The Hudson River is an estuary: a freshwater river that flows into the ocean, where it mixes with salt water. The degree of mixture between fresh and salt water is called the salinity gradient. On the Hudson, the salinity gradient at a given location can vary considerably depending on factors such as time of year, tidal flow, and precipitation. It's a different story on the Connecticut River, which was once called the Fresh River for a reason: so much water flows out through it that the water is fresh little more than a mile from Long Island Sound.
Salinity levels have an immediate effect on the surrounding ecology, since many aquatic creatures can survive only in either "salt" or "fresh" water. It's just as important for sailors (of any era), who need a fresh water supply to survive.
On this voyage, we measured salinity levels as a component of the Deck Log.