All rivers that reach the sea have ocean water at their seaward ends, and freshwater at their sources. A trip up a river takes you along a gradient of salt concentration from near 3.5 percent (the average salinity, or salt content, of seawater) to zero. There is also a gradient of tidal influence from strong near the coast to zero at some point upstream. Tidal range and salt content have controlling influences on the types of wetlands that grow along coastal rivers.
Near the mouths of such rivers, tidal currents move sediments on the river floor too frequently and too fast for large plants to become established. As a result, the inlet regions of rivers rarely have attached plant communities. A short distance upstream, however, the situation changes dramatically. Tidal currents slow as they spread laterally into protected coastal waters of bays and estuaries and the salt-tolerant plants of salt marshes can become established.
Estuarine marshes often extend over huge areas where sediments brought in by upstream currents of rising tides settle to form sand and mud flats between high and low tide level. These flats can be colonized by plants to form a salt marsh.
Further upstream in blackwater rivers the diminishing amount of seafloor sediment supplied by rising tides results in diminishing areas of salt marsh. If you look at a map of North Carolina, you will see large areas of open water in downstream stretches of our blackwater river estuaries. The sediment supply problem does not occur along the edges of estuaries — here, local runoff provides sediment to build intertidal areas suitable for salt marsh development — but the species of plants that inhabit them change as the salt content of the water declines upstream.
Near the point where salt disappears from the river altogether, there is another concentration of riverside marshes. These marshes form on sediments that settle out of the water as a result of changes in water chemistry. Where salt is present, the electrical conductivity of salt water drains away repelling charges on sediment particles to create this sediment dump at the heads of estuaries.
Upstream from estuarine marshes, ecological changes occur in the river and different plants colonize the riverbanks. Which plants grow where depends on river water chemistry and competition between species.
Let’s go look at a transect of decreasing salinity up the White Oak River.