1. Upon its organization the board may prepare a plan for the regulation of the flow of the river or rivers in its district. Such plan shall show the available and feasible reservoir sites on such river or rivers, whether any reservoir now existing can be and should be enlarged and to what extent, the approximate number of acres of land flowed or to be flowed by each reservoir, whether any of such land is owned by the state and if any, how many acres, the number of acres of private land required, together with the estimated value of the land, and how many acres, if any, of such land are lands of the state within the forest preserve. Such acreage shall include such quantity of land surrounding the reservoir to be created or enlarged as would in the judgment of the board be required for the protection, maintenance and operation of such reservoir. The plan shall be accompanied by such maps, profiles and other data and descriptions as may be necessary to set forth properly the location and character of the work and of the property to be taken or damaged, and by estimates of cost of each such reservoir. In case maps, plans and specifications for any such reservoir at or near the site thereof, are on file in the office of the department, and in case any records in any other department of the state government would be an aid in the preparation of such plans, these shall be available to the board for such purpose.
2. Upon the completion of such plan if the board approves of such plan it shall certify it to the department for its approval, and the department within forty days thereafter may approve the same, or modify it and approve it as so modified. As so approved by the department it shall be known as the “official plan” for the regulation of the flow of the river or rivers of the district. Copies thereof shall be filed in the offices of the board, the department, and of the county clerk of each county wholly or partly within the district. Such plan in like manner may be altered from time to time provided such alteration shall not affect any assessment already made.