Site Description

The Reserve encompasses 12,327 acres of tidal marshes and wetlands. North Inlet-Winyah Bay NERR features high quality, ocean-dominated waters and salt marshes in North Inlet, contrasting with the brackish waters and marshes of Winyah Bay. The bay's estuary is dominated by riverine discharges from a watershed impacted by agricultural, municipal and industrial development. Former rice fields and canals provide another system for study within the Reserve.

Watershed

The Winyah Bay watershed is approximately 18,000 square miles. Four major rivers drain into the system. More than 16,000 square miles of this drainage area is associated with the Pee Dee-Yadkin river system which originates in the Blue Ridge Mountains area of North Carolina. Water from this area flows across the Piedmont region of both North and South Carolina, and into Winyah Bay through the Pee Dee River. The Waccamaw River also receives water from the Pee Dee as the poorly defined, shallow, wide, swampy waterways merge. The Black and Sampit rivers drain much smaller watersheds.

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Tidal Range

The Reserve experiences a regular semi-diurnal tidal pattern. Mean tidal amplitude is on the order of 1.4 meters at the ocean end of Winyah Bay and North Inlet, and 1 meter at the Sampit River (1.6 m and 1.2, respectively on the spring tide.) Maximum amplitude near the ocean is about 2.2 meters. Due to its shallow character, North Inlet is thoroughly flushed by the tides, with about 50% of its water ebbing into the ocean twice per day.

In Winyah Bay, a salt wedge effect occurs as heavier salt water moves up-estuary along the bottom with the flooding tide, even though the overlying fresh water may be flowing toward the ocean. During periods of low freshwater inflow, flooding tides move salt water more then fifteen miles upstream of the Highway 17 bridges, but under average river flow, the penetration is usually within a mile of the bridges. Differences between surface and bottom salinities during these periods may be more than 20 ppt.

River Flow

Fresh water input into Winyah Bay estuary ranges from 2,000 to about one million cubic feet per second (cfs), and the mean runoff is approximately 15,000 cfs. Riverine influence is strong enough to limit ocean water penetration to the lower bay, especially during winter and spring.

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Soils

Most of the salt marsh soils in North Inlet and Winyah Bay are classified as "Bohicket silty clay loam". These poorly drained soils have a high sulfur content and are flooded twice daily by the tides. Former rice field marshes along Winyah Bay are edged with "Levy silty clay loam" soils. Marsh islands in Winyah Bay created by dredging activity include Bohicket silty clay loam soils and areas of "Udorthents, loamy" in places where fossilized shell material has been piled up.

Geology

North Inlet waters drain a very large marsh located between Debidue Island, North Island and the mainland. The mainland consists of Pleistocene Storm Beach Terrain with ridges oriented in a northeasterly-southwesterly direction. These ridges intersect the Atlantic ocean at the north end of Debidue Island. These superficial mainland features are underlain by a complex sequence of older coastal plain sediments, a sequence which is poorly understood in the immediate area at the present time.

Debidue and North Island represent part of the Holocene Barrier Beach System. This system has migrated southwest in recent times, with principal evidence here being the major spit along the northern entrance to Winyah Bay, and the smaller spit migration land forms along the northern border of North Inlet.

North Inlet drains numerous tidal creeks, and two of these extend back though the marsh to lie in close proximity to the Pleistocene mainland. The creeks are very shallow in depth, never exceeding thirty feet below sea level, commonly showing floors which are occupied by sand bars. The marsh areas are underlain by silts and clays which extend to an unknown depth below the surface. Relief is generally flat; the western third of the peninsula has the most relief with bluffs adjacent to Winyah Bay as high as fifteen meters. Geologically, Winyah Bay represents a drowned river basin and receives water from an extensive drainage basin. North Inlet is a bar-built/barrier beach estuary, while Winyah Bay is a salt wedge estuary.

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