The unit of the North Bay Aqueduct as set forth in Bulletin No. 60 of the Department of Water Resources entitled “Salinity Control Barrier Investigation,” dated March 1, 1957, subject to such modification thereof as the department may adopt, and such unit or portions thereof may be constructed by the department and maintained and operated by it to such extent and for such period as the department may determine, as a feature of the California Water Plan, and as a unit of the Central Valley Project, separate and apart from any or all other units thereof.