The county treasurer shall act as the custodian of sinking funds, or funds created for the payment of bonds, except that in case an irrigation district is located in more than one county the board, by recorded resolution, shall designate the county treasurer who shall act as the custodian of the funds. Upon the adoption of the resolution, the county treasurer so designated shall act as the custodian. A copy of the resolution and a copy of each and every contract, trust agreement, or trust indenture relating to the issuance and payment of bonds must be filed with the county treasurer.