The terms used in this Act shall be construed as follows:
(a) The term "waterway" shall refer to "The Illinois Waterway;"(b) The term "appurtenances" when used with reference to or in connection with the term "The Illinois Waterway" or in connection with the term "waterway," shall comprehend all dams, locks (including the lock connecting the Sanitary District Channel with "The Illinois Waterway,") power plants, docks, dry docks, terminals, landings, walls, dykes, embankments, tunnels, basins, pools, laterals, buildings, structures or appliances necessary, convenient or incident to the construction, operation or maintenance of "The Illinois Waterway," provided the term "appurtenances" as used herein shall not include or apply to any property, real, personal, or mixed, of any existing sanitary district, which has a population of one million or more within its territorial limits.