S.C. Code § 4-3-60
Barnwell County is bounded as follows: on the northwest by Aiken County, from which it is divided by a line beginning at the point upon the Savannah River, below the mouth of the Upper Three Runs Creek, as established by the survey of J. Seth Mixon, under act approved March 14 1874, and following said line to A. J. Weathersbee's old mill, then to the South Edisto River, in the direction of the point on the North Edisto River (mouth of Cedar Branch), where the line between Orangeburg and Lexington Counties, drawn from Big Beaver Creek, touches said river; on the northeast by Orangeburg County, from which it is separated by the South Edisto River as far down as the northeast corner of Bamberg County; on the east by the line separating it from Bamberg County; on the southeast by Allendale County; and on the southwest by the state of Georgia, from which it is separated by the Savannah River.
S.C. Code § 4-3-60