The Panama Canal Zone was an unincorporated territory of the United States from 1903 to 1979, centered on the Panama Canal and surrounded by the Republic of Panama. The zone consisted of the canal and an area generally extending on each side of the centerline, excluding Panama City and Colón, which otherwise would have been partly within the limits of the Zone. Its border spanned two of Panama's provinces. When reservoirs were created to assure a steady supply of water for the locks, those lakes were included within the Zone.