Simplify the following text for a Grade 12 native English-speaking person.
Original: The Santee and its tributaries provide the principal drainage and navigation for the central coastal plain of South Carolina , emptying into the Atlantic Ocean approximately 440 m from its farthest headwater on the Catawba River in North Carolina , Much of upper river is impounded as the expansive horn-shaped Lake Marion reservoir , formed by the 8 m long Santee Dam , built during the 1930s as a WPA project to provide a major source of hydroelectric power for the state .
Simplified: The Santee and its tributaries give the main drainage and navigation for the central coastal plain of South Carolina , emptying into the Atlantic Ocean about 440 mi ( 710 km ) from its farthest headwater on the Catawba River in North Carolina . Much of the upper river is in the expansive horn-shaped Lake Marion reservoir .