Dyke Marsh is located on the west bank of the Potomac River south of Alexandria, Virginia between Old Town Alexandria and Mount Vernon. Dyke Marsh consists of about 380 acres (1.5 km²) of tidal marsh, floodplain, and swamp forest. Forming 5,000 to 7,000 years ago, Dyke Marsh is one of the largest remaining pieces of freshwater tidal wetlands left in the Baltimore-Washington Metropolitan Area. The marsh is managed by the National Park Service. Dyke Marsh contains a wide array of plant and animal life. "Haul Road" is a trail that leads visitors into the marsh. It is frequented by area birdwatchers.