spacer  
spacer

Introduction

Edisto River Sub-basin

Combahee-Coosawhatchie River Sub-basin

Future Water Demands

References

General Introduction | History | Environmental Conditions | Biological Resources | Species Gallery | Socioeconomic Assessment | Resource Use | Resource Management | Synthesis Modules | Community Perspectives | Image Atlas | GIS Data | Bibliography | Glossary | About This CD-ROM | ACE Contacts | Site Map | Search

Surface Water Resources

Introduction

The ACE Basin Characterization study area is entirely contained within the lower coastal plain of South Carolina. However, the headwaters of many of the streams within the study area originate in the middle or upper coastal plain. The topographic slope of this region is moderate. All of the surface water in the ACE Basin study area eventually flows into St. Helena Sound and the Atlantic Ocean (South Carolina Water Resources Commission 1972). Drainage is southeasterly with a principal stream gradient of about 69 cm/km (3.5 ft/mi) (Bloxham 1979). Stream gradients range from about 375 cm/km (20 ft/mi) near the upper boundary to 19 cm/km (1 ft/mi) near the coast (Bloxham 1981). The Land, Water, and Conservation Division of the South Carolina Department of Natural Resources has divided the ACE Basin into two river sub-basins, the Edisto and the Combahee-Coosawhatchie. Approximately 32 km (20 miles) of the Intracoastal Waterway flows through the study area and connects the North Edisto River to St. Helena Sound at the mouth of the Combahee River (Intracoastal waterway {map icon}). (See related section: Geomorphology: Physiographic Regions.)


Edisto River Sub-basinEdisto sub-basins

The Edisto River sub-basin extends northward from St. Helena Sound and lies entirely within South Carolina (U.S. Army Corps of Engineers 1991). The Edisto River sub-basin is drained by four tributary rivers: the South Fork Edisto River, North Fork Edisto River, Edisto River, and Four Hole Swamp (See the surface water sites {map icon}). The North and South Fork Edisto rivers originate within and pass through the sandhills of Aiken and Lexington Counties in the upper coastal plain and join near Orangeburg to form the Edisto River in the middle coastal plain (U.S. Army Corps of Engineers 1991). Four Hole Swamp is a system of creeks which drains extensive swamp land. This swamp originates in Orangeburg County in the upper coastal plain and discharges into the Edisto River near Givhans. Near the coast, the Edisto River diverges to form the North and South Edisto rivers. These tidally influenced estuaries drain bordering salt marshes and tidal creeks (South Carolina Water Resources Commission 1983).

There are no large reservoirs in the sub-basin, and the largest, lake (unnamed) is located in Dawhoo Swamp and has a surface area of 160 hectares (395 acres) and a volume of 1 cubic kilometer. The second largest lake, Reynolds Pond in Giddy Swamp, is smaller in area, 80 hectares (198 acres), but larger in volume (2.5 cubic kilometers) (South Carolina Water Resources Commission 1983).

Saltwater Intrusion
The Edisto River is tidally-influenced below river mile 38 (river miles are statute miles measured upstream from the mouth and follow the curves of the river) and the saltwater interface extends to river mile 19.5 during high tide. For any given tidal stage, lower freshwater inflow allows saltwater intrusion farther upstream. Thus, during periods of very low flow, the saltwater interface can intrude to river mile 32, near Jacksonboro (Johnson 1977).

Streamflow Measurements
Presently, streamflow and water quality are monitored by U.S. Geological Survey at eight gaging stations: three on the South Fork Edisto River, and one each on the Edisto River, the North Fork Edisto River, Cow Castle Creek, Dean Swamp Creek, and McTier Creek (Gaging stations {short description of image}). Streamflow on the Edisto River is substantial and fairly consistent. These well-sustained flows are due primarily to discharge from ground-water reserves in the upper coastal plain region, in which over one-half the Edisto sub- basin is located (South Carolina Water Resources Commission 1983). Average annual streamflow on the Edisto River near Givhans is 74 m³/s (2,614 cubic feet per second (cfs)). Average flow in the major tributary streams is 21.6 m³/s (766 cfs) on the South Fork Edisto near Denmark and 22.1 m³/s (783 cfs) on the North Fork Edisto at Orangeburg. Cow Castle Creek exhibits variable flows typical of most middle and lower coastal plain streams, where flow is more dependent on rainfall and direct runoff. Average flow on this stream is 0.57 m³/s (20.3 cfs) (Edisto streamflow {short description of image}) (Cooney et al. 1998).

Water Use
The Edisto River and tributary streams in the upper coastal plain exhibit well-sustained flows and provide a reliable water supply. Tributary streams in the middle and lower coastal plain region, however, have more variable flows and provide limited surface-water during periods of low rainfall (South Carolina Water Resources Commission 1983). In 1983, total gross use in the Edisto sub-basin was 999 million liters per day (mld) (264 million gallons per day (mgd)), and 36 percent of this was “lost”. Surface water can be lost through evapotranspiration, transfer to another watershed, or leaching into groundwater systems. Surface water supplied 93 percent of total water demand excluding thermoelectric-power water use. Leading water users in the Edisto sub-basin are thermoelectric power, public supply (domestic, industrial, or commercial uses), and agricultural irrigation. A thermoelectric plant near Givhans operated by South Carolina Electric and Gas accounted for 60% of the gross water use in 1983. Large withdrawals by the City of Charleston, upstream of Givhans, accounted for most of the water use. Approximately 246 mld (65 mgd) of Edisto River water was diverted into the Ashley-Cooper sub-basin for public water supply and industrial use by the City of Charleston. Agricultural irrigation accounted for about six percent of gross water use in 1983. Surface water withdrawals for irrigation in 1983 were the greatest in the state with over a quarter of the irrigated acreage in the state located in this sub-basin (South Carolina Water Resources Commission 1983). Total water use in the Edisto sub-basin is projected to increase by 52% (1,518 mld or 401 mgd) by the year 2020. Agricultural and thermoelectric power plant uses are expected to remain the leading gross water users (South Carolina Water Resources Commission 1983).

Only one public facility that uses more than 1.1 mld (0.3 mgd) occurs in the study area portion of the Edisto sub-basin. The town of Edisto Beach uses 2.9 mld (0.76 mgd). Ground water is, however, the source for this user (Newcome 1995).

Water Quality
The Department of Health and Environmental Control (DHEC) monitors water quality at 37 stations in the study area of the Edisto River sub-basin (Monitoring stations {short description of image}). In most of the ACE Basin region, no major water quality problems have been identified, although high levels of fecal coliform bacteria have occurred near the highly populated areas of Edisto Island.

Development
Surface-water resource development in the Edisto River sub-basin is limited to several navigation and flood control projects. Three navigation projects have been undertaken by the U.S. Army Corps of Engineers in this sub-basin, but none of these projects is maintained (South Carolina Water Resources Commission 1983). Three small flood control projects, two in St. George and one in Givhans, have been supervised by the Natural Resource Conservation Service (NRCS) in this sub-basin. The NRCS provides engineering assistance and funding appropriation assistance for municipalities. These projects are designed to prevent flooding in rural communities and are maintained by the municipalities (Edwards, pers. comm.).

Back to Top


Combahee-Coosawhatchie River Sub-basin

Combahee-Coosawatchie River sub-basin

The Combahee-Coosawhatchie River sub-basin is contained primarily in the middle and lower coastal plain. The major freshwater streams draining this sub-basin are the Salkehatchie River, the Coosawhatchie River, and the Ashepoo River (Site map {short description of image}). The Salkehatchie River and the Little Salkehatchie are the major tributaries to the tidally influenced Combahee River. The Combahee and Ashepoo rivers drain into St. Helena Sound but the Coosawhatchie River drains outside of the study area, into Port Royal Sound. The coastal area of this sub-basin contains the most extensive estuarine waters in the state. These coastal waters are dominated by St. Helena Sound and Port Royal Sound and include numerous locally interconnecting tidal creeks and rivers (South Carolina Water Resources Commission 1983). There are no large reservoirs and only five lakes greater than 80 hectares (200 acres) in this sub-basin. An unnamed lake on the Ashepoo River is the largest lake at 324 hectares (800 acres) in the sub-basin (South Carolina Water Resources Commission 1983).

Saltwater Intrusion
The Combahee River is tidally influenced for most of its length, and, during periods of low freshwater flow, salt water can intrude as far inland as Yemassee (Hazen and Sawyer Engineers 1956). The Ashepoo River is also tidally influenced, and, during periods of low flow, salt water can intrude to the headwaters beyond U.S. Highway 17 (Johnson 1977).

Streamflow Measurements
Streamflow has been monitored on the Salkehatchie since 1951. Average annual streamflow on the Salkehatchie River near Miley is 9.8 m³/s (346 cfs) and 90 percent of the time, it is at least 2.8 m³/s (98 cfs) (Cooney et al. 1998). This well-sustained flow is probably due to discharges from several ground-water fed headwater streams located in the upper coastal plain region (South Carolina Water Resources Commission 1983). Other than these ground-water fed streams, the amount of fresh surface water available within the sub-basin is limited. Flow in the tributary and main stem regions of the Combahee and Ashepoo rivers is more variable than in the Salkehatchie River, and, in the Great Swamp region of the Ashepoo River, flow is often non-existent during the summer and fall (Combahee streamflow {short description of image}).

Water Use
irrigationIn 1983, total water use within the Combahee sub-basin was estimated at 110.9 mld (29.3 mgd), with 50 percent of the withdrawal being lost. Major water users were public supply, agricultural irrigation, self-supplied domestic, and self-supplied industrial. Ground water supplied the majority of the water for these uses. Only 18 percent of the gross use was supplied by surface water. Approximately 23.5 mld (6.2 mgd) of surface water from the Lower Savannah River sub-basin was diverted into this sub-basin for public supply use. Agricultural irrigation supply was the largest surface water withdrawal in this sub-basin, with 46% of irrigation water being supplied by surface water. When averaged over the five-month growing season, irrigation water use accounted for 41% of the gross use and 66% of the 'lost’ water. Total use in this sub-basin is projected to increase by 220% by the year 2020. Most of this increase will likely be due to increases in agricultural irrigation and most of the demand in the Combahee-Coosawhatchie sub-basin will be supplied by ground water (South Carolina Water Resources Commission 1983).

Within the ACE Basin study area of this sub-basin there is only one facility, the city of Walterboro, that uses more than 1.1 mld (0.3 mgd). Walterboro uses 7.3 mld (1.92 mgd), all of which is supplied by ground water (Newcome 1995).

Water Quality
The DHEC monitors water quality at 24 sites within the study area of this sub-basin. As in the Edisto River sub-basin, no major water quality problems have been identified at these stations except for high levels of fecal coliforms in the Ashepoo River downstream of Walterboro and in the Coosaw River near Beaufort.

Development
Surface-water resource development in the Combahee-Coosawhatchie River sub-basin consists primarily of navigation projects in the coastal waters, although some flood-control projects occur throughout the area. The U.S. Army Corps of Engineers has been involved in seven navigation projects in this sub- basin, four of which were within the study area. None of these is currently active. A number of flood-control projects have been undertaken in the sub-basin. Since 1980, the Natural Resource Conservation Service (NRCS) has conducted four flood control projects (White Hall, Campus A Middle School, Highway 363 Church of God, and Yemassee) to provide flood prevention to rural communities (Edwards, pers. comm.). The Willow Swamp region of Colleton and Bamberg Counties is the site of the only large completed flood control project conducted by NRCS in this sub-basin (South Carolina Water Resources Commission 1983).

Back to Top


Future Water Demands

Surface water demands in both the Edisto and Combahee-Coosawhatchie River sub-basins will increase substantially by the year 2020. However, the streamflows in these regions are projected to adequately meet these demands at least 95% of the time. Although no gaging stations exist in the lower reaches of streams in the ACE Basin study area, estimations of water availability in the lower reaches indicate that water supplies in the Combahee-Coosawhatchie could be limited in some tributary streams during periods of low flow. A site-specific hydrologic analysis to determine the quantity of streamwater available during rainfall events should be performed where small tributary streams are considered for water supply sources (South Carolina Water Resources Commission 1983).

NEXT SECTION: Ground Water


Authors

B. Badr, SCDNR - Land, Water, and Conservation Division

L. Zimmerman, SCDNR - Marine Resources Research Institute


References

Bloxham, W. M. 1979. Low-flow frequency and flow duration of South Carolina streams. South Carolina Water Resources Commission Report No. 11. Columbia, SC.

Bloxham, W. M. 1981. Low-flow characterization of ungaged streams in the piedmont and lower coastal plain of South Carolina. South Carolina Water Resources Commission, Report No. 14. Columbia, SC.

Cooney, T. W., K. H. Jones, P. A. Drewes, J. W. Gissendanner, and B. W. Church. 1998. Water resources data, South Carolina, water year 1997, U.S. Geological Survey, Columbia, SC. Edwards, S. personal communication. Natural Resource Conservation Service. 1999.

Hazen and Sawyer Engineers. 1956. Report to the Bureau of Yards and Docks, Department of the Navy, on water supply in the vicinity of Beaufort, South Carolina. New York, NY.

Johnson, F. A. 1977. A Reconnaissance of the hydrology of the Edisto and Ashepoo estuaries, South Carolina. South Carolina Water Resources Commission, Report No. 6. Columbia, SC.

Newcome, R., Jr. 1995. The 100 largest public water supplies in South Carolina-1995. SC Department of Natural Resources, Water Resources Division Report No. 3.

South Carolina Water Resources Commission. 1972. ACE framework study: Ashley-Combahee-Edisto River Basin. Columbia, SC.

South Carolina Water Resources Commission. 1983. South Carolina state water assessment. Report No. 140. Columbia, SC.

United States Army Corps of Engineers. 1991. Water resources development in South Carolina 1991. U.S. Army Corps of Engineers, Charleston, SC.

General Introduction | History | Environmental Conditions | Biological Resources | Species Gallery | Socioeconomic Assessment | Resource Use | Resource Management | Synthesis Modules | Community Perspectives | Image Atlas | GIS Data | Bibliography | Glossary | About This CD-ROM | ACE Contacts | Site Map | Search

Site MapSearchHelpReturn to TopGlossaryBibliographyGIS DataImage AtlasCommunity PerspectivesSynthesis ModulesResource ManagementResource UseSocioeconomic AssessmentSpecies GalleryBiological ResourcesEnvironmental ConditionsHistoryGeneral Introduction