Tidal Rivers and Main Bay watershed estuary tributary . The tidal portion of the Chesapeake Bay watershed is subject to the ebb and flow of ocean tides. This area encompasses all of the mainstem Bay and the area north and east to the fall line. The fall line is a physical barrier on the Bay's larger tributaries marked by waterfalls and rapids. One of the most important characteristics of the tidal area is the wide range of salinity from the fall line to the Bay's mouth at the Atlantic Ocean.
Salinity also affects the physical processes in the Bay.
As freshwater from the tributaries flows toward the Bay, it mixes with saltwater. That generally means that the closer water is to the Bay's mouth and the Atlantic Ocean, the saltier it will be. Saltwater also is denser than freshwater. Flows tend to higher in the spring and lower in the summer. In deeper areas of the Bay, this difference in density can lead to the formation of a pycnocline, which is a layer in the water column separating two areas of different density.