Neither of the projects for the flood control of the Saint Francis River or the Yazoo River, authorized by sections 642a, 702a–1, 702a–2 to 702a–12, 702g–1, 702j–1, 702j–2, 702k–1, and 702k–2 of this title, shall be undertaken until the States, or other qualified agencies, shall have furnished satisfactory assurances that they will undertake, without cost to the United States, all alterations of highways made necessary because of the construction of the authorized reservoirs, and meet all damages because of such highway alterations, and have agreed also to furnish without cost to the United States all lands and easements necessary to the construction of levees and drainage ditches constructed under this project: Provided, That the reservoirs for control of headwater flow of the Yazoo River system may be located by the Chief of Engineers, in his discretion: And provided further, That the Chief of Engineers may, in his discretion, substitute levees, floodways, or auxiliary channels, or any or all of them, for any or all of the seven detention reservoirs recommended in his report of February 12, 1935, for the control of floods of the Yazoo River: And provided further, That the Chief of Engineers, with the approval of the Secretary of the Army, may modify the project for the flood control of the Saint Francis River as recommended in said report, to include therein the construction of a detention reservoir for the reduction of floods, and the acquisition at the cost of the United States of all lands and flowage necessary to the construction of said reservoir except flowage of highways: Provided further, That the estimated cost to the United States of the project is not increased by reason of such detention reservoir.

The Chief of Engineers may, in his discretion, modify the project for the control of floods on the Yazoo River, as authorized by paragraph (1) of this section, to substitute therefor a combined reservoir floodway and levee plan: Provided, That the total cost thereof does not exceed the present authorization as estimated in House Committee on Flood Control Document Numbered 1, Seventy-fourth Congress, first session: Provided further, That the modified project shall be subject to the following conditions of local cooperation:

No work shall be undertaken until the States or other qualified agencies have furnished satisfactory assurances that they will—