The board, upon its own motion or upon a petition filed with the district by at least five owners of land within the district and after a public hearing, may adopt or amend flood control improvement corridor maps which show the watercourses as defined in section 31-202 within the district or the reaches of watercourses which the district in the future may determine to improve with levees or other flood control improvements. The maps shall show the corridors of land on either side of the centerlines of the watercourses which the board determines should be reserved for the future construction, operation, or maintenance of flood control improvements and shall show the approximate location of the corridors on each parcel of land traversed.