Oysters prefer brackish and salty water. A salinity range between 10-28 parts per thousand (ppt) provides the best conditions for oysters. Oysters can die if salinity drops below 5 ppt for extended periods of time.

Scientists use a small, long, hand-held device called a refractometer to measure salinity.

The Chesapeake Bay’s overall average salinity ranges from 13-17 parts per thousand (ppt), although salinity in the Bay varies widely depending on location, weather and season.

Fresh water enters the Chesapeake Bay primarily from its rivers and streams. Fresh water also enters via rain, snow and groundwater.

Salt water enters the Chesapeake Bay from the Atlantic Ocean, at the mouth of the Bay near Hampton Roads, Virginia.

In general, the lower Chesapeake Bay is salty and the upper Bay is fresh. Salinity gradually decreases as you move north and increases as you move south.

The Chesapeake Bay’s salinity ranges from fresh water (less than 0.5 parts per thousand) to salt water (25-30 ppt). The Bay's salinity is highest at its mouth, where water from the Atlantic Ocean enters. The water at the head of the Bay is fresh. The water in the middle portion of the Bay is brackish: a mixture of salt and fresh water.

The Chesapeake Bay’s salinity gradually decreases as you move north and increases as you move south. Salinity also varies widely from season to season and from year to year, depending on the amount of fresh water flowing from the Bay’s rivers.

The water at the mouth of the Chesapeake Bay is salty because it is mostly made up of salty water from the ocean. As you move north in the Bay, the water becomes less salty. Most of the Bay is brackish (a mixture of fresh and salty water).

The Chesapeake Bay is saltiest at its mouth, near Hampton Roads, Virginia. This is where salty ocean water comes into the Bay. The Bay’s salinity decreases as you move north.