of and interest on the bonds for the benefit of the bondholders and shall apply the same pursuant to the resolution authorizing the issuance of the bonds or to the resolution as modified. The local agency may invest funds held in reserve, or in any sinking fund, or funds not required for immediate disbursement, in property or securities in which cities may legally invest funds subject to their control. No such investment shall be made in contravention of any covenant or agreement in any resolution authorizing the issuance of any outstanding bonds.