There is hereby granted to persons, firms and corporations organized among other things, for irrigation and power purposes, the right to construct and maintain dams and works incident thereto over, upon and across the beds of the rivers of the state of Washington in connection with such power and irrigation purposes, and there is hereby granted to such persons, firms and corporations an easement over, upon and across the beds of such rivers for such purposes. Such easement shall be limited however, to so much of the beds of such rivers as may be reasonably convenient and necessary for such uses. All such dams and works shall be completed within five years after the commencement of construction work upon the same. The rights and privileges granted by this section shall inure to the benefit of such persons, firms or corporations from the date of the commencement of construction work upon such dams and works incident thereto, and such construction work shall be diligently prosecuted to completion, and the rights, privileges and easements granted by this section shall continue so long as the same shall be utilized by the grantees for the purposes herein specified, and the failure to maintain and use such dams and works after the same shall have been constructed, for a continuous period of two years, shall operate as a forfeiture of all the rights hereby granted and the same shall revert to the state of Washington: PROVIDED, That nothing in this section shall be construed in such a way as to interfere with the use of said rivers for navigation purposes, and all of such rights, privileges and easements granted hereby shall be subject to the paramount control of such rivers for navigation purposes by the United States: AND, PROVIDED FURTHER, That the use and enjoyment of the grants and privileges of this section shall not interfere with the lawful and rightful diversion of the waters of said rivers by other parties under water appropriations in existence at the time any such persons, firms or corporations shall avail themselves of the benefits and privileges of this section, but no such persons, firms or corporations shall have any right to construct any such dams or works over, upon or across the land between ordinary high water and extreme low water of any river of this state without first having acquired the right to do so from the owner or owners of the lands adjoining the land between ordinary high water and extreme low water over or across which said dam or works are constructed.