Carolina Environmental Diversity Explorations
A blackwater river from sea to source · By Dirk Frankenberg
The White Oak River: Introduction
(Photograph by the author. More about the photograph)
One of the interesting things to do during field study of natural areas is to follow an environmental gradient across the landscape. This is particularly rewarding if your gradient extends up a river, as the exploration takes on the aura of a classic “search for the source of the Nile.” Admittedly, the White Oak River that borders between Onslow and Carteret Counties is not exactly the Nile. But it flows through preserved natural areas in state and national forests and meets the sea at Hammocks Beach State Park, so it is about as natural as North Carolina’s rivers get these days.
The White Oak is also an excellent example of a blackwater river, one of the two types of rivers that cross our coastal plain. Blackwater rivers extend inland only as far as the coastal plain. Their source is in coastal plain wetlands, and they drain wetlands throughout their course. Brown water rivers extend further inland, with sources in the piedmont or the mountains.
Blackwater river waters are usually clear, but their waters are commonly stained dark brown by organic matter and plant pigments from the wetland plant communities through which they flow. From a distance these rivers look black, hence their name. Brown water rivers, by contrast, are rarely clear because of the silt and clays that they carry from their upland sources. Since most of these silts and clays are brownish red, the rivers appear brown to the eye.
The difference in source and sediment load of these two river types affects their geology and ecology. Blackwater rivers have few sandbars or shallow areas in their freshwater reaches, whereas the higher sediment loads of brown water rivers makes these features common. Both types of rivers are flanked by wetlands in the coastal plains, but the larger flow volumes of brown water rivers makes them more likely to have steep banks. The banks of blackwater rivers are low making them interesting to view from a boat as you can see into the interiors of forests and marshes along your way.
