“R” is for Rivers. South Carolina has an abundance of rivers that originate within the state or that enters from Georgia and North Carolina and drain land as far away as Virginia. These rivers flow generally from the northwest to the southeast. The Santee River system is the largest on the east coast. The Savannah River forms the western boundary of South Carolina. The third river system is the Pee Dee, which is the only river system in the state left undammed. Some of the most beautiful rivers in the state are those that begin in the coastal plain. Tannic acid in the organic matter found in these rivers gives them a dark cast, and so they are known as “black rivers.” The Edisto is the largest of the black rivers in South Carolina and one of the most pristine.