Why does seawater near large rivers often have lower salinity?

Seawater near large rivers often has lower salinity because of the dilution caused by the influx of fresh water. While open ocean water typically contains about 3.5 percent dissolved salts, river water has a much lower salt content, averaging only about 0.012 percent.123

When these two water types meet in an estuary, the fresh water—being less dense—tends to form a distinct layer that floats on the surface of the more saline, denser seawater. The extent to which this salinity is lowered depends on the volume of river discharge and the degree of mixing between the two water bodies. In areas where the ocean is partially enclosed or isolated by land geography, such as the Baltic Sea, the impact of river runoff on lowering salinity is particularly pronounced.1425