An average area of not less than one hundred acres per mile in fee simple, plus scenic, landscape, sightly or safety easement control up to a total average of fifty acres per mile, shall be provided for the gross length of the parkway in this state for parkway purposes. At no point shall the width of such parkway area through state or private lands be less than two hundred feet, except as the secretary may determine in particular cases. If the total acreage acquired by this state and its political subdivisions or other parties and transferred to the United States for parkway purposes is less than the above stated minimum per mile, then other public areas adjacent to or near the parkway having scenic, recreational, conservation, floodway or historic value, shall be owned, acquired, or administered satisfactorily to the secretary or leased thereto sufficient to bring the total acreage up to the minimum. The area boundaries need not be mechanically determined or measured by a uniform distance from the center line of the parkway. The variation of the width shall be dependent upon the topographical and other natural conditions, requirements of design, easements, and time and cost of acquisition. Owing to proximity in numerous locations of vertical picturesque bluffs close to the Mississippi River at high water stages and the narrow range of location of existing railway facilities, the parkway road may be constructed over or under such precipitous cliffs and caverns and around and across river bends and through back country as will best diversify the rugged, wooded and open pastoral scenery and water vistas, and also tend toward economical cost of land acquisition and parkway road construction and maintenance. The state may include in its fee simple parkway right-of-way requirement, such nearby areas of the Mississippi River within its boundaries, other than the navigable channel thereof at normal river stage, provided the nearest edge of such supplementary water, island, flowway or floodway areas shall be within five hundred feet of the center line of the parkway road and shall not exceed a width of one thousand feet.