It shall be unlawful for any person to damage, alter, change or interfere with any works constructed under the provisions of title 23 or to open or close, or cause to be opened or closed, a gate or gates in any dam or dike or a gate in any head race or obstruct any channel constructed under title 23 so far as the same relates to such improvements, without the consent of the department. After the improvement of any channel or watercourse has been authorized it shall be unlawful for any person or public authority to construct any dam or other structure on the bed of such stream or any bridge across it until the department has found that such construction or bridge will not unduly affect the projected or completed improvement.