N.H. Bishop, 1878, Voyage of the Paper Canoe

The Waccamaw drainage system is composed of many parts. All of these parts are intimately interrelated and operate in consort with each other like the parts of a living organism. Therefore, to understand and manage this drainage system we must first comprehend the complex plumbing system. The plumbing consists of numerous inputs and outputs that determine the ultimate composition and volume of water flowing through the piping system.

The WDS consists of three headwater streams: White Marsh Swamp on the west, Juniper Creek and the associated Green Swamp on the east, and the upper Waccamaw River which drains Lake Waccamaw and surrounding Friar, Boggy, and River Swamps in the north and central portions of the drainage system (Fig. 3-3). These three drainages combine south of Crusoe Island to form the lower Waccamaw River. The lower Waccamaw flows southwest into South Carolina where it joins the Pee Dee River to form Winyah Bay, a drowned-river estuarine water body which ultimately flows into the Atlantic Ocean south of Georgetown, South Carolina (Fig. 3-2). For the purposes of this report, the North Carolina portion of the WDS will be subdivided into four physiographic components: Green Swamp, Lake Waccamaw, upper Waccamaw River, and lower Waccamaw River (Fig. 3-3).

The North Carolina portion of the WDS is approximately 1,044 mi² or 668,160 acres in size, including 1,028 mi² of land and 16 mi² of water (NC DWQ, 1999) situated primarily within Brunswick and Columbus Counties. Table 3-1 compares the general pattern of land use in the two counties between 1982 (NC DEM, 1994) and 1992 (NC DWQ, 1999). The 1990

population of the WDS was approximately 48,586 as compared to 42,691 in 1970 (NC DWQ, 1999). This represents a 12.1% growth rate as compared to the statewide average of 12.7% (NC DWQ, 1999). The projected population

growth rates for Brunswick and Columbus Counties to year 2015 are 79.3% and 3.5%, respectively (NC DWQ, 1999). Most of the projected growth in Brunswick County will occur along the Atlantic coast and within the Coastal Drainage System, rather than within the WDS. However, there will be long-term indirect consequences of this growth to both the surface- water and ground-water systems within the WDS as the demand for and use of inland fresh water increases along the coastal zone.

________________________________________________

TABLE 3-1. General land uses in the North Carolina

portion of the Waccamaw watershed in 1982 (NC DEM,

1994) and 1992 (NC DWQ, 1999).

________________________________________________

1982 1992

Forest 63.2% 66.5%

Agriculture 27.1% 24.4%

Urban Development/Roads 3.7% 3.4%

Other 2.1% 5.7%

________________________________________________

The WDS is situated primarily within Brunswick and Columbus Counties, which consist of 58% hydric soils. Hydric soils are defined as "soils that are saturated, flooded or ponded long enough during the growing season to develop anaerobic conditions in the upper part" (US SCS, 1987). The water content of hydric soils is generally sufficient to support wetland vegetation. A large portion of the hydric soils within the WDS remain as wetlands in spite of the severe ditching and draining done for expansion of the agriculture and forestry industries.

The WDS includes many blackwater streams bordered by extensive bottomland swamp forests, a very large bay lake, and vast areas of poorly drained interstream divides called pocosins. Pocosin is an Algonquin word that means "swamp-on-a-hill" (Richardson et al., 1981). Since the entire WDS is situated on the outer coastal plain, it is a black-water drainage system characterized by low hydraulic gradients and dominated by wetlands. All of these wetland habitats are characterized by high water tables, long hydroperiods, and organic-rich, hydric soils overlying clay-based sediments. The WDS is "a showcase of biological richness" due to the diverse and extensive wetland habitats characterized by a wide variety of plant communities(NC DWQ, 1999). The extensive character of these wetlands resulted in minimal growth within the basin with development limited largely to forestry and agriculture.

Wetlands are generally transitional lands between dry upland regions and aquatic habitats. They have a water table that is usually at or near the land surface and often are covered by shallow water. The WDS contains three general types of wetlands (Cowardin et al., 1979): palustrine, riverine, and lacustrine.

1. Palustrine wetlands account for most of the wetland acreage in the Waccamaw and are dominated by emergent vegetation that includes swamp forests, scrub-shrub swamps, savannas, and fresh-water marshes. Palustrine wetlands form as pocosins, Carolina bays, and riverine floodplains.
2. Riverine wetlands are the freshwater habitats within the river channels including the margins that may contain emergent vegetation. All major drainages and associated tributary streams within the Waccamaw watershed fall into this category of wetland. Many portions of small stream channels within the Green Swamp pocosin disappear as the water shifts from channel flow to sheet flow, causing riverine wetlands to grade into palustrine wetlands.
3. Lacustrine wetlands include the natural lakes and reservoirs dominated by submergent vegetation within the water and emergent vegetation around the perimeter. Lake Waccamaw is a natural Carolina bay and represents the only major lacustrine habitat within the WDS.

The Waccamaw River below Old Dock is a well-developed stream that drains an extensive and complex set of upland swamp areas or pocosins (Fig. 3-3). Above the "narrows" at Old Dock and Crusoe Island, is a very large pocosin system that will be referred to generally as the Green Swamp pocosin. Green Swamp is a very young and immature drainage system that formed during the last few tens of thousands of years. The resulting flat, perched wetlands are contained totally within small basins defined by the underlying geologic framework and have formed in response to the climatic fluctuations during the last glacial and interglacial episodes.

Both the Waccamaw River and Lake Waccamaw are nationally famous for their diversity and abundance of faunal populations. "Perhaps no other water body in eastern North America contains such a number of endemic fishes and mollusks" (NC DEM, 1995), along with a high diversity of other vertebrates including snakes, alligators, and wading birds. Fuller (1977) summed up the importance as follows. "The Waccamaw basin in southeastern North Carolina and northwestern South Carolina supports more unique non-marine mollusks than any other locale in the state....protection of Lake Waccamaw is the single most important goal of conservation in the state." Portions of the upper Waccamaw River drainage were previously considered for inclusion in the North Carolina Natural and Scenic Rivers System, however, no formal proposal was ever made due to local opposition.

The waters in the Waccamaw River are generally classified as C Sw, whereas Lake Waccamaw is classified as B Sw (NC DEM, 1994). The water quality classification of C represents secondary recreation waters, while the B classification represents primary recreation waters capable of supporting swimming. The Sw designation is the supplemental classification for swamp waters characterized by low velocity flows, low pH and dissolved oxygen, and high organic content. The waters in both the Waccamaw River and Lake Waccamaw have been under consideration for reclassification as Outstanding Resource Water (ORW) by the NC DEM. This supplemental classification would designate these water bodies as "unique and special waters having exceptional water quality and being of exceptional state or national ecological or recreational significance".

In 1987 NC DPR recommended that Lake Waccamaw, the Waccamaw River, and portions of White Marsh and Bogue Swamp be given Outstanding Resource Water (ORW) designation to provide additional protection for the water quality in this system. This recommendation was based on the large number of rare and endangered aquatic species and importance of associated bottomland forest ecosystems, which possess qualities of state and national significance. In 1995 NC DEM agreed with this recommendation for Lake Waccamaw. However, they did not recommend ORW designation for the Waccamaw River or its tributaries because only one river site had an "Excellent Bioclassification". All other river sites were classified as "Good", and "many tributary streams were difficult to rate because of very low pH values (Juniper Creek) or ephemeral flow (Big Creek, Bogue Swamp)". NC DEM (pers. Comm., 1997) did not pursue ORW status for any portion of the WDS due to a 1994 limited fish consumption advisory that included Lake Waccamaw (NC DEM, 1997). The advisory was based upon elevated mercury levels in fish tissues within the entire Lumber River basin. However, in response to public concerns, ORW designation was formally established in 1999, but only for Lake Waccamaw.

According to Rudek et al. (1998) "the biggest threat to North Carolina’s lakes and rivers is---dirt---sediment washed into streams from adjacent land." Sediment appears to be a benign substance; however, it can dramatically impact water quality, stream dynamics, and the aquatic organisms living within stream systems. Rudek et al. (1998) claim that "sediment pollution remains the primary cause of stream degradation in North Carolina." The leading causes of sediment pollution are agriculture, forestry, and urbanization. Agriculture and forestry generate sediment pollution in three ways: clearing and plowing open the soil and subject it to erosion, drainage ditches deliver sediment laden runoff directly into streams, and stream bank destabilization leads to severe erosion. Urbanization increases the amount of land clearing and subsequent construction, resulting in significant increase in impervious surfaces and sediment-laden storm water runoff.

According to the NC DEM (1994) "sediment is the most widespread cause of impairment to stream water quality and biological integrity in the (Lumber River) basin. While much has been done to reduce sedimentation resulting from construction, agriculture and other land-disturbing activities....further improvements and/or more widespread application of sediment control measures....are needed." Upland regions within and around the swamps and pocosins of the WDS are utilized largely for agricultural production while large portions of the lowland regions are extensively utilized for timber production. Since both the uplands and lowlands are poorly drained, they generally require extensive ditching and draining. Consequently, drainage programs have essentially been in effect in much of Brunswick and Columbus Counties since European settlement. The long-term result of these drainage programs is the lowering of the groundwater table and the shunting of surface water directly through ditches into the adjacent streams. Once the vegetative cover is broken, then surface sediment is eroded and delivered to the streams along with the surface water.

Table 3-2 demonstrates the severity of the soil erosion problem within the North Carolina drainage basins. The table summarizes the average amount of sediment that is eroded per acre of land annually for different geographic regions of the state. These data are presented to give an overview of the general volumes of sediment and the potential extent of the sediment pollution problem. The two categories that are most relevant to the WDS are the coastal plain and coastal flatwoods regions with an average of 3.9 and 3.2 tons of sediment/acre/year, respectively. Within the WDS, the coastal plain category would include most of the upland agricultural areas, whereas the coastal flatwoods would include most of the drained lands utilized for forestry purposes. Once the sediment is eroded off the land, suspended sediment in the water column and subsequent sedimentation processes can have significant

TABLE 3-2. Soil erosion trends in North Carolina. Data are from

1992 National Resources Inventory (USDA-NRCS, 1994).

REGIONAL TONS SEDIMENT/ACRE/YEAR

CATEGORY 1982 1987 1992

BLUE RIDGE MOUNTAINS 12.7 20.8 18.3

PIEDMONT 12.3 12.0 10.5

SAND HILLS 6.0 5.6 5.1

COASTAL PLAIN 3.9 3.9 4.0

COASTAL FLATWOODS 3.2 3.1 3.2

TIDEWATER AREA 1.4 1.5 1.6

impacts upon drainage systems. Some of the major types of impacts are summarized in Table 3-3.

According to the NC DEM (1994) the problem of sedimentation is too widespread and is caused by too many sources to present sediment control strategies for targeted water bodies. Thus, they describe more than 19 State and Federal sediment control-related programs. Sedimentation, "deposition of sediment in surface waters", is a widespread nonpoint source-related water quality problem which results from land-disturbing activities including agriculture, forestry, land development, and mining. The NC DEM (1994) recommends the installation of acceptable best

TABLE 3-3. Potential impacts of suspended and bed load

sediments and the processes of sedimentation to riverine

and lacustrine systems.

1. Fills river, stream, and ditch channels and diminishing discharge capacity
2. Fills lakes and diminishes holding capacity
3. Decreases light penetration diminishing plant production
4. Increases water temperatures effecting type and functions of organisms
5. Smothers benthic organisms
6. Clogs gills of nektonic animals and chokes filter-feeding benthic organisms
7. Changes the texture and composition of benthic habitats
8. Buries hardbottom habitats
9. Decomposing organic sediments reduce oxygen concentrations
10. Carries and stores pollutants

1. Nutrients (P and C)
2. Toxic Metals (Pb, Hg, Cu, Cd, etc.)
3. Toxic Organic Compounds (PCB, dioxin, etc.)
4. Pesticides and Herbicides

1. Increases cost of water treatment

management practices (BMPs). BMPs are aimed at minimizing the area of

land-disturbing activity and the amount of time the land remains unstablilized; setting up barriers, filters or sediment traps to reduce

the amount of sediment reaching surface waters; and recommending land management approaches that minimize soil loss such as conservation

tillage, terraces, diversions, crop conservation grasses and trees, filter strips, field borders, grass waterways, water-control structures, and livestock exclusions. In spite of all the efforts of numerous State and Federal programs, there were still 116 miles of streams in the Lumber Basin found to be impaired by sediment, and thus underscoring the need for improvements in sediment control (NC DEM, 1994).

Citizens of Brunswick and Columbus Counties expressed concern about water quality, stream flow characteristics, and the generally poor water and biological conditions in the Waccamaw River. Along with these concerns is the perception that the Waccamaw River is filling with sediment. Residents describe "deep fishing holes that are no longer there" and "river reaches that are no longer navigable during low water conditions". However, there is no documentation concerning sediment filling or studies to define the source and rates of this process of sedimentation.

According to the NC DEM (1994) "sediment is the most widespread cause of water quality use support impairment in the Lumber Basin." Much of this sediment pollution is related to major changes in land-use patterns, increasing the role of both point and nonpoint sources entering the river. During the recent past there has been an increased utilization of these poorly drained lands for expanded agricultural and timber production, along with growing industrial and urban development. This has lead to an accelerated rate of ditching and draining. In addition, growth and development throughout the region has lead to major increases in groundwater withdrawals. All of these changes impact the water flow patterns of the surface drainage system and natural role of intervening swamplands.

To help understand the problem and develop possible solutions for future management of the Waccamaw drainage system, it is imperative to understand both the dynamics of the natural drainage system and define the societal demands and alterations to this system. As one of a series of studies funded by the North Carolina General Assembly to address these concerns, the Geology Department of East Carolina University, in cooperation with the North Carolina Department of Environment, Health, and Natural Resources, Division of Water Resources, initiated two investigations in 1995 concerning the Waccamaw River. The two studies concerning the Waccamaw River and their objectives were as follows.

1. SEDIMENTATION IN THE WACCAMAW RIVER DRAINAGE SYSTEM: to define the problem of sediment pollution by relating it to the geologic framework and changing land-use patterns, and to evaluate the long-term impact upon the drainage system.
2. LAND-USE PATTERNS AND GEOLOGIC FRAMEWORK OF THE WACCAMAW DRAINAGE SYSTEM: to determine the general pattern of land-use change during the past 60 years and relate this change to the geologic framework.

To realize these objectives, we 1) verified the extent of the problem; 2) collected, analyzed, and evaluated the supporting data, and 3) developed this report with a set of recommendations for potential corrective measures. Implementation of the recommendations was not within the purview of these studies.

Our study of the WDS was carried out in four phases during the time period of 1995 through 1999. Following an initial literature review (Phase I), we undertook a major field program during the first two years of the study (Phase II). A laboratory analysis of the field data, along with an analysis of old aerial photograph sets (Phase III) was carried out during the second two years. This report represents the final product of Phase IV of the project, which was the synthesis and integration of data resulting from the previous phases.

When funded, the study was to be a two-year project considering only the lower Waccamaw River. After our initial evaluation at the end of Phase I, it became clear that we had to expand the study to include a larger portion of the WDS. Consequently, we expanded the field and laboratory analysis (Phases II and III) to include the upper Waccamaw River, Green Swamp, and Lake Waccamaw regions. In order to meet our initial project objectives, we also expanded our efforts to consider and understand the geologic framework and hydrodynamics of larger WDS, which more than doubled the time commitment for the project.

Table 4-1 summarizes the data base developed for the study of the North Carolina segment of the WDS. To develop the basic framework understanding of the WDS dynamics, we carried out seven different subprograms that included both field and laboratory components. The field component included the following: 1) evaluation of the North Carolina portion of the WDS under different hydrologic conditions, 2) establishment of riverine stations, 3) measurement of cross-sectional profiles at the riverine stations, 4) obtaining vibracores for sedimentologic and stratigraphic control, and 5) running ground-penetrating radar surveys for subsurface stratigraphic information. The lab component included the following: 1) analyses of sediment texture and composition, 2) evaluation of stratigraphic units from vibracore and GPR data, 3) development of geologic cross-sections through key segments of the drainage system, 4) analyses of aerial photo sets through time, and 5) a re-evaluation of the USGS hydrologic data.

Since the first project objective was to evaluate the source, transport, and deposition of sediment within the Waccamaw River, we considered all potential sediment sources and their pathways into and through the entire WDS. Consequently, we carried out appropriate field investigations within each of the following morphological regions of the Waccamaw drainage basin to address this objective.

1. The Waccamaw River channel, floodplain swamp forests, and tributary stream systems.
2. Lake Waccamaw and the surrounding areas of urban development.
3. Associated pocosins that are extensively drained and used for timber production.
4. Adjacent upland regions that are extensively used for agricultural production.

Sediment pollution is intimately tied to processes associated with different land uses and patterns of changing land use, which in turn is directly dependent upon the geologic framework of the WDS. Thus, we used a two-pronged approach to our study. First, we defined the regional morphological units, defined the underlying stratigraphic framework, and then developed a plausible evolutionary history for the region. By doing this we defined the inter-relationship between the different morphological features and specific ecosystems such as the different kinds of wetlands, parts of the riverine system and lakes, and the different land uses including urban, agricultural, and silvacultural uses.

____________________________________________________________

TABLE 4-1. Summary of the data base developed by the present

study for the North Carolina portion of the Waccamaw drainage

system.

____________________________________________________________

1. VIBRACORES 80 Cores

Lake Waccamaw 26

Upper Waccamaw River and Green Swamp 14

Lower Waccamaw River 35

Antecedent Floodplain 5

____________________________________________________________

2. GROUND-PENETRATING RADAR 100 km

Lake Waccamaw 20

Green Swamp 22

Waccamaw River 26

Antecedent Floodplain 32

___________________________________________________________

3. RADIOCARBON AGE DATES 31 Dates

Lake Waccamaw 9

Green Swamp 6

Waccamaw River 7

Antecedent Floodplain 9

___________________________________________________________

4. SEDIMENT ANALYSES 94 Samples

___________________________________________________________

5. GEOLOGIC CROSS-SECTIONS 41 Sections

Lake Waccamaw 10

Green Swamp 4

Waccamaw River 15

Regional 9

Highway Bridges 3

___________________________________________________________

6. AERIAL PHOTO ANALYSES 6 Sets

1938, 1951, 1955,

1981, 1983, 1988 ___________________________________________________________

The second approach evaluated aerial photo surveys through time. Within the WDS, ditching and draining of wetlands and modification of the natural drainage system was of greatest importance to the problem of sediment pollution. These modifications greatly changed the processes and rates of drainage and systematically lowered the groundwater table, allowing the basic changes in land use to take place. Consequently, we used the changing pattern of drainage ditches and associated roads through time as the indicator for the second objective.

The Waccamaw River is relatively remote with few access points, has dramatic variations in water level and flow conditions, and large segments are dominated by severe channel meandering. These latter segments are extremely narrow with extensive point-bar sand shoals and rapidly eroding cut banks that frequently cause large trees to block the channel and trap extensive sand shoals. Low water usually occurs in the dry months of late spring to fall except when tropical storms influence the drainage basin. Moderate to high water flow occurs after tropical storms in the dry months and during the wet months from late fall to early spring. During low-water stages we used a light weight, 14-foot aluminum jon boat with a 4 hp engine that could readily be portaged over sand shoals and downed trees. During high-water stages, a heavy gauge, 16-foot aluminum jon boat with a 25 hp engine was used to navigate the fast-flowing water. Canoes were used on small streams characterized by channeled flow. On Lake Waccamaw we used the 16-foot jon boat and a 25-foot boat with a 50 hp engine and cabin.

The vast wetlands surrounding the riverine channels are criss-crossed with logging roads supplying basic access into the wetlands. However, the presence of deep drainage ditches on one or both sides of all roads and very dense vegetation often limited direct access into the wetlands. In addition, this severe ditching and draining led to major land use changes in which logging and planting procedures broke down the organic soil and destroyed the original surface morphology of subtle features such as the fossil scroll-bar topography.

River profiles were surveyed along the lower Waccamaw River during low-water conditions of summers 1995 and 1996 (Fig. 4-1). The 15 profiles represent straight and highly sinuous channel morphologies and were measured from a marked horizontal rope installed perpendicular to the channel. River water levels were used as a base line for measuring bank heights, with changing water levels corrected relative to water level measured daily at a water-level gauge attached to the NC Highway 130 bridge. Depth from the horizontal ropes to the channel bottom was measured using a marked stainless-steel rod at regular intervals across the channel.

The geologic section on both banks and along the bottom of the channel was described and sampled for subsequent sediment analysis. Four types of sediment samples were collected. Hand samples were obtained from the exposed banks, ponar grab samples were obtained from the subaqueous river channel. Auger and vibracore samples were obtained on the natural levee on each side of the channel (Fig. 4-1).

The 7.6 cm diameter by 5-9 m long aluminum irrigation pipe was used to obtain the vibracores. Length of the vibracore samples obtained varied tremendously depending upon the sediment type. The vibracore pipe was driven into the sediment using a 3 hp engine attached to a modified cement vibrator. The vibrator generated a low amplitude standing wave that fluidized and displaced sediment adjacent to the core pipe, permitting the pipe to penetrate with minimal resistance and sediment disturbance. The upper empty portion of the core pipe was cut off, filled with water, and sealed with a plumber’s helper to create a vacuum and aid in holding the sediment during retrieval. The cores were retrieved using a 4-ton winch attached to a 5 meter high stainless-steel tripod. The bottom of the core barrel was capped, the core was labeled and cut into 1.5 m sections, and transported to the lab for subsequent analysis. We obtained 30 vibracores along the 15 river profiles and an additional 5 cores in paleo-ridge and swale structures in the antecedent floodplain (Fig. 4-1).

4.2.2. Lake Waccamaw Profiling and Sampling

Lake Waccamaw profiles were obtained during the summer of 1995 using a recording fathometer and a GPS navigational system for location. The fathometer transducer was mounted to the side of 25-foot boat and ten bathymetric profiles were run on a grid across the lake (Fig. 4-2). Twenty six vibracores were obtained in the lake at key locations on the fathometer profiles (Fig. 4-2). The cores in the lake were driven into the lake bottom and retrieved by SCUBA divers using the same equipment described in section 4.2.1. The work was done off the stern of the anchored boat, which was used as a platform for the equipment. From these profiles and vibracores, we produced geologic maps of the lake bottom and cross sections across Lake Waccamaw.

Fourteen vibracores were obtained at several key locations within Green Swamp using the same procedures outlined in section 4.2.1 (Fig. 4-3). These vibracores were limited to a few key areas due to the problems of access along the upper Waccamaw River and within Green Swamp. Consequently, most shallow subsurface samples and stratigraphic information was obtained utilizing hand augers. From these cores and samples, three geologic cross sections were produced across the upper Waccamaw River and one across Driving Creek.

A 100 km ground-penetrating radar (GPR) survey was carried out on many segments of the WDS (Table 4-1; Fig. 4-4). The GPR survey utilized a Geophysical Survey System Inc. (GSSI) Subsurface Interface Radar System-2 (SIR-2) with 100 and 200 mHz antennas. On the roads the SIR-2 was used out of the back of a 4-wheel drive pickup truck towing the antenna at about 3 mph. The water segments of the survey were run from a 16-foot jon boat with a 25 hp engine. The survey was run with the current at just above idle speed and towed an 8-foot Avon with the antenna resting directly on the rubber bottom. Figure 4-5B displays the uninterpreted GPR data obtained in the river and Figure 4-5A shows the geologic interpretation of the data. From these data, interpreted cross-sectional profiles were constructed along the roads and bridges across various portions of the Waccamaw drainage system (Fig. 4-4).

All sets of aerial photography available in Brunswick and Columbus Counties were initially evaluated. Many sets were incomplete or in poor condition. Therefore, the photo sets were selected based upon both availability of high quality and completeness, and time intervals that showed the dramatic changes for the Waccamaw drainage system. The following photo and data sets were utilized for this study.

The relevant black and white photo sets that existed within the U.S. Department of Agriculture offices of Brunswick and Columbus Counties were photo copied utilizing bulk 35 mm black and white film and standard copy stands. The film was processed and photos printed to form a duplicate set of the relevant photo series. Mosaics of each photo set were produced for photo analysis. Specific frames were scanned into the computer for subsequent production of photographs for this report.

To develop the history of drainage ditches and roads in the Waccamaw drainage system through time, three time periods were utilized: 1938, 1955, and 1990. The data sources for producing these maps include the following.

1938: Data was digitized from aerial photos obtained by the U.S.

Department of Agriculture in March and April 1938; and

all weather road designation based on the 1941 U.S.

Geological Survey topographic maps.

1955: Data was digitized from aerial photos obtained in April

1955 by the U.S. Department of Agriculture.

1990: Data from the U.S. Department of Commerce, Bureau of Census,

Washington DC.

The vibracores were split, photographed, described, and subsampled for subsequent analysis which included the following: microscope description, sieving for textural characterization, organic carbon composition, and radiocarbon age dating. Representative sediment samples obtained from the vibracores and surface samples were analyzed utilizing standard laboratory procedures as delineated in Folk (1974). All sediment data were entered into Microsoft Excel spreadsheets for statistical analysis based upon Krumbein and Pettijohn (1988).

Thirty one, in situ and stratigraphically-constrained subsamples obtained from vibracores were submitted to Beta Analytical, Inc., Miami, FL for radiocarbon age dating (Table 4-2). Samples included calcareous shells, organic-rich mud, peat, and specific wood fragments. Dating was based on carbon remaining after relevant pretreatment procedures for calcite shell and organic material. Both standard and accelerator mass spectrometer (AMS) techniques were used to provide age dates used to establish an absolute chronostratigraphy for the WDS.

All age dates used in the present study are calibrated ¹⁴C years before present (cal BP), where present is 1950 AD based upon the

Table 4-2. Radio carbon data.

Pretoria Calibration Procedure (Vogel et al., 1993). Beta Analytical,

Inc. supplied the conventional ¹⁴C ages and the conversions to calibrated ages in years before present. The mean of the calibrated age range was used in this study (Table 4-2).

Sediment lithofacies were assigned to specific sediment units based upon the textural, compositional, and sediment structural analyses. All stratigraphic data were put into Corel Draw for development of interpretive maps and cross-sectional profiles. Stratigraphic correlation was based upon integrating the lithofacies analysis with the interpreted GPR profiles and based upon time control developed by radiocarbon age dating.

The Waccamaw drainage system (WDS) is situated on the outer coastal plain and on top of a major structural feature called the Carolina Platform (Fig. 5-1). This structural high in the crystalline basement rocks separates the adjacent Southeast Georgia Embayment to the south and Salisbury Embayment to the north (Grow and Sheridan, 1988). The Carolina Platform is interpreted to be an Early Mesozoic syn-rift, tectonic block left behind during the continental breakup of North America and Africa as rifting began about 225-200 million years ago (Grow and Sheridan, 1988; Riggs et al., 1990; Olsen et al., 1991; Snyder, 1994). The Carolina Platform is responsible for creating the major seaward protrusion along the mid-Atlantic continental margin that forms North Carolina and it’s unique coastal system.

Extensive seismic studies on the modern continental shelf suggest the Carolina Platform is a fairly stable structural feature with only minor instability through most of the Tertiary (Mateucchi, 1984; Popenoe, 1990; Riggs et al., 1990; Snyder, 1994). These researchers believe that the Carolina Platform is a topographically high erosional feature that formed an oceanic headland and controlled coastal deposition and development of the Carolina continental margin for the last 100 million years (Figs. 5-1 and 5-2). Historically, the high portion of the Carolina Platform has been called the Cape Fear Arch. However, since the Carolina Platform is an structural block with an eroded paleotopographic surface, Snyder (1982) renamed the topographically highest portion as the mid-Carolina Platform High (Snyder, 1982).

Many researchers believe that the emerged coastal plain has been tectonically active along the Cape Fear Arch initially upwarping during the Cretaceous and continuing to rise sporadically through to the Holocene (Zullo and Harris, 1979; Winker and Howard, 1977; Sollier, 1988; Prowell and Obermier, 1991; and Soller and Mills, 1991). Winker and Howard (1977) demonstrated that elevations of the Orangeburg, Surry, and Suffolk scarps and associated sediments were topographically higher on the western flank of the arch and decreased away from the arch. They defined three upwarping events during the Pleistocene. Zullo and Harris (1979) established that elevations of the Hanover, Suffolk, and Alligator Bay scarps and associated sediments were topographically higher on the east flank of the arch. They hypothesized three upwarp episodes at three million, 75,000, and 30,000 years ago. Soller (1988) delineated five river terraces within the Cape Fear River valley that

formed in response to episodic uplift of the river during the Pleistocene. Location of the river terraces along the northeast side of the river valley and the ongoing southwest movement of the river within its valley, suggested to Soller that uplift of the arch occurred throughout the Quaternary and continues into the Holocene. Cronin (1981) used biostratigraphic data from marine deposits throughout the southeastern US coastal plain to calculate a net post-Miocene, vertical upwarp rate of 1-3 cm/1000 years.

Table 5-1 is a geologic column showing the basic stratigraphic units that occur within the subsurface or crop out within the WDS. This is the terminology that will be used in the present report.

The Cretaceous stratigraphic units were deposited over the Carolina Platform and constitute an extensive sediment sequence that forms the geologic framework underlying the WDS (Figs. 5-1 and 5-2). Three Cretaceous stratigraphic units form the basement sequence of sediments that occur as seaward dipping units as depicted in Figure 5-3. Only the youngest Pee Dee Formation crops out and has a direct impact upon the modern dynamics within the WDS.

The Pee Dee Fm. is generally a 400 foot-thick unit of interbedded sequences of 1) dark green to gray glauconitic clayey sand, 2) massive dense clay, and 3) calcareous sand that grades into impure limestone (Swift and Heron, 1969). Most of the lithofacies contain concentrations of microfossils that range up to 25% of the total sediment. Dr. Scott W. Snyder (East Carolina University) prepared numerous samples of these sediment facies for micropaleontological examination. Based upon the foraminiferal assemblages, Dr. Snyder determined that all samples evaluated were Cretaceous in age (Pers. Comm., 2000).

Most of Green Swamp, associated streams, and Lake Waccamaw are either underlain by or incised into the Cretaceous Pee Dee Formation, a thick sequence of calcareous cemented sandstone, sandy moldic limestone, and tight mudstone. The clays of the Pee Dee Formation act as a seal for much of Lake Waccamaw and the surrounding Green Swamp pocosin. However, the Pee Dee sandstone and limestone aquifers discharge significant volumes of ground water into the surface water system wherever the unit is dissected by the surface drainage system. The surface of this very tight mud has a significant amount of paleotopography that probably reflects the incisement of an earlier phase of the modern drainage system.

Throughout the Tertiary, shallow marine and coastal sediments were deposited around the headland of Cretaceous rocks occurring on the mid-Carolina Platform High (Riggs et al., 1990). Most Tertiary units on the North Carolina coastal plain occur as a seaward thickening sedimentary wedge deposited off the northeast flank of the Cretaceous units. These Tertiary units crop out on the continental shelf as they wrap around the seaward nose of the structure (Figs. 5-1 and 5-2) (NCGS, 1985; Riggs et al., 1990; Snyder et al., 1993). However, the WDS is situated high and along the axis of the mid-Carolina Platform High with no Tertiary units of Paleocene through Miocene age occurring within the region. Thus, there was up to 60 million years of time when the Cretaceous sediments were severely weathered and eroded.

The first Tertiary deposits preserved in the WDS were deposited during the Pliocene (sometime after 5 million years ago) when major sea-level oscillations alternately flooded and drained the WDS (Table 5-1). Coastal marine sediments were repeatedly deposited during sea-level highstands and severely eroded during subsequent sea-level lowstands. The result is a highly dissected series of Pliocene and Quaternary coastal sediments perched on top of a severely eroded surface with significant paleotopography and a paleodrainage system cut into the Cretaceous sediments.

The sediments lying on top of the Pee Dee Fm. within the WDS are of either Pliocene or Quaternary age resulting in a major erosional unconformity between the two sets of sediments. Since the top of the Pee Dee Fm. has been severely eroded for up to 60 million years during the Tertiary, the paleotopography on the Pee Dee surface exerted a major control over the modern drainage system, including the deposition, erosion, and preservation patterns of all subsequent sediment units.

TABLE 5-1. Geologic column for the Waccamaw drainage system as used in this report.*

QUATERNARY HOLOCENE Holocene GMU < 10,000

PLEISTOCENE Late Wando GMU 10-100 thous

Middle Socastee GMU 100-400 thous

Penholoway GMU 400-700 thous

Early Waccamaw Fm. 0.7-1.7 mill

TERTIARY PLIOCENE Late Bear Bluff Fm. 2.2-2.0 mill

Duplin Fm. 3.0-3.5 mill

CRETACEOUS Late Maastrichtian Pee Dee Fm. 75-66 mill

Campanian Black Creek Fm. 84-75 mill

Santonian Middendorf Fm. 88-84 mill

* Data composited from Heron and Wheeler (1964); Swift and Heron (1969);

DuBar et al. (1974); NCGS (1985); Sollier (1988); Owens (1991); Sohl

and Owens (1991); Ward et al. (1991).

Unconformably overlying the Pee Dee Fm. are a series of very fossiliferous sandstone and sandy limestone units defined as the Pliocene Duplin and Bear Bluff Fm. and the Early Pleistocene Waccamaw Fm. (Table 5-1). These units are irregularly preserved throughout the WDS. Figure 5-4 is a geologic map of the Waccamaw watershed showing the local areas where Pliocene and early Pleistocene sediments are preserved.

The Pliocene and early Pleistocene sediments are generally thin (< 5 m) and basically occur in the subsurface in the higher topographic regions. The Waccamaw Fm. crops out and forms cliffs in two places including the north shore of Lake Waccamaw and west margin of the Waccamaw River at Old Dock (Johnson and DuBar, 1964). Due to the high concentration of fossil marine shells, these units are commonly quarried for use as aggregate or as sources of lime (e.g., Crusoe and Snake Islands).

Superimposed upon the geologic framework formed by the Cretaceous and Pliocene stratigraphic units, is a surficial layer of Pleistocene and Holocene sediments. These surficial deposits consist of a complex sequence of interbedded sand, mud, and peat sediments. This surficial layer of sediments occurring throughout the upland areas was interpreted by DuBar et al. (1974) to be a series of barrier and back-barrier deposits that formed in response to high sea-level events during Pleistocene interglacials.

These Pleistocene deposits were subsequently reinterpreted by Sollier (1988), Owens (1991), and Sollier and Mills (1991) based upon their geomorphic character and elevation. The basal Pleistocene sediment sequence contains marine fossils and was defined as the Waccamaw Formation. The overlying Pleistocene sediments consisted of a complex sequence of nonfossiliferous sand and mud facies and were subdivided into three formations based largely upon elevation and equated to age. The units include the Penholoway, Socastee, and Wando Formations that occurred at +15 to +21 meters, +9 to +15 meters, and +6 to +9 meters above mean sea level, respectively.

Our research suggests that the Penholoway, Socastee, and Wando Formations are lithologically indistinguishable and little can be done lithologically without a major subsurface research effort involving extensive drilling and remote sensing. Since these three units are not lithologic formations, we suggest that they are at best geomorphic units (GMU). Consequently, we will consider the Penholoway, Socastee, and Wando to be GMUs in this report (Table 5-1). Each unit has been defined on the basis of elevation with the highest being the oldest and sequentially younger units cut into or occurring below the previous. Resolving the complex stratigraphy of the Penholoway, Socastee, and Wando GMUs is beyond the scope of this study. The remainder of this report will focus on the Wando and Holocene GMUs that are incised into the Penholoway and Socastee GMUs.

Figure 5-5 is a map showing the distribution of the four geomorphic units within the WDS. Also on this figure are the locations of four schematic geologic cross sections through the WDS that display the relative spatial relationships between the four GMUs and the

Cretaceous Pee Dee Fm. The four sections are presented in Figures 5-6 and 5-7. Contacts between the GMUs and the Cretaceous are generalized and are based upon many different lines of evidence including field relationships from outcrop data along rivers and in quarries, vibracore and auger holes, ground-penetrating radar surveys, NC Dept. of Transportation bridge bore holes, and topographic characteristics. Most of these relationships for the upland areas and the Penholoway and Socastee GMUs are speculative at best; the relationships of the Wando and the Holocene GMUs are fairly well documented.

The uplands around the Green Swamp Pocosin and Waccamaw River are referred to as the Penholoway Geomorphic Unit (Figs. 5-5, 5-6, and 5-7). This uppermost plain was interpreted to be a marine terrace formed during the Sangamon interglacial when sea level was up to 27-30 meters above present (Cooke 1931; Doering 1960; Johnson and DuBar 1964; DuBar et al. 1974). The Penholoway GMU includes all land in the WDS that is greater than 15 meters above mean sea level and extends southeast from the Surry Scarp and Effingham Sequence of sand ridges of Winker and Howard (1977) and includes their Chatham Sequence of sand ridges. The latter sand ridges parallel the Atlantic Ocean and constitute the high land on the landward side of the modern estuarine system and upon which much of U.S. highway 17 is situated.

The Socastee GMU is defined by the land areas that occur between +9 and +15 meters above mean sea level. Figures 5-6 and 5-7 demonstrate that the Socastee is a narrow zone that is totally incised into the Penholoway GMU and occurs along the outer perimeter of the very broad, paleo-Waccamaw River valley. The Penholoway and Socastee GMUs together form the upland regions around the WDS. They are characterized by extensive flat, poorly drained areas that are now dominated by agriculture. Scattered and irregularly shaped sand ridges, perched pocosins, and Carolina bays occur on the surface throughout the distribution of these units.

Incised within the Socastee is what appears to be the modern Waccamaw River with its very broad floodplain (Figs. 5-6 and 5-7). However, this valley fill does not all represent modern floodplain and can be further subdivided into the Wando GMU and Holocene GMU. Most of the valley fill represents the Wando GMU, an antecedent floodplain that has slightly higher elevations (+6 to +9 meters above mean sea level) and is dominated by paleo-channels and associated pointbar scrolls. The Holocene channel and active floodplain constitute a small portion of the total valley fill occurring less than 6 meters below mean sea level. Wherever the modern Waccamaw River is incised down into the Wando GMU the channel is wide, deep, and fairly straight with broad sweeping meanders. These channels are usually rock bound along one or more sides and are very high so that they are not overtopped during normal flooding conditions.

The Holocene GMU and the associated sediments of the Waccamaw River occur within those river segments dominated by highly meandering channels bounded by active point bars and low cut banks. These active sediment-choked portions of the Waccamaw River have narrow, shallow channels with low banks and broad modern floodplains. Since these floodplains are flooded during most of the yearly wet season, they are characterized by wetland vegetation and are rarely converted to forestry use.

The Waccamaw drainage system is defined for this report as that portion of the Waccamaw River watershed occurring within Brunswick and Columbus Counties of North Carolina. The WDS and associated habitats and sediments are either incised into or perched on top of an inherited geologic framework of older stratigraphic units. Composition, distribution, morphology, and evolutionary history of these older stratigraphic units control the geometry and dynamics of the modern WDS.

The lower Waccamaw River today is an underfit stream with a modern channel that is significantly smaller than the overall river valley or the paleochannels that occur within the antecedent floodplain of the Wando GMU. The lower Waccamaw River drains the Green Swamp pocosin through the Old Dock narrows formed by the Crusoe Island ‘cork’. This ‘cork’ is the result of more resistant deposits of Plio-Pleistocene sediments that occur on both Crusoe Island and adjacent upland areas (Fig. 3-3). Consequently, the entire character of the drainage system changes dramatically from a broad sheet-flow dominated pocosin system above Crusoe Island to primarily an incised riverine system below the Island.

The upper portion of the Waccamaw drainage system consists of the Green