It's probably Catawba culture pottery. Check out the book Woodland Potters and Archaeological Ceramics of the North Carolina Coast on Amazon. That should be a good guide for you. When I was an archaeologist, my specialty was more with the Saponi culture in the norther part of the state.
The second photo looks to be net impressed pottery. If you are searching by rivers, definitely look for natural bluffs above the river banks...ones that can withstand the river flooding. That is where you will find remnants of villages and their middens. Be careful though, with middens, you often find burials if the soil conditions are right (sandy, no clay).
If you do run into more than just shards, you may want contact an archaeologist. It's tough finding good sites and there is still much to learn. My buddy/former professor is one of the leading prehistoric Indian archaeologists in the region, so just let me know if you ever want me to hook you up with him.
Phil
