There are hereby authorized to be appropriated for fiscal year 1979 and ensuing fiscal years such sums as may be necessary and, effective October 1, 1983, not to exceed an additional $650,000,000 (October 1, 1983, price levels), and, effective October 1, 2000, not to exceed an additional $95,000,000 (October 1, 2000, price levels), and, effective October 1, 2001, not to exceed an additional $32,000,000 (October 1, 2001, price levels), and, effective October 1, 2003, not to exceed an additional $540,000,000 (October 1, 2003, price levels), and effective October 1, 2015, not to exceed an additional $1,100,000,000 (October 1, 2003, price levels), plus or minus such amounts, if any, as may be justified by reason of ordinary fluctuations in construction costs as indicated by engineering cost indexes applicable to the types of construction involved herein, to carry out the provisions of this subchapter to remain available until expended if so provided by the appropriations Act: Provided, That no funds exceeding $20,000,000 (October 1, 2003, price levels), as adjusted to reflect any ordinary fluctuations in construction costs indicated by applicable engineering cost indexes, shall be obligated for carrying out actual construction to modify an existing dam under authority of this subchapter prior to 30 calendar days from the date that the Secretary has transmitted a report on such existing dam to the Committee on Natural Resources of the House of Representatives and the Committee on Energy and Natural Resources of the Senate. The report required to be submitted by this section will consist of a finding by the Secretary of the Interior to the effect that modifications are required to be made to insure the safety of an existing dam. Such finding shall be accompanied by a technical report containing information on the need for structural modification, the corrective action deemed to be required, alternative solutions to structural modification that were considered, the estimated cost of needed modifications, and environmental impacts if any resulting from the implementation of the recommended plan of modification. For modification expenditures between $1,800,000 and $20,000,000 (October 1, 2015, price levels), the Secretary of the Interior shall, at least 30 days before the date on which the funds are expended, submit written notice of the expenditures to the Committee on Natural Resources of the House of Representatives and Committee on Energy and Natural Resources of the Senate that provides a summary of the project, the cost of the project, and any alternatives that were considered.