31.32 Dams not to injure other dams or sites. No such dam shall be erected to the injury of any mill lawfully existing, either above or below it on the same stream; nor to the injury of any mill site on the same stream on which a mill or milldam shall have been lawfully erected and used or is in the process of erection, unless the right to maintain a mill on such last-mentioned site shall have been lost or defeated by abandonment or otherwise; nor to the injury of any such mill site which has been occupied as such by the owner thereof, if such owner, within a reasonable time after commencing such occupation, completes and puts in operation a mill for the working of which the water of such stream shall be applied.