If a sanitary drainage district has constructed one or more channels, drains, or ditches from a city of the primary class to or beyond the boundaries of the district downstream and there remains from the lower terminus of such improvement a portion or continuation of the watercourse unimproved, the Department of Natural Resources shall investigate the conditions of such watercourse, and if the department determines that further improvement in such watercourse downstream is for the interest of lands adjacent to such watercourse below the point of the improvement, the department shall file a plan of such improvement in the office of the county clerk of each of the counties in which any of the lands to be benefited are situated and in which any portion of the watercourse to be improved is located. Such plan shall describe the boundaries of the district to be benefited and shall contain an estimate of the benefits that would accrue to the sanitary district by reason of such improvement as well as the cost thereof and an estimate of the special benefits that would accrue to lands adjacent to the watercourse by reason of improved drainage, such estimate being detailed as to the various tracts of land under separate ownership as shown by the records of the county in which such lands are situated.