Gov. Code, § 6573

So long as any bonds or interest coupons thereof are outstanding and unpaid, the revenues and interest thereon shall not be used for any purpose not authorized by the indenture, unless the authority to make such use of revenues shall be authorized by the bondholders pursuant to the provisions of the indenture, and during such period the joint powers agreement shall be irrevocable and may not be amended or modified in any manner to the detriment of the bondholders. Any bond for the payment and discharge of which, upon maturity or upon redemption prior to maturity, provision has been made through the setting apart in a reserve fund or special trust account created pursuant to this article to insure the payment thereof, of moneys sufficient for that purpose or through the irrevocable segregation for that purpose in some sinking fund or other fund or trust account of moneys sufficient therefor, shall be deemed to be no longer outstanding and unpaid within the meaning of any provision of this article.