Zekiah Swamp is a swamp in Charles County, Maryland, in the United States, formed by the Zekiah Swamp Run, a braided stream which becomes Allens Fresh Run as it debouches into the Wicomico River, a left tributary of the Tidal Potomac River. The Zekiah Swamp Run stretches 21 miles (34 km), the length of Charles County. The swamp's southern end is protected as the Zekiah Swamp Natural Environment Area, which has an area of 450 acres (180 ha) and an elevation of three feet (0.91 m) above sea level. The Maryland Department of Natural Resources has the authority to purchase an additional 5,000 acres (2,000 ha) for the Natural Environment Area. Charles County has moved to protect an additional 65,000 acres (26,000 ha) of the wetland. The headwaters of the swamp are protected as part of Cedarville State Forest. "Allen's Fresh" was first accurately surveyed in 1835, by John Henry Alexander, Maryland's official cartographer.