Conjunction of the Cape Fear River and the Northeast Cape Fear River

Figure 3. The confluence of the two rivers. (Photograph by the author. More about the photograph)

The town of Wilmington is located at the junction of the Northeast Cape Fear and Cape Fear rivers. In this photo the Cape Fear River is entering from the bottom. The water in the Cape Fear River is just turning salty as it reaches Wilmington, the zero salinity line being about two miles upstream. I say “about” because the salt content of the water depends not only on the river flow but on the input from the ocean some 28 miles away. During very high tides or periods of low rainfall, this section of the river may experience salinities of several parts per thousand (35 parts per thousand is full-strength seawater). After heavy rains the zero salinity line can be found as far as 15 miles to the south, halfway to the river’s end.

This has serious consequences for the organisms living in the estuary, who must be tolerant of varying salt concentrations, or eurohaline. Saltwater has high concentrations of ions, which causes water to move out of organisms living in the water. Estuarine and marine organisms have developed adaptations to overcome the dehydrating effect of saltwater, but estuarine organisms have the tougher task of coping with conditions which can change hourly with the tides.

This is as good a time as any to describe the classification of the Cape Fear River estuary. Scientists make a living classifying things, so we’ll keep a couple employed. Estuaries can be classified by their method of formation and by the balance of river flow and tides. The Cape Fear estuary is a good example of a drowned river valley estuary. Another type of estuary common to North Carolina is the bar built estuary, which is caused by sand being moved alongshore by ocean currents, eventually closing off sections of water from direct contact with the ocean. The movement of coastal sediments is another very dynamic process. Masonboro Sound, just east of Wilmington, is a good example.

Most North Carolina estuaries were formed by a combination of river-drowning and sediment deposition — the Outer Banks and the enclosed Pamlico and Albemarle Sounds come to mind. Of course, scientists live to argue as well as classify, so anything that doesn’t neatly fit a classification neatly is exciting! Two types of estuaries not found in North Carolina are the tectonically formed estuary and the fjord. (Tours to California and Alaska will have to be found elsewhere.)

As sea level has been rising during the Holocene, the coastal plain is slowly being flooded with seawater. The encroaching sea fills up the valleys dug by rivers, which at one time ran fresh to points many miles past the current coastline. The estuaries are always “catching up” to the current sea level. That is, as the sea level moves, it takes many years for the plant and animal life along the estuary to adjust to the changing conditions. And during those years sea level has changed some more, so the organisms have to keep adjusting. Estuaries are thus prime examples of systems in dynamic equilibrium.

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