CAPE LOOKOUT Barrier Island Ecology of Cape Lookout National Seashore and Vicinity, North Carolina NPS Scientific Monograph No. 9

INLET DYNAMICS

Inlets, permanent or temporary, are an integral part of the barrier-island environment. Permanent inlets are usually opposite the mouths of major rivers and let river water into the sea. Temporary inlets shift position frequently, depending on storms and sand movement. Fisher (1962) described the historical patterns of both kinds of inlets on the Outer Banks and showed that nearly all the islands have been broken by inlets, even though only a few existed at any one time; 14 have been present at various times between Ocracoke Inlet and Barden's Inlet. Temporary inlets form when a storm first drives high water across an island. As the storm passes and the wind blows from the opposite direction, water in the sound is forced back over the island at low places, and may divide the beach on the way. Water can then flow back and forth until sand carried by the littoral drift eventually plugs the inlet. While the inlet is open, exchange between the sound and the sea benefits both ecosystems. The life of the new inlet depends on tidal flow, the depth of the sea and sound waters, the frequency of storms, and the amount of littoral transport. Broad shoals build up in the sound behind the inlet until they impede tidal scouring. The updrift side of the inlet usually migrates in the direction of the littoral flow while the downdrift side erodes. If migration of one side is faster than erosion of the other, the inlet eventually closes. The shoals behind are then invaded by salt marsh grass, and new marshes appear over a wide area that was once open water. Overwash and dune growth build up the new land and soon all superficial traces of an inlet disappear.

Such a pattern may be seen on Core Banks where Drum Inlet used to be. This inlet, opposite Atlantic, N.C., has had the usual history of opening and closing. It was open in the early 1800s and closed in the mid-1800s, only to open again in 1933 during the great hurricane of that year. It remained open until 1971, migrating southward nearly a mile, and building broad shoals in the sound (Fig. 38). The tides no longer regularly flooded these shoals, and small dunes started in the middle of the shoals where sand blown off the bare flats accumulated around sprigs of Spartina patens (Figs. 39, 40). The shoals are now being colonized rapidly by Spartina alterniflora (Fig. 41). Figure 44 is a map of the areas being colonized by plants and the locations of cores which demonstrate that the shoal material was definitely marine. In December 1971, the U.S. Army Corps of Engineers opened a new inlet about 4.5 km south of the old inlet and reestablished a regular tidal cycle in Core Sound.

Fig. 38. Drum Inlet closing. (A) 28 October 1958, Drum Inlet was opened by the great 1933 hurricane and was then dredged from time to time. When this photograph was taken, several severe storms created numerous inlets which soon closed. Littoral drift is from the right to the left. A spit can be seen forming on the updrift side of the inlet; large deltaic shoals have appeared. Southwest is to the left in this photograph. (B) 24 August 1963. The updrift spit has migrated nearly all the way across the inlet. Tidal channels have become sinuous and shoals have continued to build. The downdrift side has been eroded. Buried layers of marsh peat were exposed where the channel cut through the berm, as well as on the beach; this is further evidence of general barrier island retreat. (C) 11 February 1971. The updrift spit migrated faster than the downdrift side eroded, and finally sealed off the inlet in January 1971. Overwash began filling in the low areas behind the former channel. With the growth of the updrift spit, the inlet channel migrated 2 km southwest since 1958, following the normal pattern of inlet migration toward the south or west depending on the orientation of the islands.

Fig. 39. View of Drum Inlet in June 1971, halfway down the spit which closed the inlet. Much of the berm is barren, although dunes are growing up along the backside. Dunes are well established where the spit is oldest. Beyond the old channel are broad shoals exposed at low tide. The towns of Atlantic and Cedar Island. North Carolina, are on the horizon.

Fig. 40. The older shoals associated with the Drum Inlet tidal delta are being invaded by Spartina alterniflora (in the foreground) and low dunes with Spartina patens, Fimbristylis spadicea, and Erigeron pusillus to the left. The broad, bare shoals provide sand for dune growth when they dry out at low tide.

Fig. 41. Large shoals behind the former inlet are being colonized by saltmarsh cordgrass, Spartina alterniflora. and blue-green algae, the dark patch in the foreground. In time, these shoals will become highly productive salt marshes. They are already heavily populated with fiddler crabs and are important feeding grounds for birds.

Maps of the early 1800s and local documents show the site of Cedar Inlet a few miles south of the Drum Inlet area. By 1850, this inlet was completely closed, although its location was marked on the map (Fig. 45). We were able to locate Cedar Inlet (Fig. 42) and make several cores through three of the marsh islands in Core Sound behind the old inlet. The layers of peat were relatively shallow; underneath we found surf shells, so these marshes had their beginnings on shoals that formed when sand and shells from the beach were carried into the inlet. From the direct evidence of Drum Inlet and Cedar Inlet, with the patterns of marsh islands behind the barriers and the deep channels between these islands that end abruptly behind the barrier (Fig. 43), it seems clear that various parts of Core Banks contained inlets at one time or another (Fig. 46), and that the closing of inlets rapidly widens low barrier islands by a factor of 2 or 3. Overwash then fills in the low places, connects marshes as the island retreats, and continues the building process. Dunes constantly form, are knocked down, and then reform. Thus inlets, overwash. and dune growth are processes by which the barriers are built, maintained, and migrate as the sea rises (Fig. 46A). Much of what we see today on the Outer Banks is of very recent origin and definitely marine, even though the islands may have been formed originally by some other means. A stump we found in place on the seaward side of Shackleford, visible only at high tide, turned out to be less than 200 years old by Carbon-14 dating (Sample W-2307, U.S. Geol. Survey) (Fig. 14).

Fig. 42. Site of Cedar Inlet, open from the 1700s to the early 1800s, The marshes and creeks follow a pattern which suggests that they developed on former tidal delta shoals. Cores from these marshes showed peat layers about 50 cm thick underlain by fine sand and shells. The shells included the same surf species as were found in the Drum Inlet shoals, which is direct evidence that the Cedar Inlet marshes developed on sand and shells that moved down the beach in the littoral drift and then into the inlet, much as at Drum Inlet. When the inlet closed, marsh growth proceeded unhindered by tidal surges. The dark areas are underwater beds of widgeon grass (Ruppia maritima) and eelgrass (Zostera marina).

Fig. 43. Guthries Hammock seems to be located at a former inlet, although there is not yet any direct evidence for this. The former channel is probably the creek at the upper left. A migrating spit sealed off the inlet on the right side and a continuous berm built up. The dark patches are stands of live oak (Quercus virginiana), holly (Ilex opaca), and other forest species growing on old dunes. It seems likely that these old dunes may have started on barren shoals much like the present sequence at old Drum Inlet. In time the dunes were colonized by the tree species, thus culminating the successionary sequence from tidal delta shoals, to grassy dunes and marsh, to forest and marsh.

Fig. 44. Map of Drum Inlet, summarizing changes shown by aerial photographs and ground surveys. Figures 40 and 41 were taken on the large shoal nearest the channel marked by the number "2." Sites 1 and 2 were locations of cores containing surf shells. Marshes are developing all around the edges of the large shoals and completely covering the smaller ones. Shoals covered by low tide are potential substrate for underwater vegetation, such as in the Cedar inlet area, and are being invaded by those species; a lush stand of eelgrass was found in the old channel near the letter "h." Inlets are thus a major means by which the barrier island system widens and large new salt marshes form.

Fig. 45. (A) The site of Cedar Inlet as shown in the 1888 issue of the U.S. Coast and Geodetic Survey chart. The former channel is well marked on this map. (B) The present site taken from a 1963 aerial photograph. What was once the channel has been filled by overwash deposits. Numbers on the marshes refer to locations of cores described in Fig. 42.

Fig. 46. Missing text. (click on image for an enlargement in a new window)

Fig. 46A. Generalized summary of harrier island dynamics and migration (vertical scale exaggerated). Stage 1 is a hypothetical barrier island which may have existed anywhere from present times to several thousand years ago, with a well developed dune line, or series of dunes, and a forest behind. In stage 2, the sea level has risen slightly and storms have knocked the dune barrier back into the woodlands. By stage 3, much of the barrier island has been overwashed and the dunes pushed back. The marsh has grown vertically and been somewhat eroded, and some former uplands are now salt marsh as a result of sea level rise. In stage 4, the barrier has retreated considerably from its original position. Dune and overwash sand has moved completely over the old forest, which is now exposed on the ocean side. Marshes near the island interior have been covered as well. Further retreat places sand completely over the original marsh surface and into the lagoon behind, where new marshes form. At stage 6 an inlet has opened and a typical tidal delta has appeared behind it. The temporary inlet has closed in stage 7, and the tidal delta now supports salt marsh and low dunes. Overwashes have tied the marsh islands to the main barrier and have filled in the old channels in stage 8. The salt marshes are now well developed on the old tidal delta, woods have grown upon the low dunes on these marsh islands, the salt marsh fringe behind the barrier is expanding, and on the barrier itself new dune lines and woodlands have formed where only a short time ago there was water. (click on image for an enlargement in a new window)