615 ILCS 5/23
It shall be the duty of the Department of Natural Resources to maintain stream gauge stations, and to make careful investigations of the streams of the State with reference to the carrying capacity of all such streams in times of flood and under normal conditions; to prevent the carrying capacity of streams to be limited and impaired by fills, deposits, obstructions, encroachments therein, deposit of debris or material of any kind, including trees, tree limbs, logs, shrubbery, or related growths and trimmings therefrom in or upon the bank of any waters and water courses or in such proximity to such waters and water courses or any tributary thereto where the same shall be liable to be washed into or deposited along such waters and water courses, either by normal or flood flows, as a result of storms or otherwise, which may in any manner impede or obstruct the natural flow of such waters and water courses, or bridges over same, to an extent where the same cannot safely dispose of the flood waters which may naturally, lawfully, and properly be discharged therein; to require such changes in bridges across any navigable waters or streams, or bodies of water made navigable, necessary to meet the demands of navigation and commerce thereon; and to establish by regulations water levels below which water cannot be drawn down behind dams from any stream or river within the State of Illinois, in order to retain enough water in such streams to preserve the fish and other aquatic life in the stream, and to safeguard the health of the community. If the capacity of any stream is limited and impaired by reason of any of the Acts or construction in this Act provided, so as to constitute a menace to property along the course of said stream or safety of the people of the State, or results in damage, overflow, or an interruption to navigation, or if water is being drawn down, or is about to be drawn down in contravention of the water level regulations established by the Department, the Department of Natural Resources shall take such action as may be required, by injunction or otherwise, to prevent such encroachments or the erection of such structures, or compel the removal or modification of same, or to prevent water being drawn down below the levels established by the Department. It shall be unlawful for any person, persons, corporations, counties, cities, municipalities, or other agency to make any fill, deposit, or encroachment in, deposit or placement of felled or trimmed woody plant upon or along the bank, or erect any bridges over a stream that has a drainage area of one square mile or more in urban areas or 10 square miles or more in rural areas, until plans, profiles and specifications and other data which may be required, have been first filed with the Department of Natural Resources of this State, and a written permit received therefor.
The Department of Natural Resources is authorized, in case of existing dams, to require that the dams be maintained in a proper state of repair, and at a height for proper control of water levels in the disposal of flood waters and at normal stages, and for such purposes to require changes and modifications therein, and to compel the installation of fishways in dams wherever deemed necessary; provided, however, that the enactment by any governmental unit of any new rule, regulation, ordinance, law, or other requirements passed after the construction of an existing dam, shall not be construed to give the Department of Natural Resources the power or authority to require changes in the design, structure, or construction, of any existing dam in conformance with all applicable standards existing at the time of its construction and which is in good repair.
615 ILCS 5/23