Wetland Inventory, Mapping, and Description of the Potomac Gorge

Mangi Environmental Group; Erdmann, Linda

Despite its urban location in the Washington metropolitan area, the Potomac Gorge harbors an abundance and significant variety of wetlands. The Gorge extends from Great Falls to Theodore Roosevelt Island and encompasses approximately 9,700 acres in Virginia, Maryland and the District of Columbia (DC). Little data was available characterizing the locations, sizes, types, and health of these wetlands, so the National Park Service (NPS) and The Nature Conservancy (TNC) contracted Mangi Environmental Group, Inc. to inventory, map, and characterize wetlands in the Gorge in 2002 and 2003. The project was a joint effort of the C&O Canal National Historical Park (C&O Canal NHP) and George Washington Memorial Parkway (George Washington Memorial Parkway) and TNC, with participation by National Capital Region staff.The project was conducted in two phases. In Phase I, a desktop identification and mapping of all wetland types within the Potomac Gorge site boundary was conducted using satellite imagery, ancillary Geographic Information Systems (GIS) wetland indicator data, and field data. Phase I included data gathering and assessment, desktop wetland identification and mapping, and preliminary groundtruthing. In Phase II, palustrine wetlands were field delineated and characterized, and representative riverine and lacustrine wetlands were field verified and characterized. All delineated palustrine wetlands were surveyed using a Global Positioning System (GPS). By utilizing this two phased approach, actual field efforts were focused to less than 10 percent of the entire project area reducing labor effort and cost, while still providing an accurate and comprehensive inventory of the wetlands of the Potomac River Gorge.A total of 374 palustrine wetlands comprising 133 acres were field surveyed in the Potomac Gorge. Deciduous forested wetlands are the dominant palustrine wetland class in the Gorge, representing 62 percent of the total surveyed acreage of palustrine wetlands and 65 percent of the total number of surveyed wetlands found in the Gorge. Forested wetlands occur throughout the Gorge but are absent on the frequently flooded scour bars within the river channel. Emergent wetlands represent the second most common class, comprising 21 percent of the total surveyed acreage. Emergent wetlands are primarily found in the Chain Bridge Flats area. The remaining 17 percent of wetlands were made up by open water, unconsolidated bottom, scrub-shrub, and rock bottom classes.A breakdown of ownership of palustrine wetlands in the Gorge indicates that the National Park Service owns approximately 60 percent of the total surveyed and non-surveyed wetland acreage in the Gorge. Thirty-eight percent are privately owned and another 2 percent are county owned.Four major threats to wetlands were evident from the field survey: deer, recreation, invasive species, and stormwater. Large local deer populations appear to be a problem throughout the Gorge, most notably in the Gold Mine Tract and Great Falls Park in Virginia and Maryland. The Potomac Gorge is a heavily used recreational area. Hiking trails occur throughout National Park lands and cross wetlands in many areas. Both designated and undesignated trails adversely impact wetlands by trampling vegetation, introducing invasive species, compacting and eroding soils, and altering hydrology. Invasive non-native weeds are a major threat to wetlands in the Gorge. In some floodplain wetlands, they have become more abundant than native plant species. Japanese stiltgrass (Microstegium vimineum) is the most serious threat in floodplain wetlands. Stormwater runoff from developed areas within the Gorge has caused wetland sedimentation and water quality degradation.

Wetland Inventory, Mapping, and Description of the Potomac Gorge

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