It shall be the duty of the county commissioners of any county of this state to which this act applies now or hereafter having therein any public bridge or bridges over any navigable river, which bridge or bridges have been ordered or shall hereafter be ordered or required by the secretary of war of the United States, or the board of directors of any drainage district organized under the laws of this state, in any county having an assessed valuation for taxation purposes of over ninety million dollars, to be removed, relocated, reconstructed or improved, to take possession of such bridges and to forthwith remove, relocate, reconstruct or improve the same, as may be necessary and proper, and the ownership and control of such bridges is hereby vested in such counties, and the board of county commissioners thereof are hereby authorized and empowered to use any part or portion of any such bridge in the reconstruction of such bridges or of any other bridge or viaduct situated in their county, or to sell or dispose of the materials composing any such bridge, and to use the proceeds of such sale in the reconstruction or improvement of bridges over rivers situated in said county. The provisions of this act shall apply to all public bridges and approaches thereto and grade separation structures connected therewith, and all parts thereof, used by any street railway company or any other corporation, and to all public bridges and approaches thereto and grade separation structures connected therewith heretofore constructed over any such river by counties, either with or without the aid, assistance or co-operation of any street railway company or other person or corporation, and also to public bridges and the approaches thereto and grade separation structures connected therewith situated in or constructed by or under the control of any city or any other municipality.

Should any relocation, change or alteration of the channel of any river by any competent authority necessitate the removal, relocation or reconstruction of bridges and the approaches thereto and grade separation structures connected therewith over the channel of such river as relocated or altered, such work shall be done and such improvements made under the provisions of this act. Bridges and the approaches thereto and grade separation structures connected therewith, constructed, reconstructed, improved or relocated under the provisions of this act shall be under the control of and maintained by the board of county commissioners of the county, and such county commissioners shall also have power to contract with any city in which any such bridge is situated, or with any corporation using any such bridge, for the maintenance or repair thereof.