Whenever any subdivision is traversed by a watercourse, drainageway, channel or stream, the subdivider shall, at his or her own expense, make adequate provision for straightening or widening the channel so that it will properly carry the surface water, and shall also provide and dedicate to the city an easement along each side of the stream, which easement shall be for the purpose of widening, improving or protecting the stream. The width of such easement shall not be less than 20 feet, and the total width of the easement shall be adequate to provide for any necessary channel relocations and straightenings.