[2] Much of the upper river is impounded by the expansive, horn-shaped Lake Marion reservoir, formed by the 8-mile (13 km)-long Santee Dam.

The dam was built during the Great Depression of the 1930s as a Works Progress Administration (WPA) project to provide a major source of hydroelectric power for the state of South Carolina.

The Santee is formed in central South Carolina 25 miles (40 km) southeast of Columbia by the confluence of the rivers Wateree and Congaree.

The two channels reach the ocean at Santee Point, approximately 15 miles (24 km) south of Georgetown, and not far from the outlet of the Pee Dee River.

It allowed direct water transportation between the upcountry of central South Carolina and the port city Charleston, at the outlet of the Cooper River.

The main source of electric power for the utility was a hydroelectric system inland from Charleston, built by the federal government during the administration of President Franklin D. Roosevelt.

During the 1980s, the Army Corps of Engineers built a "rediversion" canal to send most of the water back into the Santee, partially mitigating this problem.