The Santee River is a river in South Carolina in the United States, 143 miles (230 km) long. The Santee and its tributaries provide the principal drainage for the coastal areas of southeastern South Carolina and navigation for the central coastal plain of South Carolina, emptying into the Atlantic Ocean approximately 440 miles (708 km) from its farthest headwater on the Catawba River in North Carolina. The Santee River is the second largest river on the eastern coast of the United States, second only to the Susquehanna River in drainage area and flow. 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.