Their primary purpose was to provide hydroelectric power to the Midlands  and Lowcountry . Their secondary purpose was to provide a navigable channel from Columbia  down to Charleston . That said, most folks value Lakes Marion and Moultrie because together they comprise one of the best freshwater fishing locales in America, chalking up many world record-holding catches. And because of their location in the deep South, they never freeze over and thus are fishable all year.
Wild-looking Lake Marion on the Santee River, known for its plethora of striped bass, is the largest and northernmost of the two, covering over 100,000 acres. More manicured Lake Moultrie on the Cooper River covers about 60,000 acres and is known for its massive catfish (blue cats over 40 pounds are not uncommon, and some over 50 pounds have been caught).
Since its inception in 1934, the Santee Cooper project’s credo has been “for the benefit of all.” Because of this, you’ll find no elitism, no exclusivity, nor any serious attempt to limit access to the lakes. There are numerous marinas, public landings, and fish camps all around the circumference of both.