tidewater, in U.S. history, that part of the Atlantic coastal plain between the shoreline and the farthest upstream points in rivers reached by oceanic tides. In many cases the fall line is given as the western boundary. The tidewater, with its good harbors readily accessible to the ocean, was settled first by European colonists. Later the Southern tidewater became one of the many regions of large plantations as well as an area of important commercial towns.