Salinity is a key factor influencing the physical make-up of the Bay. Salinity is the number of grams of dissolved salts in 1,000 grams of water. Salinity increases with depth. Therefore, freshwater tends to remain at the surface. Seawater from the Atlantic Ocean enters the mouth of the Bay.
Salinity is highest at that point and gradually decreases as one moves north.
Salinity levels within the Chesapeake vary widely, both seasonally and from year to year, depending on the volume of freshwater flowing into the Bay. On a map, isohalines or salinity contours mark the salt content of surface waters. Because the greatest volume of freshwater enters the Bay from northern and western tributaries, isohalines tend to show a southwest to northeast tilt. The rotation of Earth also drives this salinity gradient. Known as the Coriolis Force, it deflects flowing water to the right in the Northern Hemisphere so that saltier water moving up the Bay is deflected toward the eastern shore.