Carolina) fall into this second, partial- 

 ly mixed classification at least season- 

 ally. 



Most estuaries in the study area are 

 classified as vertically homogeneous 

 (Pritchard 1967, 1971; Schubel 1971), 

 where tidal mixing is the dominant physi- 

 cal process. These systems receive fresh 

 water primarily though local precipitation 

 via tidal creek drainage systems particu- 

 larly during spring floods. Sapelo Sound, 

 Georgia, and the lagoon-marsh complex ad- 

 jacent to North Inlet, South Carolina 

 (Finley 1975), are two examples of verti- 

 cally homogeneous systems. Lagoon-marsh 

 complexes in southern North Carolina are 

 not fed by major streams (Cleary et al. 

 1979); therefore, they can also be consid- 

 ered vertically homogeneous. 



In estuaries not directly influenced 

 by large riverine sources, estuarine cir- 

 culation patterns are largely determined 

 by tides, wind, and by the water storage 

 capacities of lagoon-marsh complexes 

 (Oertel 1975). The lagoon-marsh complexes 

 in Georgia, for example, are extensive and 

 average 6,5 to 7.5 km (4.0 to 4.6 mi) in 

 width. These areas store large volumes of 

 water during high tide, and during tidal 

 drainage they contribute significantly to 

 water circulation and nutrient exchange 

 within the estuarine ecosystem. These 

 large lagoon marshes generally occupy a 

 major portion of the watershed of the es- 

 tuarine basins, and therefore direct rain- 

 fall is the major source of freshwater to 

 these sytems (Tom Williams, Clemson Uni- 

 versity, Georgetown, South Carolina; pers. 

 comm. ) 



Estuarine Sedimentation 



The origin of sediments in estuaries 

 and the processes that affect their dis- 

 tribution and deposition have been the 

 subject of extensive research and scien- 

 tific debate for over 25 years (Guilcher 

 1967). Estuarine sedimentation patterns 

 are complex and influenced by tidal cycle, 

 wind direction and duration, waves, sea- 

 sonal riverine flooding, water storage 

 capacity of lagoon-marsh complexes, and 

 sediment availability. The biological 

 animal-sediment interactions (bioturba- 

 tion) and chemical factors are also impor- 

 tant (Howard 1975). These factors may vary 



continuously in space, time, and intensity 

 (Oertel 1974). 



The processes of sedimentation can 

 best be understood if the estuarine system 

 is divided into three parts, based on gen- 

 eralized physical and hydrographic charac- 

 teristics: (1) the lower sound and inlet 

 entrance; (2) the middle region of the es- 

 tuary, including the main rivers feeding 

 the sound; and (3) smaller tidal creeks 

 draining the marsh complex. Naturally oc- 

 curring oyster reefs can be found in each 

 of these main zones in the study area. The 

 three estuarine sedimentation zones are 

 illustrated in Figure 2. 



The area of the lower sound and inlet 

 entrance is influenced primarily by marine 

 processes. Wind-wave and tidal ly generated 

 currents exert the greatest influence in 

 the lower sound, creating a relatively 

 high energy sedimentary system. Where a 

 sufficient sediment supply is present, 

 this area is characterized by medium- to 

 coarse-grained and commonly cross-bedded 

 sands. Where the lower sound is less in- 

 fluenced by strong tidal currents, bottom 

 sediments consist of a mixed medium- to 

 fine-grained muddy sand. These sands be- 

 come progressively finer grained and in- 

 terbedded, or mixed with mud farther in- 

 land. This is particularly common in estu- 

 aries without fluvial sources of coarser- 

 grained sediment. Near the mouth of the 

 sound, influence of the adjacent shoreface 

 is indicated by the increasing grain size 

 and higher energy bedforms, sand ripples, 

 etc. (Mayou and Howard 1975). Sandflats 

 and mudflats frequently characterize the 

 intertidal margins of the lower sound. 



In estuarine systems characterized by 

 large riverine freshwater input, the ver- 

 tically stratified lower sections of the 

 estuaries become natural traps for fine- 

 grained sediment (Schubel 1971). Fine- 

 grained sediment transported in the upper 

 freshwater layer frequently will settle 

 into the lower saline layer and then be 

 carried back inland. Suspended sediment 

 may, therefore, be transported back and 

 forth many times within the lower section 

 of an estuary before it is finally depos- 

 ited (Postma 1967). 



The middle region of the estuarine 

 sedimentary environment includes the 



