National Park Service

U.S. Department of the Interior

Natural Resource Program Center

A hydrogeomorphic map of Assateague Island National 

Seashore, Maryland and Virginia

Natural Resource Report NPS/NRPC/GRD/NRR—2010/215
ON THE COVER

Tidal marsh dominated by Spartina alterniflora that developed in a low swale between 

sets of beach ridges that form Green Run Island. The ridges are occupied by maritime 

forest with a fringe of salt-tolerant shrubs including Baccharis, Iva, and Myrica.
A hydrogeomorphic map of Assateague Island 

National Seashore, Maryland and Virginia

Natural Resource Report NPS/NRPC/GRD/NRR—2010/215

David E. Krantz

Department of Environmental Sciences

University of Toledo

Toledo, Ohio 43606

June 2010

U.S. Department of the Interior

National Park Service

Natural Resource Program Center

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Program Center publishes a range of reports that 

address natural resource topics of interest and applicability to a broad audience in the National 

Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate high-priority, current natural resource 

management information with managerial application. The series targets a general, diverse 

audience, and may contain NPS policy considerations or address sensitive issues of management 

applicability.

All manuscripts in the series receive the appropriate level of peer review to ensure that the 

information is scientifically credible, technically accurate, appropriately written for the intended 

audience, and designed and published in a professional manner. 

This report received informal peer review by subject-matter experts who were not directly 

involved in the collection, analysis, or reporting of the data. Data in this report were collected 

and analyzed using methods based on established, peer-reviewed protocols and were analyzed 

and interpreted within the guidelines of the protocols.

Views, statements, findings, conclusions, recommendations, and data in this report do not 

necessarily reflect views and policies of the National Park Service, U.S. Department of the 

Interior. Mention of trade names or commercial products does not constitute endorsement or 

recommendation for use by the U.S. Government.

This report is available from the Natural Resource Publications Management 

(http://www.nature.nps.gov/publications/NRPM) and the Geologic Resources Division websites 

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm)

Please cite this publication as:

Krantz, D. E. 2010. A hydrogeomorphic map of Assateague Island National Seashore, Maryland 

and Virginia. Natural Resource Report NPS/NRPC/GRD/NRR—2010/215. National Park 

Service, Fort Collins, Colorado.

NPS 622/104170, June 2010
iii

Contents

Page

Figures............................................................................................................................................ v

Tables........................................................................................................................................... vii

Abstract......................................................................................................................................... ix

Acknowledgments........................................................................................................................ xi

Introduction: Developing the Map.............................................................................................. 1

Characterizing the Surficial Aquifer with Geophysical Surveys............................................. 3

Creating the Map Units.......................................................................................................... 13

Primary Map Units..................................................................................................................... 17

Island Core - IC ..................................................................................................................... 18

Overwash - OW..................................................................................................................... 19

Tidal Marsh - TM .................................................................................................................. 21

[Former] Inlet - IN................................................................................................................. 23

Washaround - WA ................................................................................................................. 24

Ponds - PD............................................................................................................................. 25

Defining the Map Subunits ........................................................................................................ 27

Categories for Storms and Vegetation................................................................................... 28

Island Core............................................................................................................................. 30

Subunits IC3 and IC4........................................................................................................ 30

Subunit IC5 ....................................................................................................................... 31

Subunits IC1 and IC2........................................................................................................ 31

Subunit ICmm.................................................................................................................... 33

Distinguishing Island Core subunits................................................................................. 33

Overwash ............................................................................................................................... 34

Subunit OWfr (frequent) ................................................................................................... 35

Subunit OWint (intermittent)............................................................................................. 36

Subunits OWfan, OWch, and OWsw................................................................................. 37

Unit DN and DNmm.......................................................................................................... 39
iv

Tidal Marsh............................................................................................................................ 39

Subunit TM1...................................................................................................................... 39

Subunit TM2...................................................................................................................... 40

Subunits TMsr and TMfl ................................................................................................... 40

Distinguishing Tidal Marsh subunits................................................................................ 42

[Former] Inlet......................................................................................................................... 44

Subunits IN1, IN2, and IN3............................................................................................... 44

Subunits Inch and INsw .................................................................................................... 46

Distinguishing Inlet subunits............................................................................................ 46

Inlet-closure ridges........................................................................................................... 46

Washaround ........................................................................................................................... 48

Subunits WA3 and WA4 .................................................................................................... 48

Subunits WA1 and WA2 .................................................................................................... 49

Distinguishing Washaround subunits............................................................................... 49

Ponds...................................................................................................................................... 52

Subunit PD4...................................................................................................................... 54

Subunits PD3 and PD2 ..................................................................................................... 55

Subunit PD1...................................................................................................................... 55

Pond Metrics..................................................................................................................... 55

Summary...................................................................................................................................... 57

Literature Cited .......................................................................................................................... 59

Appendix A: Locations of Scenes Shown in Report Figures................................................... 61
v

Figures

Page

Abstract Image. The transition from the back-barrier tidal marsh onto the higher-elevation 

island core covered by maritime forest.......................................................................................... ix

Figure 1. Regional and local setting of Assateague Island ............................................................ 1

Figure 2. (A). Color infrared photomosaic of Assateague Island National Seashore used as 

the base for the hydrogeomorphic map. Northern section of the map area .................................... 4

Figure 2. (B) Central section of the map area, showing locations of study sites for ponds,

wells, and geophysical surveys....................................................................................................... 5

Figure 2. (C) Southern section of the map area, showing locations of study sites for ponds,

wells, and geophysical surveys....................................................................................................... 6

Figure 3. Location of focus site KM10, showing individual geophysical transects...................... 7

Figure 4. Gamma and electromagnetic induction logs for USGS well WO-Dg-23 at the 

KM10 site........................................................................................................................................ 8

Figure 5. Ground-penetrating radar profiles KM10-04 and KM10-01.......................................... 9

Figure 6. A dipole-dipole resistivity profile along transect KM10-04 ........................................ 10

Figure 7. Representative examples of Schlumberger resistivity soundings................................. 11

Figure 8. Example of a high-resolution DEM for site KM10...................................................... 13

Figure 9. Section of Assateague Island showing all the primary map units................................ 16

Figure 10. Representative section of the Island Core map unit ................................................... 17

Figure 11. Section of the island core created as a set of recurved spits adjacent to a former 

inlet ............................................................................................................................................... 18

Figure 12. Section of the island core created as storm ridges wrapped around an older part 

of the island................................................................................................................................... 19

Figure 13. Area of extensive overwash that covers the entire width of the island ...................... 20

Figure 14. Overwash deposits, distinguishing between frequent and intermittentoverwash....... 20

Figure 15. Typical Tidal Marsh setting on the back-barrier flat and adjacent low islands.......... 21

Figure 16. Tidal marsh occupying the sand bodies of the previous flood-tidal delta at Green 

Run Inlet........................................................................................................................................ 22
vi

Figure 17. The low-lying section of the island at Little Level, site of a former inlet.................. 23

Figure 18. Large washarounds on the bay side and smaller washarounds in the center of Fox 

Hills Level..................................................................................................................................... 24

Figure 19. Examples of ponds on Assateague Island................................................................... 25

Figure 20. A diversity of ponds representing the entire range from completely fresh to 

completely brackish or saline........................................................................................................ 26

Figure 21. Subunits of the Island Core map unit ......................................................................... 32

Figure 22. Ocean-side swales that receive seawater overwash intermittently............................. 35

Figure 23. A complete, isolated overwash system that was active during the March 1962 

northeaster storm........................................................................................................................... 36

Figure 24. An overwash fan on northern Assateague Island, ...................................................... 37

Figure 25. Overwash channels and swales on northern Assateague Island ................................. 38

Figure 26. Subunits of the Tidal Marsh map unit ........................................................................ 40

Figure 27. Subunits of the Inlet map unit..................................................................................... 44

Figure 28. Inlet-closure ridges and intervening swales at the former Green Run Inlet ............... 47

Figure 30. Subunits of the Washaround map unit........................................................................ 50

Figure 31. Ridge and swale sets created by storm overwash flow laterally around the higher 

section of the island core............................................................................................................... 54
vii

Tables

Page

Table 1. Study sites on Assateague Island ..................................................................................... 2

Table 2. General classification scheme for ponds based on characteristic salinity...................... 26

Table 3. Map units, suffix modifiers, and equivalencies. ............................................................ 28

Table 4. Categories of storm intensity. ........................................................................................ 29

Table 5. Categories of storm effects............................................................................................. 29

Table 6. Categories of salt tolerance in plants. ............................................................................ 29

Table 6.1. Plant species and common names............................................................................... 30

Table 7. Subunits and characteristics of the Island Core map unit. ............................................. 34

Table 8. Subunits and characteristics of the Overwash map unit................................................. 34

Table 9. Subunits and characteristics of the Tidal Marsh map unit. ............................................ 43

Table 10. Subunits and characteristics of the Inlet map unit. ...................................................... 43

Table 11. Subunits and characteristics of the Washaround map unit........................................... 52

ix

Abstract

Abstract Image. The transition from the back-barrier tidal marsh onto the higher-elevation island core 

covered by maritime forest View looking northeast toward the ocean.

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 
x

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 densitydriven 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.
xi

Acknowledgments

Supported by the Northeastern Cooperative Ecosystem Studies Unit, Great Lakes-Northern 

Forest Cooperative Ecosystem Studies Unit, and the Geologic Resources Inventory Program of 

the National Park Service.

Funding for this research was provided in part by the Geologic Resources Inventory, one of the 

twelve inventories of the National Park Service Inventory and Monitoring Program.

This mapping project benefitted tremendously from the thesis research of Sagit Hall, with 

valuable input from her advisory committee: Doug Levin, now with NOAA, Ted Mollett at the 

University of Maryland Eastern Shore, and Martin Rabenhorst of the University of Maryland.

Students from the University of Toledo who helped with various aspects of field data collection 

and data processing included Abby Norton, Geoff Wikel, Matt Seifert, and Alycia Pittenger. The 

Natural Resources staff at Assateague Island was always helpful with logistics, field support, and 

scientific input – special thanks go to Carl Zimmerman, Brian Sturgis, Courtney Schupp, Mark 

Duffy, Arty Rodriguez, Andrew Roach, and Mark Sturm.

Reviewed by Courtney Schupp, Brian Sturgis, Carl Zimmerman, and Mark Sturm of Assateague 

Island National Seashore.

Formatted to meet NPS Natural Resource Report specifications by Philip Reiker of the Geologic 

Resources Division of the National Park Service.

1

Introduction: Developing the Map

The ground-water hydrology of Assateague Island, a barrier island along the Atlantic coast of 

Maryland and Virginia (Fig. 1), is controlled largely by the geomorphology, or the landforms, of 

the island. In turn, the distribution of fresh and brackish ground water in the unconfined, surficial 

aquifer, and the geometry of the fresh ground-water lens under the island strongly influence the 

distribution of plant communities and habitat for a diversity of invertebrate and vertebrate 

wildlife. This report accompanies a hydrogeomorphic map of Assateague Island National 

Seashore (ASIS) that subdivides and categorizes the island surface in terms of its 

geomorphology as related to characteristics of the ground-water hydrology. The map itself is in a 

GIS format with digital polygons representing all of the map units and subunits discussed in this 

report overlain on a georeferenced color infrared photomosaic of the National Seashore (Fig. 2).

The map covers the entire Maryland section of the island, including the National Seashore and 

the Maryland State Park, and extends about 1 km into the Virginia section of the island.

Figure 1. Regional and local setting of Assateague Island, showing the area covered by the 

hydrogeomorphic map as represented by the individual sections of the photomosaic in Figure 2.

The conceptual framework of this map is based in part on recent studies of the ponds (Hall, 

2005) and the larger-scale geomorphology of the island (Morton et al., 2007), combined with 

geophysical surveys and observations on the island specifically in support of interpretations for 

this map. As part of the pond survey, we characterized the surficial aquifer near the seven pond 

sites, which are documented in detail by Hall (2005), using a variety of geophysical methods: 

ground penetrating radar, electrical resistivity, and borehole geophysical tools.
2

These geophysical surveys showed contrasting conditions in the surficial aquifer at the various 

sites, which tended to group in predictable patterns related to geomorphology and position on the 

island. A preliminary geomorphic map developed while conducting the pond survey helped 

relate the characteristics of the ponds to the broader setting of Assateague Island. That 

preliminary map guided additional field study to identify the end-member geomorphic and 

hydrologic settings, and to characterize the environmental gradients between those end members

All sites for the pond study and geophysical surveys are identified on the island sections 

presented in Figure 2 and summarized in Table 1. A separate report will document and interpret 

the extensive geophysical data collected on the island in support of the hydrogeomorphic map.

Table 1. Study sites on Assateague Island. (Location identifiers refer to the standard NPS kilometer 

designations for Assateague Island.)

Location Type of data Geophysical 

data Map units Comments

KM10 geophysics, 

wells GPR, resistivity IC, OW, TM S edge of extensive overwash 

area

KM11 pond IC

KM13 geophysics GPR, resistivity IC, IN behind man-made dune

KM15 pond IC

KM18 pond, 

geophysics GPR, resistivity IC, OW, WA, 

TM Tingles Island access

KM22 geophysics, 

wells GPR, resistivity IC, IN, WA, TM Pine Tree Island access;includes brine study

KM23 ponds, 

geophysics GPR, resistivity IC, OW, IN ponds in swales between storm 

ridges

KM24 pond, 

geophysics GPR IC, OW, IN North edge of Fox Hills Level

KM26 geophysics resistivity IN (INsp) Fox Hills Level salt pan

KM27-29 geophysics, well GPR, resistivity IC, OW, TM Green Run Island;

well at Valentine cottage

KM30 ponds, 

geophysics GPR, resistivity IC, OW, IN Jackson cottage access

KM33 ponds, 

geophysics GPR IC, IN historic Green Run Inlet
3

Characterizing the Surficial Aquifer with Geophysical Surveys

Sets of geophysical transects were completed at each of ten study sites on the island (Fig. 2, 

Table 1). As mentioned above, many of these sites were part of the pond study, but the remaining 

sites were chosen to characterize sections that are representative of larger areas of the island. The 

geophysical sites cover all of the primary geomorphic and hydrologic settings of the island. In 

general, transects measured conditions in the center or “core” of each primary map unit, and 

crossed the transitions or gradients within and between the map units. For example, in the section 

of the island encompassing kilometers 27 to 29, immediately north of Green Run and informally 

referred to as “Green Run Island,” four geophysical transects were completed that covered the 

width of the island from the edge of the overwash area to the bay-side salt marsh; four additional 

transects on the ocean side and bay side covered shore-parallel geomorphic transitions. These 

transects together comprise over 2.5 kilometers of continuous measurements at this locality.

Most, but not all, of the geophysical transects were along the access roads because the vegetation 

was too dense elsewhere. Because most of the roads are simply cut into the island surface or 

filled with island sediments, there is a negligible effect on the quality of the geophysical data; for 

those few road sections with thicker fill (greater than ~0.5 m) or remnants of asphalt paving, the 

geophysical surveys were taken along the edge of the road.
4

Figure 2. (A) Northern section of the map area. Color infrared photomosaic of Assateague Island 

National Seashore used as the base for the hydrogeomorphic map. The map extends from the Ocean 

City Inlet at the north end of the island to approximately 1 km south of the Maryland-Virginia border. Also 

shown are locations of pond sites (ovals), deep wells (crosses), and geophysical survey sites (rectangles) 

that are labeled by NPS kilometer designations. (All references to kilometer location on the island are 

based on the standard kilometer posts used by the NPS, starting with kilometer 0 [zero] at the Ocean City 

Inlet at the north end of the island and increasing to the south.) All aerial photographic images in this 

report are oriented with north toward the top of the page. Photomosaic provided by the ASIS GIS group.
5

Figure 2. (B) Central section of the map area, showing locations of study sites for ponds, wells, and 

geophysical surveys.
6

Figure 2. (C) Southern section of the map area, showing locations of study sites for ponds, wells, and 

geophysical surveys.
7

Several key sites – KM10, KM22, and KM28-29 (using the standard NPS kilometer 

designations) (Fig. 2) – provided critical “golden spikes” where geophysical logs down existing 

deep wells produced detailed vertical profiles of sediment type from gamma logs and groundwater conductivity (i.e., salinity) from electromagnetic (EM) induction logs. The vertical 

structure of the surficial aquifer from the well logs was mapped laterally away from the single 

point of the well by transects of ground-penetrating radar (GPR) and electrical resistivity. These 

two techniques produce two-dimensional profiles that, combined, allow the interpretation of the 

depth of fresh ground water in the surficial aquifer. Transects at several sites were re-occupied 

during successive field excursions to obtain time series showing some of the dynamics of the 

ground-water system.

Figure 3. Location of focus site KM10, showing individual geophysical transects. The locations of scenes 

presented in all figures are shown in the photomosaic panels in Appendix A.

Site KM10, along Shell Road just north of the main entrance road coming off the Verrazano 

Bridge (Fig. 3), is one of the key sites, and provides an example of how the ground-water 

hydrology is inferred from the geophysics. The gamma log from a US Geological Survey well at 

the site (Fig. 4) shows sand of the overwash sheet down to 4 m, underlain by back-barrier silt to 

about 7.5 m. The EM log shows an increase in ground-water salinity starting at about 5 m depth 

and continuing to 12 or 12.5 m, where it reverts to fresh ground water. The GPR profile KM10-

04 (Fig. 5) across the center of the island, near the well, also shows the base of the of the 

overwash sands as a strong, mostly flat-lying reflection at about 4 m. Above this boundary, the 

GPR reflections are bright and clear, but below, they are weak, indistinct, or absent because of 

attenuation of the radar waves by brackish pore water. The base of the GPR reflections gives a 

quantitative measure of the depth from the land surface to the base of the fresh ground-water 

lens. One section of the profile with weak reflections below 4 m, that occurs between 40 and 140 
8

m along the transect, is likely to have fresh (or nearly fresh) ground water penetrating down into 

the back-barrier silt deposits, similar to that seen in the EM well log between 4 and 6 m depth 

(Fig. 4). In contrast, the eastern end of GPR profile KM10-01 (Fig. 5) shows a progressively 

shallower fresh lens toward the ocean as the result of the injection into the surficial aquifer of 

seawater from overwash that percolates down through the sand and recharges the water table.

Figure 4. Gamma and electromagnetic induction logs for USGS well WO-Dg-23 at the KM10 site. The 

gamma log shows the relative proportion of fine-grained sediments (higher values) to sand (lower values).

The EM induction log primarily shows the conductivity of the pore fluids (ground water) in the sediments 

surrounding the well casing; lower values indicate fresh and higher values indicate brackish to saline 

ground water.
9

Figure 5. Ground-penetrating radar profiles KM10-04 and KM10-01 running west to east across the island at site KM10. The attenuation of radar 

reflections, indicated by the light magenta shading, marks the base of the fresh ground-water lens, which correlates closely with the depth indicated by the EM conductivity log from the well at this site. The deepest, most stable part of the fresh lens is near the center of the island. An abrupt transition to brackish ground water in the shallow aquifer coincides with the edge of the high marsh on the bay side. The fresh lens toward the ocean end of the transect becomes progressively shallower because of brackish ground water resulting from frequent overwash of seawater.
10

Figure 6. A dipole-dipole resistivity profile along transect KM10-04. In the bottom panel (Inverse Model 

Conductivity Section), the cool colors (blue and green) indicate fresh ground water, and the warm colors 

(orange and red) represent brackish to saline ground water. The vertical (depth) scale for this 

measurement has less resolution than either the EM conductivity log or the GPR profile, but the method 

gives excellent complementary information about the structure of the aquifer. The transition at 4.5 m 

depth to brackish ground water correlates well with both the GPR and well-log data. (Figure provided by 

G.L. Wikel.)

Electrical resistivity provides an independent, and complementary, measure of the structure of 

fresh and saline ground water in the surficial aquifer beneath the island. The two-dimensional 

resistivity profile presented in Figure 6 is from a dipole-dipole survey, which shows the 

continuity of the subsurface structure, but is very time-consuming to collect. This resistivity 

profile supports the interpretation of both the well logs and GPR profiles by showing a relatively 

thin layer of highly resistive materials (the equivalent of low electrical conductivity) in the upper 

4 m, with a fairly abrupt transition between 4.5 and 5 m to much lower resistance materials 

(more highly conductive) below. This layering is interpreted as fresh ground water in sand near 

the surface overlying brackish to saline ground water in silt below 4.5 m.

A second type of resistivity measurement, a Schlumberger (pronounced “schlum-bear-zhay”) 

vertical electrical sounding (VES) was used more extensively on the island because it can be 

collected more rapidly – about 1 hour per sounding compared with 8-10 hours for a dipole-dipole 

transect – and it has greater vertical resolution. The limitations of the VES are that it represents a 

single vertical point rather than a two-dimensional section, and the plotted VES data are not 
11

intuitively obvious without extensive explanation. However, as with the geophysical surveys in 

general, sites for the VES were chosen carefully to represent end-member environments on the 

island, and were always done in concert with another geophysical method.

Figure 7. Representative examples of Schlumberger resistivity soundings, showing a range of 

distributions of ground-water salinity in the surficial and shallow confined aquifers beneath Assateague 

Island and Sinepuxent Neck. (A) Log-log plot of resistivity versus depth (as AB/2 distance) for eight 

resistivity soundings that represent conditions from completely fresh to completely saline ground water.

(B) Inversion model for resistivity sounding at the NPS Headquarters on Sinepuxent Neck, with fresh 

ground water in a sandy upper unit and a silty lower unit. (C) Inversion model for a typical mid barrier 

island setting, represented by KM10, showing dry sand at the top, wet sand with fresh ground water, and 

a transition to brackish or saline ground water at 4 m depth; a lower transition back to fresh (or fresher) 

ground water in a confined aquifer occurs near 21 m depth. (D) Inversion model for a salt pan on Fox Hills 

Level, with saline to hypersaline ground water in the upper 14 m of the aquifer.
12

A subset of all the Schlumberger soundings, representing a wide range of hydrologic settings on 

and near Assateague Island, is presented in Figure 7a. The settings include the NPS Headquarters 

on Sinepuxent Neck, which is a late Pleistocene barrier island with stratigraphy similar to that 

under modern Assateague Island but with fresh ground water to depth, several sites near the 

center of Assateague Island with a fresh ground-water lens overlying brackish ground water, 

several sites with either recent or persistent seawater overwash, and one example of a subsurface 

hypersaline brine (beneath Pine Tree Island).

The data from a single sounding are plotted on log-log axes (Fig. 7a), with apparent resistivity 

versus a proxy for depth (AB/2 distance). In a simplified sense, the shape of each individual 

curve is a response to the amount of electricity that is transmitted between two electrodes 

through subsurface layers with different electrical properties. Some of the common layers 

encountered beneath Assateague Island in order from very high resistivity (very low 

conductivity) to low resistivity (high conductivity): dry sand, sand saturated with fresh water, silt 

with fresh water, and either sand or silt with saline ground water. An extreme case is the 

hypersaline brine beneath Pine Tree Island, which has twice the salinity of seawater and is nearly 

completely conductive (it acts almost like a copper wire).

To contrast two very different hydrologic settings, the resistivity curve for NPS_HQ-R-01 has 

high values, above 100 ohm-m, for almost the entire depth, whereas the curve for KM26-R-01, 

from the salt pan in Fox Hills Level, has extremely low resistivity values, generally below 1 

ohm-m (Fig. 7a). The first-order effect is that the NPS_HQ site is underlain by sand saturated 

with fresh ground water, whereas the KM26 site has saline to slightly hypersaline ground water 

in the upper several meters. The downward curvature for NPS_HQ-R-01 most likely indicates 

fresh ground water in a layer of silt below the sand, and the upward curvature for KM26-R-01 

implies fresh or nearly fresh ground water in a layer of sand deeper beneath the island.

The interpretations from the apparent resistivity curves are supported by inversion modeling of 

the data from each VES. Three examples are shown that represent the fresh ground water setting 

of Sinepuxent Neck (Fig. 7b), a typical vertical profile for the middle of Assateague Island (Fig. 

7c), and the salt pan of the KM26 site (Fig. 7d). In the inversion procedure, the observed 

apparent resistivities with depth are back-calculated from a model that successively adds layers 

with different thickness and bulk resistance. Results of the models with the lowest root mean 

squared (RMS) error are presented graphically with a preferred solution (the dark lines in panels 

7b, c, and d) and a range of possible solutions (the shaded gray areas in the same panels) where 

conditions change rapidly. In Figure 7, each solution is annotated with the most likely 

combination of sediment and ground water that would produce the layering in the inversion 

model.

The main point is that the individual VES curves (Fig. 7a) and the layered inversion model 

results (Fig. 7b, c, d) show the vertical structure of the fresh ground-water lens and underlying 

brackish ground water beneath sections of the island. The individual dipole-dipole transects and 

Schlumberger soundings were located specifically to represent end-member settings, which 

would become the primary map units, and the gradients within and between the primary map 

units, which would become the subunits and boundaries between primary units. With this 

approach, the characterization of aquifer conditions within representative sections of the island 

can be extrapolated with confidence to the entire map area.
13

Creating the Map Units

Because of the consistent, predictable hydrologic conditions at the end-member settings, these 

geomorphic environments became the six primary map units for the hydrogeomorphic map, 

which are: IC, Island Core; OW, Over Wash; TM, Tidal Marsh; IN, (former) Inlet; WA, Wash 

Around; and PD, Pond. While these primary units were not derived explicitly from the 

geomorphic categories of Morton et al. (2007), the two classification schemes are very similar 

and reflect the strong control of storm processes on forming the land surface of the island. The 

primary hydrogeomorphic units are further subdivided to express smaller-scale features of the 

island that influence the distribution of the plant communities. The map subunits reflect 

variations in elevation, specific features derived from the formation processes, such as overwash 

channels and fans, and the susceptibility of the areas to overwash or flooding by seawater.

The map was created by drawing interpretations, that is, the polygons constituting the map units 

and subunits, directly on printed panels from a color infrared (IR) photomosaic flown in October 

1993, and provided by the GIS group at ASIS. The photomosaic is georeferenced and covers 

about two-thirds of the island, from the Ocean City inlet to about 1 km south of the MarylandVirginia border (Fig. 1). Even though more recent, higher-resolution photomosaics of the island 

in natural color are available, this photomosaic was chosen in large part because the color IR 

allows distinction of subtle variations in vegetation, either cover or type, that are difficult to 

extract from the natural-color images. Lidar DEMs with a 10-cm contour interval (Fig. 8) were 

created for sections of the island from the EAARL lidar survey collected by NASA in 2002 and 

processed for bare-earth topography (Harris et al., 2003). Although the lidar DEMs were useful 

for defining dunes in the overwash area and storm ridges on the ocean side of the island core, the 

link between geomorphology and the hydrologic effects that control the distribution of 

vegetation is better expressed on the color IR image than on the lidar DEMs. For example, lowrelief ridge and swale sets could be identified on the color IR but not on the lidar images. 

Vegetation is an effective temporal integrator of hydrologic conditions.

Figure 8. Example of a high-resolution DEM created from the 2002 EAARL lidar bare-earth topographic 

data for geophysical survey site KM10. Color contour interval is 10 cm.
14

As with all maps, this hydrogeomorphic map of Assateague Island is a representation of the 

actual land surface of the island. The map expresses a conceptual model, one which has been 

developed from numerous measurements and observations on the island, and from an evaluation 

of the storm processes that have shaped the island surface. The primary map units represent the 

end-member environments identified and quantified during the field surveys. The map subunits 

are a semiquantitative means of defining variations and gradients of hydrologic conditions within 

the primary map units. If needed by the NPS for specific projects, this classification could be 

applied to localized areas with more detail and greater spatial resolution. The map has 

considerable predictive value in explaining the distribution of plant communities, invertebrates, 

and vertebrates on the island, and should be tested against other independent spatial datasets.

16

Figure 9. Section of Assateague Island showing all the primary map units. Near kilometer 16
17

Primary Map Units

The following brief summaries describe the key features of each primary map unit and explain 

the dominant processes and the typical hydrologic conditions. Figure 9 shows a representative 

section of Assateague Island that includes all of the primary map units. Each primary map unit is 

further subdivided into map subunits that express variations within the primary unit. The map 

subunits are described and explained in the next section of the report

Figure 10. Representative section of the Island Core map unit, outlined with purple, between Little Level 

and Fox Hills Level, kilometers 19 to 24.
18

Island Core - IC

The Island Core (IC) is the central part of the barrier island that has generally higher elevation 

and is typically covered by maritime forest (Fig. 10). The two most common geomorphic settings 

for the IC map unit are sets of recurved spits adjacent to a former inlet (Fig. 11), or an older 

section of the island protected on the ocean side by large storm ridges (Fig. 12). The island core 

lies landward/bayward of the primary dune system, and the storm ridges that protect the island 

core often are identified as the “secondary dunes,” although the formation processes of these two 

landforms differ substantially. Some storm ridges exceed 4 m elevation and have thick 

unsaturated sands, which commonly appear as a sparsely vegetated sand ridge. Behind the storm 

ridges, the island core has a low slope toward the back-barrier tidal marsh and lagoon. Here the 

island core typically grades into the high marsh, with subtle changes of elevation on the order of 

10 cm demarcated by changes in vegetation cover or assemblage. The central part of the island 

core has the most stable, deepest fresh ground-water lens, observed as deep as 7-8 m measured 

down from the land surface, and the most consistently fresh ground water. Ponds in the central 

island core are well protected from salt-water intrusion, permanently fresh, and particularly 

valuable wildlife habitat.

Figure 11. Section of the island core created as a set of recurved spits adjacent to a former inlet. Below 

South Beach, near kilometer 17.
19

Figure 12. Section of the island core created as storm ridges wrapped around an older part of the island.

Green Run Island, near kilometer 28.

Overwash - OW

Overwash zones on the ocean side of the island are segregated into either frequent (OWfr) or 

intermittent (OWint) subunits depending upon the relative frequency of inundation with 

seawater. Overwash zones may cover nearly the entire width of low-lying sections the island 

(Fig. 13), such as in northern Assateague, or may be localized as overwash fan complexes (Fig. 

14). The OWfr subunit comprises the berm and back-beach environments that receive seawater 

overwash during spring high tides and minor storms. The OWint subunit is delineated by near 

absence of vegetation and broad sheets of recently mobilized sand transported landward from the 

beach and berm. The OWint subunit receives seawater overwash during stronger minor storms 

and moderate storms, commonly once to several times a year. Overwashing seawater typically 

flows through low areas among the dunes, often creating channels, and will pond in swales, 

where the saltwater infiltrates the surficial aquifer. Ground water in the OW zone is brackish 

nearly year-round, and highly dynamic with the episodic input of full-salinity seawater.

Vegetation in the Overwash zone is limited to those species specifically adapted to tolerate 

frequent inundation by seawater, persistent brackish ground water, and desiccation associated 

with the relatively thick unsaturated sandy soil zone.
20

Figure 13. Area of extensive overwash, outlined 

with orange, that covers the entire width of the 

island, north Assateague, kilometers 5 to 9.

Figure 14. Overwash deposits on the seaward 

side of Fox Hills Level, distinguishing between 

frequent (OWfr) and intermittent (OWint) 

overwash. Near kilometer 26.
21

Tidal Marsh - TM

One of the most extensive features on the bay side of the barrier island, tidal marshes form on the 

intertidal zone of the back-barrier overwash flat (Fig. 15) and on sandy islands created by the 

flood-tidal delta of an inlet (Fig. 16). Subtle variations in elevation are accentuated by the 

zonation of the salt-adapted, salt-tolerant, and salt-resistant vegetation. In most settings, the 

entire TM unit varies by only a few tens of centimeters with an extremely low slope. Ponds in 

the intertidal low marsh almost universally have saline or brackish water, which may be received 

either by flow through natural tidal creeks or man-made ditches, or by ponding during spring 

high tides and high-water events. Most ponds in the high marsh are similarly brackish but may 

be flushed by fresh or reduced-salinity ground water during periods with a high water table. In 

specific settings, salt pans in the high marsh produce hypersaline brine that sinks into the 

surficial aquifer. Elsewhere in former inlets, broad salt-pan areas with elevations slightly below 

mean high water are mostly unvegetated except for cyanobacterial mats and Salicornia. Ground 

water beneath the tidal marsh is typically brackish to fully saline, although fresher ground water 

recharged from the island interior may flow shallowly beneath the marsh in discrete sand beds 

overlain by low-permeability salt marsh peat and mud.

Figure 15. Typical Tidal Marsh setting on the back-barrier flat and adjacent low islands, outlined with 

green. South of main entrance road from the Verrazano Bridge, near kilometer 11.
22

Figure 16. Tidal marsh occupying the sand bodies of the previous flood-tidal delta bayward of the historic 

Green Run Inlet. Middlemoor Islands, near kilometer 33. The broader view of this area is shown in Figure 

2c.
23

[Former] Inlet - IN

Assateague Island is cut extensively by former inlets, some of which were only shallow and 

short-lived, others were wider, deeper, and lasted longer (possibly a few hundred years). After 

most inlets close, they leave a low-elevation section of the island (Fig. 17), usually bounded 

laterally by relatively higher sand ridges or spit complexes (the Island Core map unit). At many 

sites, the tidal channels of the former inlet are prominent features cutting across the island and 

extending as deep channels into the back-barrier lagoon (Fig. 9). Associated with these crosscutting channels are longitudinal sand ridges and lateral terraces that were created by vigorous 

tidal flow while the inlet was open. On the seaward side of the former inlet throat, the trace of 

the tributary “catchment” area of the inlet mouth is usually evident, although modified by 

subsequent sand transport, inlet-closure ridges, and dunes. Former inlets typically have 

predominantly saline to brackish ground water because these are preferential pathways for both 

storm overwash and subsequent ground-water flow due to the coarse, permeable channel fill.

Consequently, most ponds within the IN map unit are brackish with varying degrees of flushing 

with fresh ground water. Vegetation in the lower-elevation sections of the IN map unit generally 

is equivalent to that of the low marsh and high marsh; salt-tolerant shrubs typically occupy the 

higher-elevation areas.

Figure 17. The low-lying section of the island at Little Level, site of a former inlet, outlined with blue. Near 

kilometer 20.
24

Washaround - WA

Washarounds are slightly higher-elevation features in the otherwise low-lying former inlets or 

areas of extensive overwash (Fig. 18). They were older sections of the island that have been 

dissected and sculpted by high-velocity flow of ocean water across the island during major 

storms. Washarounds characteristically have a hydrodynamically rounded or ovate form with a 

series of storm ridges on the seaward side of the feature, which protects the interior of the 

washaround. Washarounds vary in width and bayward length from a few tens of meters to 

several hundred meters. Many characteristics of the washarounds are analogous to the Island 

Core, but scaled down considerably. Vegetation is distinctly zoned, and the center of the feature 

may have a permanent, moderately deep (3-4 m) fresh ground-water lens. The center of the 

larger washarounds is covered by maritime forest with obligate freshwater plants. The plant 

community is steeply zoned along the seaward edges of the washaround as the elevation drops 

off the storm ridges. In contrast, the bayward side of a washaround is typically occupied by lowmarsh grasses and high-marsh shrubs and grasses in a low-slope pocket protected by the 

wrapping storm ridges. Swales and ponds in the center of a washaround are mostly fresh, 

although they are susceptible to flooding during strong moderate and major storms.

Figure 18. Large washarounds on the bay side and smaller washarounds in the center of Fox Hills Level, 

outlined with yellow. Near kilometer 27.
25

Ponds - PD

Essentially all natural ponds on Assateague Island, and there are literally hundreds present on the 

island, were formed by channelized overwash flow during storms that cut below the depth of the 

water table (Fig. 19a). Because there are no streams on the island, other than tidal creeks in the 

marshes, all ponds on the island are fed by ground-water seepage, into the pond from the upgradient end and out of the pond from the down-gradient end (Fig. 19b). Ponds are the only 

source of fresh water on the island for animal habitat and drinking. The character of the ponds 

varies dramatically depending upon the position on the island and the thickness and dynamics of 

the fresh ground-water lens (Fig. 20).

For the map, ponds are categorized by the relative frequency and extent of input of saltwater 

(Table 2), either by storm overwash from the ocean or high-water flooding from the bay. The 

assignment of ponds to one of the four categories is based on (1) the seasonal characteristics of 

pond water (specifically conductivity) as measured in eleven ponds weekly for one year by Hall 

(2005); (2) a broader qualitative survey of ponds observed while conducting field work that 

focused mostly on the vegetation in and around the ponds – for example, ponds rimmed by 

Spartina alterniflora are always brackish, those with Typha are predominantly fresh; (3) 

observations of saltwater flow into ponds associated with overwash produced by two moderate 

storms during the period of study; and (4) position of the pond relative to the primary map units 

(Fig. 20) – for example, PD4 is found almost exclusively near the center of either the Island Core 

or the larger Washarounds, whereas PD1 generally is found in Tidal Marsh or Inlet settings.

Figure 19. Examples of ponds on Assateague Island. (A) Three ponds (a, b, c) cutting across the high 

marsh on the bay side of Green Run Island. All three clearly were cut by channelized overwash flow.

Pond (a) is connected to the brackish water of Chincoteague Bay by a narrow tidal creek. (B) Similar 

ponds associated with older washarounds, and also created by vigorous storm overwash flow. Pond (d) 

has no stream connections through the marsh. The series of ponds labeled (e) illustrate the downgradient connection by ground-water flow among ponds in a swale.
26

Figure 20. A diversity of ponds representing the entire range from completely fresh (PD4) to completely 

brackish or saline (PD1). Because all ponds on Assateague Island receive water from ground water, the 

salinity characteristics of the pond reflect the relative degree of protection from seawater overwash or 

flooding as related to geomorphology and position on the island.

Table 2. General classification scheme for ponds based on characteristic salinity.

PD1 always brackish to saline 

PD2 usually brackish, may freshen from ground-water input during wet periods 

PD3 usually fresh, infrequent input of brackish to saline water either from direct overland flow or from 

input of brackish ground water from up gradient

PD4 protected ponds that remain fresh, these are particularly important for wildlife
27

Defining the Map Subunits

Within each primary map unit, the designations are refined further using numerical (1-4, or 1-5) 

and/or character suffixes. In general, the numerical suffixes grade from lower elevation (1) to 

higher elevation (4 or 5) within a primary map unit. However, the numerical suffixes do not 

indicate elevation alone, but also the degree of protection from seawater overwash or flooding by 

high water. Low but broad sections provide greater protection to the island interior than narrow 

sections of the same absolute elevation. Similarly, a relatively low-elevation area mostly 

surrounded by ridges, particularly on the ocean side, is better protected than an exposed area of 

equivalent elevation. As an example, many areas of the OWint (Overwash intermittent) subunit 

are actually higher elevation than the well-protected IC3 (Island Core 3) areas of the island 

interior.

Character suffixes are abbreviations for specific landforms, for example, ch for channel, sp for 

salt pan, and sw for swale. All suffixes used in the map are identified in Table 3. Numerical and 

character suffixes within cells relate elevation (the number) and specific landforms (the 

abbreviation), for example, a moderately high sand ridge associated with an old inlet may be 

categorized as IN3sr. Equivalencies are made between map subunits by reading across rows in 

the table. In some cases, especially for gradational changes on the island, a particular section will 

be classified as two different but essentially equivalent map units, for example, the classification 

of IC1/TM2 is applied where the bayward side of the island core (IC1) grades into the high 

marsh (TM2).

Two equivalent map subunits share the same or similar characteristics for elevation, hydrology, 

and degree of protection from overwash or flooding. Consequently, equivalent subunits generally 

have the same vegetation cover and assemblage. The distinction between equivalent subunits 

arises from the geomorphic setting and the processes of formation. To extend the example used 

above, the high marsh subunit TM2 appears in several different geomorphic settings: on the 

bayward side of the island core (IC1) or larger washarounds (WA1), intermediate elevations in 

former inlets (IN2), and higher ridges on marsh islands (TM2) such as those of former flood-tidal 

deltas. As correlated across the row in Table 3, the compound identifiers for the map subunits to 

categorize these examples are IC1/TM2, WA1/TM2, IN2/TM2, and simply TM2 (for the marsh 

island), respectively.

For the classification of ponds, the equivalencies in Table 3 indicate the map units that most 

commonly enclose a pond with a given numerical suffix. For cases where a pond straddles the 

boundary between two units, the pond will receive ground water from the higher-elevation, upgradient area, which controls the salinity characteristics of the pond.
28

Table 3. Map units, suffix modifiers, and equivalencies.

Numerical suffixes:

Within a primary map unit, such as IC, numerical suffixes range from lowest elevation (1) 

to highest elevation (4 or 5).

Abbreviated suffixes:

ch - channel fr - frequent (overwash)

fl - flat, tidal int - intermittent (overwash)

sp - salt pan fan - overwash fan

sr - sand ridge mm - man-made

sw - swale DNmm - man-made dune

wt - wetland

Equivalent map subunits are read across rows, for example, an area of high marsh on the bay side of a 

washaround may be categorized as WA1/TM2. Pond subunits defined in Table 2 correlate with the 

geomorphic map subunits that enclose the pond, for example, PD3 most commonly occurs within IC2 or 

IC3.

Categories for Storms and Vegetation

Qualitative descriptions of the strength of coastal storms and the adaptation of plant species to 

salinity are used in the explanations of the map subunits. These factors relate the dominant 

processes of storm overwash and flooding to the resulting form of the island surface, the salinity 

dynamics of ground water in the surficial aquifer, and the distribution of individual plant species 

and plant communities on the island. Detailed quantification of storm processes or the salt 

tolerance of plants was not a focus of this mapping project, although it may be a goal of future 

field study to evaluate the relations implied by the map. The following tables define the 

categorical terms used in this report to describe storm intensity and salt tolerance of plants.

Island Core Washaround Former Inlet Tidal Marsh Pond Overwash

IC WA IN TM PD OW

1 / sp 1 / fl 1

1 1 2 / ch, sw 2 / sr 1 fr / 1 ch

1 2 3 / sr 2 int / 2 ch, sw

2 2 / 3 3 3 ch, fan

3 3 3 / 4

4 4 / sr 4

5 / sr n/a DNmm
29

Table 4. Categories of storm intensity.

Beaufort Scale descriptor Maximum wind speed

Minor Near Gale 33 knots / 61 km/hr

Moderate Gale 40 knots / 74 km/hr

Major Strong Gale 47 knots / 87 km/hr

Storm >48 knots / >89 km/hr

Hurricane >64 knots / >118 km/hr

Table 5. Categories of storm effects.

Water height Recurrence Comments

Minor 0 to 0.5 m above MHW once to several times 

per year

Most common Oct-Mar; floods

high marsh and scrub fringe;

active flow across OWfr areas.

Moderate 0.5 to 1.5 m once every 2-5 years Active flow into OWint areas;

extensive back-barrier flooding.

Major >1.5 m above MHW once every 10-50 years,

depending upon magnitude

Extent of overwash depends on

magnitude; most low-lying areas

completely washed over;

seawater may flood edges of

island core and washarounds.

Table 6. Categories of salt tolerance in plants.

Representative taxa Comments

Salt-adapted Spartina alterniflora Predominant grass of salt marsh, flourishes in constant 

exposure to saltwater.

Salt-tolerant Spartina patens,

Distichlis, Juncus, 

Baccharis, Iva

Predominant plants of the high marsh and scrub 

transition; able to withstand flooding of several days 

and persistent brackish ground water.

Salt-resistant Myrica, Panicum,

Juniperus, Phragmites

Predominant plants of the scrub community between the 

high marsh and upland; also ocean-side swales that 

receive seawater overwash intermittently.

Freshwater-obligate Pinus taeda, Acer

Prunus, Smilax, 

Vaccinium, Vitis,

Toxicodendron

Predominant plants of the maritime forest; not able to 

withstand flooding by seawater; spatial distribution 

limited by shallow brackish ground water.
30

Table 6.1. Plant species and common names

Plant species Common name

Acer rubrum red maple

Baccharis halimifolia groundsel tree

Distichlis spicata saltgrass

Iva frutescens marsh elder

Juncus roemerianus needlerush

Juniperus virginiana eastern redcedar

Myrica (Morella) cerifera wax myrtle

Panicum amarum, virgatum switchgrass, panic grass

Phragmites australis common reed grass

Pinus taeda loblolly pine

Prunus virginiana common chokecherry

Smilax rotundifolia green briar

Spartina alterniflora salt marsh cordgrass

Spartina patens salt meadow cordgrass

Toxicodendron radicans poison ivy

Vaccinium angustifolium lowbush blueberry

Vitis rotundifolia wild grape

Island Core

Subunits IC3 and IC4

Island Core subunits IC3 and IC4 are generally the higher-elevation section of the island (Figs. 

10-12, 21), but more important they are protected from overwash even during moderate storms.

General characteristics for all the Island Core subunits are summarized in Table 7. In many 

settings, subunits IC3 and IC4 are protected on the seaward side by a single large ridge – usually 

an IC4 or IC5 feature – or by a series of moderately high ridges. Elevations for IC3 are generally 

1 to 1.5 m above sea level, with ridges in the range of 1.5 to 2 m. Swales within IC3 may be as 

low as 0.5 m, but are protected from saltwater flooding by the surrounding high ground, and 

usually are occupied by freshwater wetlands or seasonal ponds. The vegetation within swales 

differs from the surrounding maritime forest by adaptation to hydric soils, that is, soils that are 

water saturated throughout most or all of the year. Elevations within subunit IC4 are commonly 

1.5 to 2 m, and ridges, which are a prominent component of this feature, are upwards of 2 m.

The IC4 areas are covered by the oldest-growth and most diverse maritime forests on the island, 

largely due to the depth and stability of the fresh ground-water lens. The maximum observed 

thickness of the fresh lens is 7-8 m, which is much shallower than predicted by theoretical 

considerations but limited in depth by the low permeability lagoonal silt layer that underlies 

much of the island. Many of the IC4 areas appear to have been relatively protected during the 

1962 Ash Wednesday storm, one of the two most devastating storms of the 20th century for 

Assateague Island. Many of the prominent storm ridges that isolate the island core from the 

overwash areas were either created or enhanced during the 1962 storm.
31

Subunit IC5

Island Core subunit IC5 differs from IC4 primarily in the thickness of the unsaturated (i.e., dry) 

soil zone and the sparsity of vegetation (label [e] in Figure 21b), in particular trees and shrubs.

Hudsonia, beach heather, may have a scattered distribution across the largely barren surface of 

the IC5 subunit. In most cases, the IC5 subunit is a high-elevation ridge created by one or more 

storms and subsequent aeolian processes. Elevations of IC5sr sand ridges commonly exceed 2.5 

m and may be as high as 4.5-5 m. A second, non-natural setting for IC5 is landward of the high 

man-made dune (subunit DNmm) that fronts the island section from about kilometer 10 through 

kilometer 16, encompassing the Maryland State Park and the camping area of the National 

Seashore south to the cross-over road for off-road vehicles. The man-made dune is so effective at 

restricting overwash that the ground water in the seaward section of the island that otherwise 

would be categorized as subunits OWfr and OWint is fresh or only slightly brackish rather than 

brackish to saline.

Subunits IC1 and IC2

On the bayward side of the central island core, the island surface slopes gradually into the backbarrier tidal marsh. Subunits IC1 and IC2 span this transition, and in many cases will have a 

compound identifier such as IC1/TM2 (label [h] in Figure 21b). Geomorphically, this slope is the 

bayward section of the overwash sheet or a platform of amalgamated overwash fans that is 

slightly above mean high water, with elevations of 0.25 to about 1 m. Subunit IC2 is protected 

from saltwater intrusion except during intense moderate and major storms. In contrast, subunit 

IC1 may be flooded during strong minor and moderate storms. In the subsurface, the fresh 

ground-water lens that recharges in the central island core begins to shallow substantially, from 

2-3 m thick beneath IC2 to generally less than 1.5 or 1 m under IC1. The vegetation is similarly 

zoned in response to the thickness of the fresh lens and the frequency of inundation by seawater.

Vegetation of IC2 is a scrubby mixture of salt-resistant plants and pioneering freshwater plants.

Trees generally do not attain full size. Vegetation grades across IC1 from salt-resistant (Myrica, 

Juniperus, Phragmites) to salt-tolerant (Juncus, Baccharis, Iva) toward the tidal marsh.

Ponds within IC1 but closer to the IC1-IC2 boundary are typically categorized as PD3, 

predominantly fresh with occasional brackish periods. For these ponds, substantial fresh groundwater flow from the island core will flush any intruded saltwater relatively quickly. Ponds 

toward the tidal marsh side of IC1 are categorized as PD2 since they are more likely to be 

flooded with saltwater and typically are brackish except during wet periods with a high water 

table on the island.

In a few sections of the island, subunit IC2 appears on the seaward side of the central island core, 

or laterally adjacent to former inlets (primary map unit IN), as low-relief sets of ridges and 

swales created by storms. The ground-water hydrology of this type of IC2 area may differ from a 

bayward IC2 in that the up-gradient ground water is likely to be coming from the landward side 

of the overwash zone and would be brackish. Additionally, ridges and swales within this IC2 

setting often have markedly different vegetation because each ridge may protect a narrow, 

shallow fresh lens isolated from the immediately adjacent brackish ground water.
32

Figure 21. Subunits of the Island Core. Part of 

the view shown in Figure 10, near kilometer 23; 

access road to Pine Tree Island appears at the 

top of the scene. (A) Lidar DEM of the scene. B)

Color infrared aerial photograph without polygons 

or identifiers of map subunits; letter labels are 

used in the text for explanation of features. (C)

Map subunits for Island Core and associated 

features.
33

Subunit ICmm

Man-made features such as causeways, dikes, or spoils piles in the back-barrier may be high 

enough above the tidal marsh that they have characteristics of the IC1 or IC2 subunits. 

Distinguishing Island Core subunits

The scene presented in Figure 21 shows two sections of the island core that developed on either 

side of a small inlet that is now closed. The broader view of this area is shown in Figure 10. The 

single inlet channel, label (a) in Figure 21b, remains as a deep channel that connects with 

Chincoteague Bay but is cut off by sand ridges on the ocean side. The former inlet mouth, label 

(b), remains as a broad, fan-shaped low area on the seaward side of the inlet channel; the active 

overwash sheet is encroaching on and filling this low area. Sets of spits on either side of the inlet 

channel (two [c] labels) were formed by capture of longshore sand transport and diversion into 

the inlet when the inlet was open and active. A narrower and lower, but distinct, set of two spits 

(d) appear to have formed as the last stage in the closure of this inlet. The largest and highest 

ridge to the south of the inlet (e), as shown by the lidar DEM (Fig. 21a), formed earlier in the 

history of the inlet when the channel was relatively wider. The size of this large ridge may have 

been enhanced by aeolian transport, as suggested by what may be a wind-deflation bowl on the 

bay side, in the area of label (g).

The three prominent ridge sets, (c), (d) and (e), on the ocean side protect lower-lying sections of 

the island core to the west, areas (f) and (g), from storm surge and overwash, allowing growth of 

a mature maritime forest (f) and mixed forest and scrubland (g). Ponds in this protected area, 

such as (i), are perennially fresh and classified as PD4.

The bayward side of the island core was the former back-barrier overwash flat that is now 

occupied by high marsh (h), with the compound classification of IC1/TM2. Slightly higher 

elevation washarounds (three [j] labels), which were created before the ridge sets formed on the 

seaward side, are hummocks of mixed forest and scrubland in the high marsh behind the main 

body of the island. These three moderately large washarounds suggest a previous stage for this 

section of the island that was similar to the low area of Fox Hills Level today. Each washaround 

shows a zonation of elevation and vegetative cover, categorized as WA1 through WA3. High, 

presumably dry, sparsely vegetated ridges on two of the washarounds are classified WA4.
34

Table 7. Subunits and characteristics of the Island Core map unit.

Subunit Elevation

Relative 

frequency of 

saltwater input

Thickness of 

fresh groundwater lensVegetation Geomorphic 

setting

IC5 highest;

2.5-5 m

only during major 

storms

7-8 m (max 

observed) thick 

unsaturated zone

sparse, freshwater 

obligate, resistant to 

aridity

highest elevation 

ridges assoc. with 

recurved spits or 

storm ridges

IC4 1.5-2 m

ridges to 3 m

only during major 

storms

6-8 m (max 

observed) 

moderately thick 

unsaturated zone

maritime forest, 

freshwater obligate

sets of recurved 

spits or storm ridges

IC3 1-1.5 m

ridges to 2 m

during strong 

moderate and 

major storms

3-6 m maritime forest & 

scrubland, 

freshwater obligate

older island section, 

protected by ridges 

on ocean side

IC2 0.5-1 m during moderate 

and major storms

2-3 m scrubland with saltresistant plantsback-barrier 

overwash platform 

or spit platform

IC1 0.25-0.5 m during strong minor 

and moderate 

storms

1-2 m mixed scrubland & 

grassland, salt 

tolerant

back-barrier 

overwash platform 

or spit platform

Table 8. Subunits and characteristics of the Overwash map unit.

Subunit Elevation Relative frequency 

of saltwater input

Thickness of 

fresh groundwater lens

Vegetation Geomorphic 

setting

OWfr 0-2 m, dunes 

higher

receives overwash 

several times per 

month; spring high 

tides, minor storms

always brackish 

to saline

none, or 

pioneering 

beach/dune 

assemblage

most active 

overwash across 

and behind berm

OWint 0.5-2 m, dunes 

higher

receives overwash 

several times per 

year; strong minor & 

moderate storms

alternates 

between shallow 

fresh and 

completely 

brackish

beach/dune 

assemblage, 

Ammophila, 

Cakile, Solidago, 

Amaranthus

overwash sheet and 

channels through 

dunes

OWsw 0.5-1 m receives overwash 

several times per 

year; strong minor &

moderate storms

alternates 

between shallow 

fresh and 

completely 

brackish

salt-tolerant 

grasses Spartina 

patens, Juncus, 

Panicum

low areas of mouths 

of former inlets or 

overwash channels
35

Overwash

Subunit OWfr (frequent)

In sections of the island that are not experiencing severe erosion, the beach and back-beach 

environments accrete vertically and prograde seaward between storms, creating the berm. The 

crest of the berm may be 1.5 to 2 m above mean sea level (Table 8), and the height is influenced 

by wave action during successive spring high tides and minor storms. The island surface slopes 

gently landward away from the berm crest, in part because of the progressive loss of velocity of 

overwashing waves and in part because of the deflation caused by removal of fine sand by 

aeolian (wind) processes. The landward limit of this area may be marked by the primary dunes, 

either natural or man-made, but in many places on Assateague Island it simply grades into the 

larger overwash sheet created by the stronger storms that carried beachface sands farther toward 

the island interior. This OWfr subunit receives overwashing seawater regularly, during periods of 

spring high tide with moderate wave action, and from the water-level set-up and wave action of 

minor storms. The ground water from this subunit drains back toward the ocean, and remains 

brackish to fully saline.

In low-lying sections of the island such as northern Assateague, the OWfr subunit extends 

considerably farther landward and may cover the entire width of the island (Fig. 13). It is likely 

that these sections of the island retain brackish ground water perennially, with only minor mixing 

and freshening from precipitation.

Figure 22. Ocean-side swales that receive seawater overwash intermittently. In this area, they are 

classified as IC3sw or IC2sw depending upon the relative frequency of inundation by seawater. These 

swales have a unique salt-tolerant grass community dominated by Spartina patens, Distichlis, and 

Juncus. This scene is from the southern part of Figure 12, near kilometer 28.
36

Subunit OWint (intermittent)

The stronger minor and moderate storms have created a broader overwash sheet than that 

described for the OWfr subunit. Reworking of the sand sheet by wind creates the secondary dune 

field, which is included in this map subunit. Where the primary dunes exist, the OWint deposits 

extend landward through breaches in the dunes that were cut by channelized overwash flow 

during storms. In these settings, active overwash will follow low-lying paths through the primary 

dunes into the secondary dune field. As the surface flow loses momentum, seawater will pond in 

swales where it infiltrates into the surficial aquifer over the following days. This recharge of 

saltwater mixes with and pushes out any fresh ground water, then the brackish ground water 

flows down gradient in the subsurface. By this process, down-gradient ponds and wetlands 

receive brackish ground water, although with a lag time of days to weeks after the storm.

The swales that receive overwashing seawater several times a year tend to have a distinct salttolerant grass community dominated by Spartina patens, Juncus, and Panicum; this plant 

community has a unique visual appearance in the color IR images (Fig. 22). More generally, 

vegetation in the OWint subunit is sparse and limited to those taxa specially adapted to the harsh 

conditions that alternate between inundation by seawater and desiccation because of the thick 

unsaturated zone. These overwash areas are the habitat for American beach grass (Ammophila 

breviligulata), sea rocket (Cakile edentula), and the Federally-listed Threatened seabeach 

amaranth (Amaranthus pumilus), which colonize organic wrack and incipient dunes.

Figure 23. A complete, isolated overwash system that was active during the March 1962 northeaster 

storm, showing the catchment area with tributary flow on the ocean side, convergence to a single deep 

channel, then deposition of transported sand and distributary flow on the bay side.
37

Subunits OWfan, OWch, and OWsw

A typical overwash system has a seaward splay of shallow tributary channels crossing the berm 

and back-beach flat that converge into a single channel cutting across the center of the island 

(Fig. 23). If there is sufficient sustained water flow during the overwash event, the system 

spreads out in a distributary fan on the back-barrier flat or produces a small delta prograding into 

the lagoon itself. The deepest incision occurs at the channel throat where overwash flow 

converges, and this deepened trough is commonly preserved as a pond after the storm event. In 

the following months or years, aeolian transport of the exposed sand of the overwash sheet often 

fills or smoothes over the shallower, upstream channels of the tributary catchment area. Existing 

overwash channel systems are likely to be reoccupied during subsequent storms until the 

seaward catchment area is mostly filled with sand. Consequently, many overwash fans show 

several depositional lobes from successive storm events.

Figure 24. An overwash fan on northern Assateague Island, near kilometer 9, shown with and without 

interpretation of map subunits.
38

Overwash fans that were produced relatively recently, within the past 100 years or so, often are 

discernible on the island surface as discrete geomorphic features (Fig. 24). With time and 

subsequent storms, the sand deposited in older overwash fans is reworked and smoothed out by 

both wind and flowing water. In most areas, the body of the barrier island is actually a platform 

sloping gently toward the bay created by the coalescing of many overwash fans or a broader 

overwash sheet.

Overwash channels (OWch) and swales (OWsw) are both low areas that were created by 

overwash flow across the island surface during storms. The distinction made in assigning map 

subunits to these features is that OWch implies a fairly discrete, continuous channel that cuts 

across a significant width of the island, often crossing several map units, whereas a swale is 

usually an isolated low area (Fig. 25). Although the OWch features are relatively narrow, in 

some cases less than 10 m wide and a few meters deep, they are conduits for subsequent 

overwash flow and allow transport of seawater farther across the island than sheet flow across 

the OWint subunit. Additionally, the overwash channels connect in the subsurface to relatively 

coarser, more permeable sediments that permit more rapid flow of the saline or brackish ground 

water. As described above, the overwash swales commonly receive and allow ponding of 

overwashing seawater that then infiltrates the surficial aquifer by focused recharge.

Figure 25. Overwash channels and swales on northern Assateague Island, near kilometer 5, shown with 

and without interpretation of map subunits.
39

Unit DN and DNmm

In delineating the OWint subunit, individual dunes were not distinguished, largely because they 

cannot be clearly identified in the color IR image, although they do appear in the lidar DEM. The 

high, continuous man-made dune that fronts the Maryland State Park and the camping area of the 

National Seashore is delineated, and classified as DNmm (dune, man-made), because it so 

profoundly affects the hydrology of the island immediately behind it by restricting overwash and 

subsequent infiltration of seawater in the geomorphic setting that otherwise would be classified 

as OWint. 

Tidal Marsh

Subunit TM1

Much has been published about the tidal marsh setting, structure, and processes because of the 

importance of this environment to the barrier island and coastal bays ecosystems. For this map, 

subunit TM1 is essentially equivalent to the low marsh dominated by Spartina alterniflora.

Geomorphically, the low marsh develops on the distal, bayward margins of the overwash 

platform where its elevation drops into the intertidal zone (Fig. 26). Mean high water generally 

defines the upper elevation limit of the low marsh and the transition into the high marsh (Table 

9).

Alternative settings for the TM1 subunit include the extensive marsh islands that develop on the 

flood-tidal deltas of now-closed tidal inlets. An exceptional example of this setting is the 

Middlemoor Island complex on the bay side of the historical Green Run Inlet near the MarylandVirginia state line (Figs. 2c and 16). TM1 marshes also may occupy the low areas between the 

island core and older, antecedent sections of the island on the bay side, which can be seen in 

Figures 10, 17, and 21 from the areas around Little Level and Pine Tree Island.

Because of the near-daily flooding of TM1 with saltwater, the shallow ground water beneath the 

marsh is predominantly brackish to saline. Locally in the marsh there may be discharge of 

reduced-salinity ground water flowing from the island interior and controlled by breaches in the 

low-permeability mud and peat of the marsh. Hypersaline brines may be produced in isolated 

basins within the low marsh or near the landward edge of the marsh. During the summer, in both 

of these settings, seawater that floods these basins during spring high tides or high-water events 

is evaporated by intense solar radiation, leaving behind and concentrating the salt. Open, 

unvegetated areas in the grassy plain of the marsh may indicate production of brine that killed 

the Spartina; these are classified as salt pans, subunit TMsp. Salt pans and saltwater ponds in the 

tidal marsh (subunit PD1) almost certainly intergrade, but ponds tend to be deeper and may be 

connected to the bay by a tidal creek or man-made ditch. Salt pans also form on the broad 

intertidal flats in low-lying areas of the island such as Fox Hills Level and Little Level.
40

Figure 26. (A) Subunits of the Tidal Marsh. Part of the scene shown in Figure 15, near kilometer 11. Lidar 

DEM with 10-cm color contours.

Subunit TM2

Elevation increases of even 10 cm above the low-marsh surface allow colonization by the salttolerant plant community. This may result in a high-marsh grassland dominated by Spartina 

patens with stands of Distichlis and Juncus, or in scrubland populated by Baccharis and Iva. In 

the subsurface, this transition also may coincide with the shallow availability of relatively lowersalinity and higher-oxygen ground water, which has been shown to be a control on the 

distribution of Spartina patens. Geomorphically, TM2 is most often either the low-slope distal 

part of the back-barrier flat, produced by overwash processes, or low sand ridges deposited by 

flow through former inlets or areas of extensive storm overwash. Subunit TM2 in various 

geomorphic settings is equivalent by vegetation and hydrology to subunits IC1, WA1, or IN2 

(Table 3). Ponds within subunit TM2 are most commonly classified as PD2, filled with brackish 

water most of the year but occasionally flushed with reduced-salinity ground water recharged 

from the island core or a large washaround. Alternatively, some of the deeper ponds in subunit 

TM2, or those closer to the island core, will tap a flow of ground water that is more consistently 

fresh; these are classified as PD3.

Subunits TMsr and TMfl

The edge of the tidal marsh that is exposed to the open fetch of the coastal bay commonly has a 

rim of sand reworked by waves and built upon the marsh surface. These tidal marsh sand ridges 

(subunit TMsr) are smaller and lower than the typical ridges classified as TM2. Even so, the 

slight increase of elevation above the low-marsh surface is sufficient to allow colonization by 

Baccharis and Iva. Another subenvironment of the tidal marsh, the subtidal flat (subunit TMfl), 

lies slightly below mean low water, but above low low water. These flats do not have emergent 

vegetation, but depending upon the intensity of tidal currents and wave action, may have 

submerged aquatic grasses or macroalgae.
41

Figure 26. (B) Color infrared aerial photograph without polygons or identifiers of map subunits; letter 

labels are used in the text for explanation of features.

Figure 26. (C) Map subunits for Tidal Marsh and associated features.
42

Distinguishing Tidal Marsh subunits

The subunits of a typical tidal marsh setting are shown in Figure 26, which is zoomed in from the 

scene near kilometer 11 presented in Figure 15. The extensive low marsh in this back-barrier 

area, indicated by label (a) in Figure 26b and map subunit TM1 in Figure 26c, is cross cut by a 

pattern of mosquito ditches dug in the 1930s. This marsh is nearly a monoculture of salt marsh 

cordgrass, Spartina alterniflora. In the color IR images, the low marsh typically appears as a 

deep red, sometimes with lighter mottling that indicates areas with sand exposed at the surface.

Shallow ponds and salt pans appear in the low marsh as darker areas, possibly with a light purple 

tint that may indicate bacterial or cyanobacterial mats on the sediment surface.

In this view, the edge of the high marsh, label (b) and subunit TM2, is fairly distinct although in 

many places it is more gradational. Here, the lighter tone of the combined vegetation and land 

surface in part indicates a sandy marsh soil with minimal accumulation of mud or mud and peat, 

which implies an elevation slightly above mean high water. The textural difference between the 

low marsh and high marsh surfaces is likely to be the addition of other salt-tolerant grasses such 

as Spartina patens and Juncus, and pioneering salt-tolerant shrubs. Geomorphically, this is the 

bayward extension of the consolidated overwash sheet from the island core, and this subunit is 

given the compound identifier TM2/IC1. In this area, because of the very low slope from the 

island core, the upland boundary of the high marsh is gradational and not clearly defined (label 

[c]). The scrub vegetation gets denser, some trees appear, although they do not obtain full size, 

and the plant assemblage moves toward salt-resistant and freshwater-obligate species.

Older washarounds (subunits WA2 and WA1) appear in the high marsh as isolated islands of 

slightly higher ground (label [d]) occupied by salt-resistant scrub and small trees. Other ridges 

that project above the marsh surface may have been reworked from earlier sections of the island 

or from earlier flood-tidal delta deposits also may be designated WA2 or WA1 (label [e]). These 

sand ridges typically are occupied by the same salt-resistant scrub assemblage. Note that with the 

exception of the larger washarounds, few of the subunits of the Tidal Marsh map unit can be 

discerned from the lidar DEM although they are reasonably distinct on the color IR image.

Label (g) marks Hall’s (2005) pond KM11, which is a moderately deep overwash channel cut 

into the back-barrier overwash sheet. This pond sits near the transition from the uppermost part 

of the high marsh onto the island core. From Hall’s observations over a year, this pond was 

consistently filled with fresh water except for an influx of saltwater during one high-water event 

caused by wind set-up in Chincoteague Bay. During this event, the salinity in the pond spiked 

rapidly, but then was flushed by fresh ground water from the island core over the following 

week.
43

Table 9. Subunits and characteristics of the Tidal Marsh map unit.

Subunit Elevation

Relative 

frequency of 

saltwater input

Thickness of fresh 

ground-water lens Vegetation Geomorphic setting

TM2 mean high 

water to 

 0.2-0.3 m

during spring 

high tides and 

most storms

generally thin <1 m, 

but may be thicker 

with GW flow from 

higher ground

salt-resistant 

grasses and 

shrubs

upland edge of 

marsh; overwash and 

washaround flats

TMsr mean high 

water to 

 0.2-0.3 m

during spring 

high tides and 

most storms

brackish ground 

water

salt-tolerant 

grasses and 

shrubs

wave-reworked sand 

rims around marsh, 

low inlet ridges

TM1 intertidal, 

generally 

MLW to 

MHW

daily inundation brackish ground 

water

dominantly 

Spartina 

alterniflora

intertidal part of 

back-barrier flat and 

islands

TMfl shallow 

subtidal, -

0.2 to 

-0.3 m to 

MLW

daily inundation brackish ground 

water

no emergent 

macrophytes, 

may have SAV 

or macroalgae

subtidal part of backbarrier flat

Table 10. Subunits and characteristics of the Inlet map unit.

Subunit Elevation Relative frequency of 

saltwater input

Thickness 

of fresh 

groundwater lens

Vegetation Geomorphic 

setting

IN3 0.5-1 m, 

ridges higher

during moderate and major 

storms

2-3 m scrubland with 

salt-resistant 

plants

spit platform or 

marginal terrace 

to inlet channel

IN2 mean high 

water to 0.2-

0.3 m

during spring high tides and 

most storms

brackish 

ground 

water

salt-tolerant 

grasses and 

shrubs

marginal terrace 

to inlet channel 

& low inlet 

ridges

IN1 intertidal, to 

MHW

daily inundation brackish 

ground 

water

dominantly 

Spartina 

alterniflora

edges of inlet 

channel

INsw varies, but 

0.5 m lower 

than nearby 

surface

susceptible to direct 

overwash flow and downgradient brackish GW flow

alternates 

between 

brackish 

and brief 

periods of 

fresh

salt-resistant 

grasses and 

shrubs, 

adapted to 

hydric 

conditions

low areas 

between ridges 

of former inlet 

mouth
44

[Former] Inlet

Subunits IN1, IN2, and IN3

Locations of former inlets on Assateague Island are most commonly marked by a rounded, 

funnel-shaped low area on the seaward side of the island and a single deep channel or set of 

channels that extend from the center of the island into the back-barrier lagoon (Figs. 9, 11, and 

17). The seaward low area is the former mouth of the inlet that filled with sand by longshore 

transport as the inlet closed. While these former inlets were open and active, the flow of the daily 

tides and storm surges through the inlet created streamlined longitudinal sand bars within the 

channel and sand ridges on the channel margins. At points where the tidal channel bifurcated or 

discharged into a broader, unrestricted area of the lagoon, subtidal to intertidal bars or flood-tidal 

ramps of sand were deposited. Even while the inlet was active, some of these features became 

emergent as sand was transported and deposited above mean high water by either waves or wind.

The map subunits IN1, IN2, and IN3 delineate the progressively higher-elevation remnant 

geomorphic features left after the inlet closed completely (Fig. 27c, Table 10). Tidal marshes 

generally occupy the intertidal margins of the former inlet classified as IN1 or IN1/TM1. By 

vegetation and hydrology, subunit IN1 is functionally equivalent to TM1. 

Figure 27. Subunits of the Inlet map unit. Part of the scene shown in Figure 9, near kilometer 17. (A)

Lidar DEM with 10-cm color contours. 

The slightly higher-elevation sand ridges of subunit IN2 are most commonly covered with saltresistant shrubs and grasses, and are equivalent to TM2. However, in most cases IN2 ridges are 

roughly perpendicular to the overall orientation of the barrier island, whereas TM2 marshes most 

often are roughly parallel. Subunit IN2 is also used to indicate areas of the closed inlet mouth 

that are above mean high water but are still susceptible to overwash from strong minor to 

moderate storms. These areas typically have brackish ground water and are grasslands or 

scrublands of salt-resistant plants, such as at Little Level (Fig. 17) and the former Sinepuxent 

Inlet near kilometer 17 below South Beach (Fig. 27).
45

Higher and/or more protected areas of the margins or mouth of the former inlet are classified as 

IN3. For most of the year these areas have fresh ground water, typically flowing from the 

adjacent island core, but receive input of either overwash or brackish ground water frequently 

enough to limit growth of the maritime forest community.

Figure 27. (B) Color infrared aerial photograph without polygons or identifiers of map subunits; letter 

labels are used in the text for explanation of features. 

Figure 27. (C) Map subunits for Inlet and associated features.
46

Subunits INch and INsw

Where the geomorphic features are clearly expressed, inlet channels and swales are classified as 

INch and INsw, respectively. These designations may be accompanied by a numerical suffix that 

indicates elevation and relative protection from overwash.

Distinguishing Inlet subunits

Some of the geomorphic setting of a former inlet was described for Figure 21 and the Island 

Core subunits. The scene presented in Figure 27 is similar, but with multiple inlet channels (four 

[a] labels) and a more complex set of ridges. This former inlet below South Beach, near 

kilometer 17, is likely to be the historical Sinepuxent Inlet that was open from the 1600s into the 

early 1800s. Shallow remnants of inlet-mouth channels on the ocean side (two [b] labels) are 

interspersed between the spit ridges that finally closed off the inlet (two [c] labels). The low 

areas of the former inlet mouth are generally designated subunits IN2 or IN3 depending upon 

relative elevation (Fig. 27c), or INch if clearly a channelized feature. Larger, higher, and sparsely 

vegetated spit ridges are classified as the IC5sr subunit of the Island Core, whereas lower sand 

ridges within the former inlet mouth may be classified as INsr.

Sand that entered the open, active inlet from the ocean was reworked into longitudinal bars and 

ridges within and between the tidal channels (label [d]); these are designated IN2 or IN3, again 

depending upon relative elevation. The former inlet is flanked by recurved spit complexes, as 

shown in Figure 11, which are a major component of the Island Core map unit. Typically the 

updrift (northern) side of the inlet has the more extensive spit complex. Individual spit ridges 

(label [e]) may be significantly higher than others in the set, and are categorized as IC5sr.

Intertidal spit platforms and supratidal terraces along the margins of the inlet channels were 

subsequently occupied by low-marsh grassland or high-marsh mixed grassland and scrubland, 

designated IN1/TM1 and IN2/TM2, respectively. Larger reworked sand bodies within the former 

inlet are generally classified as washarounds (map unit WA), as marked by label (g). Lowerelevation features (label [h]) with a hydrodynamic shape similar to a washaround are most 

commonly reworked from the sand deposits of the flood-tidal delta (refer also to the Middlemoor 

Islands shown in Figures 2c and 16).

Inlet-closure ridges

At a few locations on the island, inlet-closure ridges – a set of low, arcuate, nearly parallel ridges 

– create a ridge and swale barricade across the former inlet mouth (Fig. 28). Inlet-closure ridges 

are best expressed at kilometer 33, the site of the historical Green Run Inlet (Fig. 28b) that 

shallowed and closed between 1880 and 1900.

The ridges and swales preserved as the island surface at the KM33 site (Fig. 28a) show the 

sequence of closure for Green Run Inlet. The spit marking the southern flank of the inlet appears 

as a sparsely vegetated sand ridge (label [a]) that hooks around to the northwest then west. Hall’s 

(2005) pond site KM33 is in the wooded swale south and east of label (a). The northern margin 

of the inlet is the fishhook-shaped higher ground identified by label (b) and shown on the 

historical chart (Fig. 28b).
47

The first stage of inlet closure at this site proceeded with sets of spits prograding from the north, 

indicated by labels (c) and (d). As the mouth of the inlet shallowed dramatically, lower, more 

continuous ridges (e), possibly produced during storm high-water events, spanned the entire 

width of the former inlet. The low areas between these inlet-closure ridges are now swales with 

perennial wetlands occupied mostly by grasses with a few shrubs. The outermost (eastern) swale 

receives sufficient seawater, primarily from brackish ground-water flow from the up-gradient 

overwash zone, to have a predominantly salt-tolerant plant community, similar to the swales 

shown in Figure 22.

Figure 28. (A) Inlet-closure ridges and 

intervening swales at the former Green 

Run Inlet. Modern setting, near 

kilometer 33.

Figure 28. (B) Navigation chart from 1880 showing Green 

Run Inlet. (NOAA archives.)
48

Washaround

Subunits WA3 and WA4

Because of their process of formation by reworking of sand during successive major storms, the 

larger washarounds (wider than about 75 m) have a series of arcuate ridges on the seaward side 

(Figs. 18 and 29). The tallest of the ridge sets are commonly well above sea level, 1.5 m to as

much as 3.5 m (Table 11), and protect the interior of the washaround as an island usually 

surrounded by brackish environments. As a result, and very similar to the island core setting, the 

fresh ground-water lens is deepest and most stable in the center of the washaround, with an 

abrupt shallowing immediately outside of the outer ridge. This effect is expressed as a coincident 

sharp transition, often within 5-10 m, from freshwater-obligate maritime forest to salt-tolerant 

scrub to high marsh. Map subunits WA3 and WA4 constitute the protected interior of the 

washaround.

Most of the ponds associated with washarounds lie in the swales between storm ridges on the 

ocean side of the feature (Fig. 29). However, in a few cases, swales in the interior of the larger 

washarounds are deep enough to be ponds, at least seasonally. More commonly, the swales are 

seasonal freshwater wetlands with a distinct plant community that may include Osmunda ferns.

Some of these swales that become seasonally dry support trees and shrubs, whereas others that 

are more persistently wet are covered predominantly by grasses.

Figure 29. An older large washaround sitting behind the present island core. Near kilometer 19.
49

Subunits WA1 and WA2

Subunits WA1 and WA2 occur as low-lying ridges around the perimeter of the larger 

washarounds (the main feature in Fig. 30), and as the body of smaller washarounds (eastern side 

of Fig. 30). Many of the washarounds wider than about 100 m have a fringe of low ridges that 

grade down in elevation to the high marsh (Fig. 29). These ridge sets are categorized as WA1 

and WA2, and sometimes are given a compound designation such as WA1/TM2. In these 

settings, the vegetation makes the transition from the salt-resistant scrub, generally on ridges of 

WA2, through the salt-tolerant vegetation typical of the high marsh occupying WA1, into the 

salt-adapted grasses of the tidal marsh itself. As with the lower-elevation subunits of the island 

core, the designation of washaround subunits WA1 and WA2 in part depends on the width of the 

feature, which affects the likelihood of complete flooding during storms.

The smaller washarounds, from less than 100 m to about 10 m wide, appear as isolated tufts or 

hummocks in low-lying sections of the island that are frequently overwashed. Other studies, such 

as those on the Virginia barrier islands, have called these features “marsh pimples.”

Classification of these smaller washarounds depends largely on the width of the feature and the 

character of the surrounding island surface. For example, washarounds of 10-20 m width in the 

frequently washed over sections of Fox Hills Level and Little Level are classified as WA1, 

whereas slightly larger washarounds of 20-40 m may have a core of WA2 with a rim of WA1.

Hydrologically, these smaller washarounds have a localized shallow lens of fresh ground water 

that may be only about 1 m deeper than the surrounding land surface and dissipates close to the 

edge of the feature. However, this isolated pocket of fresh ground water may be sufficient to 

allow growth of salt-tolerant shrubs.

Distinguishing Washaround subunits

As mentioned previously, the larger washarounds of Assateague Island are effectively islands 

themselves, with many of the characteristics of the island core, just scaled down. The single large 

washaround presented as an example in Figure 30 lies immediately north of similar washarounds 

on the bay side of Fox Hills Level shown in Figure 18. All of the larger washarounds are 

composed of a series of sand ridges molded by high-velocity flow across the island during storm 

surges and overwash events. The main body of the washaround, label (a) in Figure 30b and 

subunit WA3 in Figure 30c, is covered by maritime forest. Individual higher ridges, three (b) 

labels, are sparsely vegetated because of a relatively thick unsaturated soil zone. Lower-lying 

ridges on the ocean side of the washaround, label (c), were created by major storms in 1952 and 

1962, based on aerial photographs taken soon after these events.

Because this washaround is on the bay side of Fox Hills Level, the inlet salt pan, label (d) and 

subunit INsp, abuts the ocean side of the washaround. Commonly on the bay side of 

washarounds, the wrapping storm ridges create a sheltered low area (label [e]) that may be a 

shallow cove, or occupied by tidal marsh with brackish ponds. Smaller sand bodies within or 

along the flanks of the tidal channels may have substantial sections above sea level, such as 

marked by label (f), that are classified as small washarounds. Similarly, flow-sculpted, round or 

ovate hummocks within the broad flat of Fox Hills Level are also small washarounds (two [g] 

labels, but many more shown). Many of these smaller features have a central core that may be ~1 

m above the surrounding island surface, and may be partly covered by salt-tolerant grasses and 

scrub; these are classified as WA2 with a lower fringe of WA1.
50

Figure 30. Subunits of the Washaround map unit. This area near kilometer 26 is immediately north of the 

scene shown in Figure 18. (A) Lidar DEM with 10-cm color contours.
51

Figure 30. (B) Color infrared aerial photograph without polygons or identifiers of map subunits; letter 

labels are used in the text for explanation of features.

Figure 30. (C) Map subunits for Washaround and associated features.
52

Table 11. Subunits and characteristics of the Washaround map unit.

Subunit Elevation

Relative frequency 

of 

saltwater input

Thickness of 

fresh groundwater lensVegetation Geomorphic setting

WA4 1.5-2 m, 

ridges to 3 m

only during major 

storms

3-5 m maritime forest, 

freshwater 

obligate

center of washaround 

protected by storm 

ridges

WA3 1-1.5 m, 

ridges to 2 m

during strong 

moderate and major 

storms

2-3 m maritime forest & 

scrubland, 

freshwater 

obligate

near center of 

washaround, lower 

elevation than WA3, 

closer to marsh

WA2 0.5-1 m during moderate 

and major storms

1-2 m scrubland with 

salt-resistant 

plants

generally the margin of 

the washaround

WA1 0.25-0.5 m during strong minor 

and moderate 

storms

1 m or less mixed scrubland 

& grassland, salt 

tolerant

low margin of 

washaround, 

slightly above MHW 
53

Ponds

Ponds on Assateague Island occur where an erosional process has cut down through the surface 

sediment to a base below the water table. In practically all cases, this formation process was 

associated with channelized flow of overwash during a storm. In a variation on this theme, 

existing ridges or dunes that dam the overwashing seawater during the storm may be breached at 

a low point. The channelized flow, with considerable velocity through the constriction, will scour 

a substantially deep trough; this is the formation process inferred for the ponds at Hall’s (2005) 

site KM33 (location shown on Fig. 2c).

In other settings where the high front of the island core stands as a barrier to overwash flow 

across the island to the lagoon, flow is diverted laterally, usually toward an adjacent low-lying 

area such as a former inlet. The diverted flow follows the low swales between previous storm 

ridges. The geomorphic result of this process can be seen clearly at the northern edge of Fox 

Hills Level near kilometer 24 (Fig. 31). Similar to the effect of flow through an overwash 

channel, high-velocity flow along a swale between bounding ridges also creates localized deep 

scour troughs that become ponds. Many of the ponds associated with the larger washarounds are 

similarly situated in swales between ridges (Fig. 29).

Ponds that lie in line within either an overwash channel system or a ridge and swale system are 

linked by subsurface flow of ground water. The up-gradient, seaward end of either system is 

likely to continue to receive overwash. During moderate storms that produce overwash, surface 

flow of seawater may not continue through all of the aligned ponds in a swale, but may lose 

momentum and pond in the seaward end of the swale. Here, the saltwater infiltrates into the 

surficial aquifer, mixes with the existing ground water, which may be relatively fresh, then flows 

down gradient in the subsurface. The down-gradient ponds, even though they did not receive 

direct overwash, will become brackish as this slug of saline ground water moves through the 

system. Eventually, freshwater from precipitation fills the ponds and pushes the brackish ground 

water toward the bay.

From the pond study, as summarized by Sagit Hall in her 2005 thesis, ponds on Assateague 

Island receive salt by several processes. The one most often cited in other publications, input 

from salt spray from the ocean, is actually a relatively minor, although real, effect. Salt from salt 

spray is most important during periods of drought when evaporation removes a large volume of 

the pond water and concentrates the salt. As described above, the dominant input of saltwater 

into a subset of ponds on the ocean side of the island is from direct overwash flow. The indirect 

process of subsurface flow of brackish ground water is important toward the landward edge of 

the Overwash unit and, especially, for ponds in the Inlet unit and seaward side of the Island Core. 

Something of the converse process occurs with ponds on the bay side of the island. Ponds may 

be flooded by high water in the coastal bay, which is most commonly caused by wind set-up 

within the bay. Then, instead of receiving brackish ground water from up gradient, these flooded 

ponds may be flushed by fresh, or at least reduced-salinity, ground water flowing down gradient 

from recharge areas on the island core.
54

Figure 31. Ridge and swale sets created by storm overwash flow laterally around the higher section of 

the island core. Northern margin of Fox Hills Level, near kilometer 24.

Subunit PD4

Ponds within the higher, protected center of the island core and the larger washarounds are the 

most persistently fresh and are classified as PD4. These fresh ponds are heavily utilized by the 

larger vertebrates of the island and commonly have paths leading to them that were created by 

horses and deer. The surrounding high ground not only protects the pond from direct surface 

overwash of saltwater, but maintains an elevated water table and thick freshwater lens with 

sufficient hydraulic head to protect the pond from incursion of brackish ground water.
55

Subunits PD3 and PD2

Ponds classified as PD3 and PD2 are similar in that both are brackish during part of the year and 

are flushed by either fresh or reduced-salinity ground water from up gradient, which is usually 

the central part of the island core or a large washaround. The distinction made between these two 

categories is whether they are predominantly fresh (PD3) or brackish (PD2) for most of the year.

Realistically, because of the temporal dynamics, these partly fresh / partly brackish ponds 

represent a continuum rather than discrete categories. Further, any individual pond will become 

fresher or saltier through periods of above average rainfall or drought, respectively.

In geomorphic setting, PD3 ponds are most commonly situated within the mid-elevation sections 

of the island core, subunits IC3 and IC2, whereas PD2 ponds generally lie within the lowelevation fringe of the island core, subunit IC1, or occasionally at the edge of the high marsh 

(TM2). The PD2 designation is also applied to ponds on the ocean side of the island core that 

frequently receive overwash or brackish ground-water flow.

Subunit PD1

As mentioned in the discussion of the tidal marsh, practically all ponds in the low marsh (TM1), 

and most in the high marsh (TM2), are either saline or brackish and are classified as PD1. At a 

few sites, adjacent ponds near the boundary of the high-marsh and upland may have different 

designations based on connection to bay water by a natural tidal creek or man-made ditch (Fig. 

19a). Isolated ponds near the upland edge of TM2 are likely to receive reduced-salinity groundwater flow from up gradient.

Pond Metrics

Most of the ponds on Assateague Island that are not in the tidal marsh should be identified on the 

hydrogeomorphic map. However, it was not always possible to distinguish between ponds and 

shadows of large trees, especially on images collected in late afternoon, and some smaller ponds 

may have been missed. Further, the polygons on the map are coarsely drawn relatively to the size 

of the actual ponds, and should not be used to calculate pond area. Instead, if more detailed 

information about the ponds is valuable to the NPS, additional GIS work with the more recent, 

higher resolution natural color aerial images would be appropriate. By zooming in, pond margins 

could be more accurately delineated, which should yield better metrics such as the distribution of 

pond size and the density of ponds on different sections of the island or within map units
.
57

Summary

The basic principle underlying this hydrogeomorphic map of Assateague Island National 

Seashore is that the geomorphology of the island in conjunction with the active processes 

associated with precipitation, evapotranspiration, and inundation with seawater by storm 

overwash from the ocean side or high-water flooding from the bay side largely control the 

ground-water hydrology, which has a primary influence on the distribution of the plant 

communities. The vegetation responds to the frequency and magnitude of saltwater input by 

overwash or flooding, and the duration of inundation or brackish ground-water conditions, which 

are controlled by geomorphology and position on the island.

The large-scale features of the island surface have been created by several major storms over the 

past few centuries, and have been modified by more-frequent but lower-intensity moderate 

storms. Storms have breached the island at many places to create short-lived tidal inlets, the 

remnants of which persist as low-lying island sections that remain susceptible to overwashing 

seawater. Storms also created the higher-elevation sections of the island core by reworking sand 

into protective ridges. Storm processes that operated for only a few days have produced 

landforms on the island that persist for decades to centuries, and strongly affect the function of 

the ecosystem.

Actual observations of the structure of the surficial aquifer and fresh ground-water lens under the 

island deviate substantially from conventional conceptual models. Rather than having a single, 

deep, connected fresh ground-water lens that stretches the length of the island, the fresh lens 

occurs in segments and is much shallower than predicted by theoretical considerations. Higher, 

protected sections of the island core produce and sustain a moderately deep fresh ground-water 

lens, observed to be as thick as 7-8 m. Because Assateague is a transgressive barrier island, a 

low-permeability layer of lagoonal silt underlies most of the island, which physically limits the 

depth of the fresh lens. At the edges of the island core, the fresh lens shallows in response to the 

attenuation of hydraulic head and infiltration of saltwater into the aquifer from ocean-side 

overwash and bay-side flooding. The lateral extent of the fresh lens along the length of the island 

is similarly restricted by the low-lying areas of former inlets where the ground water is 

predominantly brackish. The ocean-side section of the island comprising the beach, berm, and 

primary and secondary dune fields has an extremely dynamic ground-water system with 

alternating inputs of freshwater from precipitation and saltwater from overwash, and preferential 

subsurface flow of brackish ground water following storm-created channels and swales.

59

Literature Cited

Hall, S. Z. 2005. Hydrodynamics of freshwater ponds on a siliciclastic barrier island, Assateague 

Island National Seashore, Maryland. Thesis. University of Maryland, College Park, USA.

Harris, M. S., J. C. Brock, M. Duffy, and A. Nayegandhi. 2003. Lidar application for vital signs 

monitoring at Assateague Island National Seashore. Inaugural National Conference on 

Coastal and Estuarine Habitat Restoration, Baltimore, Maryland, USA.

Morton, R. A., J. E. Bracone, and B. Cooke. 2007. The geomorphology and depositional subenvironments of Assateague Island, Maryland and Virginia. U.S. Geological Survey, Denver, 

USA. Open File Report 2007-1388.

National Oceanic and Atmospheric Association (NOAA). 1880. Navigation Chart P1554: From 

Isle of Wight to Green Run Inlet, Maryland. U.S. Department of Commerce, NOAA. Office 

of Coast Survey, Historical Map and Chart Project, Washington, D.C.

(http://historicalcharts.noaa.gov) Accessed June 2009.

61

Appendix A: Locations of Scenes Shown in Report Figures

Appendix A1. Northern section of Assateague Island National Seashore.
62

Appendix A2. Central section of Assateague Island National Seashore.
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Appendix A3. Southern section of Assateague Island National Seashore.

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and 

other information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated 

Island Communities.

NPS 622/104170, June 2010
National Park Service

U.S. Department of the Interior

Natural Resource Program Center

1201 Oakridge Drive, Suite 150

Fort Collins, CO 80525

www.nature.nps.gov

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