A Hydrogeomorphic Map of Assateague Island National Seashore, Maryland and Virginia

Krantz, David

The landforms and hydrology of Assateague Island National Seashore are interpreted and categorized in a hydrogeomorphic map of the barrier island. This report accompanies a digital map in GIS format, defines the primary map units and subunits, and explains the approach for classifying sections of the island. The base for the map is a color infrared photomosaic of the National Seashore that covers two-thirds of Assateague Island. A lidar digital elevation model of the island with 10-cm vertical resolution was used as a supplement to the photomosaic for interpretation of landforms. The interpretation of island hydrology relies on a previous study of ponds on the island and geophysical surveys of representative sites to delineate the vertical character and horizontal continuity of the surficial aquifer and fresh ground-water lens. Geophysical methods included gamma and electromagnetic induction logging of existing deep wells, ground-penetrating radar, and electrical resistivity.The surface of Assateague Island has been sculpted mostly by vigorous overwash flow during major storms and the cycle of inlet formation and closure. The associated storm and inlet processes have created an island surface with alternating low sections and higher-elevation uplands that are protected by sand ridges. The water table, which is the top of the surficial (unconfined) aquifer, generally follows the topography of the island surface, and its elevation above sea level in part controls the depth of the fresh ground-water lens beneath the island. Consequently, geomorphology related to storm processes is linked to the distribution of fresh ground water on the island, which in turn is a primary control on the plant communities. Vegetation, both cover and assemblage, is an excellent temporal integrator of hydrologic conditions, and changes in vegetation commonly coincide with geomorphic boundaries on the island.The spatial distribution and dynamics of fresh and brackish ground water beneath the island are strongly affected by the frequency and magnitude of the input of saltwater onto the island surface and into the surficial aquifer by storm overwash from the ocean side and high-water flooding from the bay side of the island. Both the ocean and bay sides of the island have highly dynamic brackish zones in the aquifer produced by surface inundation of saltwater and deeper density-driven ground-water flow.Six primary map units are defined for the island based on consistent geomorphic and hydrologic character: Island Core (IC), Over Wash (OW), Tidal Marsh (TM), [Former] Inlet (IN), Wash Around (WA), and Ponds (PD). Each primary map unit is divided into map subunits based on relative elevation and degree of protection from saltwater inundation. Numerical suffixes are used to indicate relative elevation within a map unit, grading from low (1) to higher (4 or 5); for example, subunit TM2 represents the high marsh. Character suffixes of subunit identifiers are abbreviations that indicate specific landforms such as swales (sw) or overwash channels (ch). Certain map subunits may be equivalent in hydrologic and vegetative character to a subunit of another primary map unit. For example, the high marsh TM2 may appear in several different geomorphic settings including the bay side of the island core, and low areas of washarounds and former inlets. In these cases, compound identifiers are assigned, such as IC1/TM2. Subunits and characteristics of the primary map units, and equivalencies among subunits are summarized in tables.This hydrogeomorphic map has substantial explanatory and predictive value for evaluating the spatial character of many components of the barrier-island ecosystem, including the distribution of plant species and communities, fresh-water resources for large vertebrates, and living and breeding habitat for numerous invertebrates and smaller vertebrates.

A Hydrogeomorphic Map of Assateague Island National Seashore, Maryland and Virginia

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