765 ILCS 135/1
That in any and all cases where different persons have the right to use, in separate or distinct quantities or proportions, the water or water-power furnished by a dam across any river in this state, it shall be lawful for such person and the owner or owners of such dam and water-power to fix and determine upon some fair, impartial and reasonable mode and manner by a system of weirs, or weirs and floats, or other reasonable plan of measuring and delivering to each person entitled to use the water or water-power furnished by such dam, his just share or proportion thereof, which regulation, when made and acknowledged by such persons and the owner or owners of such dam and water-power, and recorded in the recorder's office of the county in which such dam is situated, shall be binding upon all persons entitled to use the water or water-power furnished by such dam; and in case any such dam is owned by a corporation, such corporation may by by-law or resolution entered upon a book kept by the corporation and subject to the free inspection of all persons interested make such regulations: Provided, however, that in all cases the regulations made for measuring and delivering such water or water-power shall fairly and impartially apportion the same to each person entitled to the use of the same according to his just share thereof: And, provided, further, that nothing in this act contained shall be construed to impair any covenant, contract or agreement heretofore executed to any person or corporation for water or water-power or their legal rights thereto.
765 ILCS 135/1