1. Subject to paragraphs 2 and 3, the occupier of any land on or through which an undesignated watercourse runs, or of which such a watercourse forms a boundary, shall so scour out and cleanse and maintain the portion of the watercourse running on or through, or bounding, the land occupied by him that the efficiency of the watercourse, for the due discharge of any water that may flow into it, is not impaired by reason of any act or omission of his or of anyone acting under his authority.N.I.