Cal. Gov. Code § 54421

By resolution the legislative body may pledge, place a charge upon, and assign all or any part of the gross revenues of the enterprise and of any and all funds referred to in Section 54478 for the security of the bonds. The gross revenues of the enterprise include revenues of improvements and extensions later constructed or acquired and revenues of existing systems, plants, works, or undertakings to be improved or extended or for the improvement or extension of which the bonds are to be issued.