Idaho Code § 50-221

Cities situated on navigable lakes and streams, when the corporate boundaries or limits of such cities extend to the shorelines of such lakes or streams, shall have power by ordinance to fix, determine or extend its corporate boundaries or limits over the waters of such lakes or streams for a distance of one fourth (1/4) of a mile from the low-water mark of such navigable lakes, and for a distance of seventy-five (75) feet from the low-water mark of such navigable streams.