S.D. Codified Laws § 5-12-33
Contemporaneously with the issue of revenue bonds as provided by this chapter, all interim notes, even though they may not then have matured, shall be paid, both principal and interest to date of payment, from the funds derived from the sale of revenue bonds for the permanent financing and such interim notes shall be surrendered and canceled, or the proceeds at the sale of said bonds (excepting the accrued interest received) shall be used to purchase direct obligations of the United States of America so long as such obligations will mature at such time or times, with interest thereon or the proceeds received therefrom, to provide funds adequate to pay when due or called for redemption prior to maturity the notes to be refunded, together with the interest accrued thereon and any redemption premium due thereon, and such proceeds or obligations of the United States of America shall, with all other funds legally available for such purpose, be deposited in escrow with a banking corporation, or national banking association, located in and doing business in the State of South Dakota, with power to accept and execute trusts, or any successor thereto, which is also a member of the federal deposit insurance corporation and of the federal reserve system, to be held in an irrevocable trust solely for and until the payment and redemption of the notes so to be refunded, and any balance remaining in said escrow after the payment and retirement of the notes to be refunded shall be returned to said authority to be used and held for use as revenues pledged for the payment of said refunding bonds.
SDCL 5-12-33