Every person owning property through which a watercourse passes, or such person’s lessee, shall keep and maintain that part of the watercourse within the property below the elevation of the 100-year flood free of trash, debris, grass clippings or other organic wastes and other obstacles that would pollute, contaminate or significantly alter the quality of water flowing through the watercourse. In addition, the owner or lessee shall maintain existing privately owned structures within or adjacent to a watercourse, so that such structures will not become a hazard to the use, function or physical integrity of the watercourse.