The Southern Maryland Heritage Area, consisting of Calvert, Charles, and St. Mary's Counties, lies entirely within the Atlantic Coastal Plain and is bounded by the Chesapeake Bay on the east and by the Potomac River on the west and south. While the land area covers over 1,100 square miles, some 150 square miles of water further extend the physical boundaries of the area.

Distinctive natural features of the area are defined by the three primary watershed regions: the rivers and streams draining the Lower Potomac River; the streams and stream valleys associated with the Patuxent River; and the Chesapeake Bay shoreline with its three primary stream systems in the upland plateau and two stream systems in the lowland plain areas.