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Edisto River Sub-basin
Future Water Demands
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
). (See related section: Geomorphology: Physiographic Regions.)
Edisto River Sub-basin
The Edisto River
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
). 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
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).
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
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
) (Cooney et al. 1998).
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).
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
). 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.
DevelopmentBack to Top
Surface-water resource development in the Edisto River sub-basin
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.).
Combahee-Coosawhatchie 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
Salkehatchie River and the Little Salkehatchie are the major tributaries
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
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).
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 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
In 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
(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
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
DevelopmentBack to Top
Surface-water resource development in the Combahee-Coosawhatchie River
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).
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
B. Badr, SCDNR - Land, Water, and Conservation Division
L. Zimmerman, SCDNR - Marine Resources Research Institute
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.