No money appropriated under sections 702g-1 and 702k-1 of this title shall be expended on the construction of any reservoir project herein authorized until States, political subdivisions thereof, or other responsible local agencies have given assurances satisfactory to the Secretary of the Army that they will (a) provide without cost to the United States all lands, easements, and rights-of-way necessary for the construction of the project, except as otherwise provided herein; (b) hold and save the United States free from damages due to the construction works; (c) maintain and operate all the works after completion in accordance with regulations prescribed by the Secretary of the Army: And provided, That the construction of any dam authorized herein may be undertaken without delay when the dam site has been acquired and the assurances prescribed herein have been furnished, without awaiting the acquisition of the easements and rights-of-way required for the reservoir area: And provided further, That whenever expenditures for lands, easements, and rights-of-way by States, political subdivisions thereof, or responsible local agencies for any individual project or useful part thereof shall have exceeded the present estimated construction cost therefor, the local agency concerned may be reimbursed one-half of its excess expenditures over said estimated construction cost: And provided further, That when benefits of any project or useful part thereof accrue to lands and property outside of the State in which said project or part thereof is located, the Secretary of the Army may acquire the necessary lands, easements, and rights-of-way for said project or part thereof after he has received from the States, political subdivisions thereof, or responsible local agencies benefited the present estimated cost of said lands, easements, and rights-of-way, less one-half the amount by which the estimated cost of these lands, easements, and rights-of-way exceeds the estimated construction cost corresponding thereto: And provided further, That the Secretary of the Army shall determine the proportion of the present estimated cost of said lands, easements, and rights-of-way that each State, political subdivision thereof, or responsible local agency should contribute in consideration for the benefits to be received by such agencies: And provided further, That whenever not less than 75 per centum of the benefits as estimated by the Secretary of the Army of any project or useful part thereof accrue to lands and property outside of the State in which said project or part thereof is located, provision (c) of this section shall not apply thereto; nothing herein shall impair or abridge the powers now existing in the Department of the Army with respect to navigable streams: And provided further, That nothing herein shall be construed to interfere with the completion of any reservoir or flood control work authorized by the Congress and under way on June 15, 1936.