Most biologists will tell you that the Coastal Plain is where things get interesting. The place where a river interfaces with the ocean is called an estuary, and it’s perhaps the most interesting place of all. Estuaries are heavily tidal in nature (indeed, the word derives from aestus, Latin for tide), and feature brackish water and heavy silt content.
South Carolina  typically has about a six to eight-foot tidal range, and the coastal ecosystem depends on this steady ebb and flow for life itself. At high tide, shellfish open and feed. At low tide, they literally clam up, keeping saltwater inside their shells until the next tide comes.
Waterbirds and small mammals feed on shellfish and other animals at low tide, when their prey is exposed. High tide brings an influx of fish and nutrients from the sea, in turn drawing predators like dolphins, who often come into tidal creeks to feed.
It’s the estuaries that form the most compelling and beautiful sanctuaries for the area’s incredibly rich diversity of animal species. Many estuaries are contiguous with those of other rivers. (Charleston Harbor, formed by the confluence of the Ashley and Cooper Rivers, is an excellent example of that phenomenon.)