For the purpose of avoiding an unduly high operation and maintenance assessment in any one year and to keep the operation and maintenance charges in connection with the Arrowrock Division of the Boise reclamation project within the ability of the water users to pay, the Secretary of the Interior is authorized to allow the irrigation districts of the said Arrowrock Division and the irrigation districts ditch companies, and water users who have assumed obligations to pay proportionate parts of the estimated cost of the operation and maintenance of the Arrowrock Reservoir, to pay the costs, as determined conclusively by said Secretary, incurred in the repair, resurfacing, and improvement of the Arrowrock Dam and in increasing the height thereof (to provide additional capacity to offset past and, to some extent, future losses of capacity resulting from the deposit of silt in the said reservoir) in twenty annual installments instead of requiring the payment of all of such operation and maintenance costs in one year as provided in section 492 of this title: Provided, That such costs, for the purpose of any amendatory contracts affecting the construction charges of Arrowrock Dam that may be entered into as authorized by subchapter X of this chapter, may, in the discretion of the Secretary, be treated as part of the construction charges of said dam, and as payable in the same manner as such charges.