Carolina Environmental Diversity Explorations
Cape Fear estuaries · By Steve Keith
Moving south and to the opposite bank of the river, we come across Allen Creek. This side of the river is much less accessible, and wide expanses of marsh and swamp remain. Note the distinct tree line indicating a rather homogeneous change in elevation.
In this photo the river inlet is to our right, so the river should, in theory, flow from left to right. Something seems amiss, as the Allen Creek effluent is definitely moving right to left.
I mentioned that estuaries can be classified by their balance of riverine and tidal flows. An estuary dominated by river discharge is a stratified or salt wedge estuary. Since fresh water is lighter than sea water, river flow coming down the estuary will override the salt water, and salt water being pushed upriver by tides will sink. Looking from the side, the salt water forms a wedge underneath the fresh water. The fresh and salt waters form distinct layers within part of the estuary. Eventually the salt and fresh waters mix, although fresh water may remain at the surface for some miles outside the river inlet.
The Cape Fear estuary is a well mixed estuary. In this type of estuary the tidal forces are stronger, so salt water flows with more volume and velocity up the river. This increased flow causes turbulence within the estuary, which mixes the water so that no layers are formed. In this type of estuary the entire water column, not just a bottom layer of salt water, moves upriver during flood tides. So the mystery of Allen Creek is solved. (Of course, over time, the net movement must be out to sea.)