When the Secretary determines that lands and waters, or interests therein, have been acquired by the United States in sufficient quantity to provide an administrable unit, he shall declare establishment of the Ozark National Scenic Riverways by publication of notice in the Federal Register. The Secretary may thereafter alter such boundaries from time to time, except that the total acreage in the Ozark National Scenic Riverways shall not exceed sixty-five thousand acres, exclusive of land donated by the State of Missouri or its political subdivisions and of federally owned land transferred pursuant to section 460m-1 of this title.