The Secretary is authorized to provide for modifications of the projects authorized by this subchapter to the extent he determines appropriate for purposes of meeting the international settlement objective of this subchapter at the lowest overall cost to the United States. No funds for any such modification shall be expended until the expiration of sixty days after the proposed modification has been submitted to the appropriate committees of the Congress, unless the Congress approves an earlier date by concurrent resolution. The Secretary shall notify the Governors of the Colorado River Basin States of such modifications.