Salinity

Here at Beczak, we are often asked the question “Is there salt in the Hudson River?” The answer is yes and no. There are actually three types of water in the Hudson, each one containing different amounts of salt: freshwater, saltwater, and brackish water.

Freshwater enters the river through precipitation (rainfall and snowmelt) and also through groundwater. Freshwater contains very little or no salt. Saltwater enters the river from the Atlantic Ocean. At high tide, saltwater flows north from the ocean into the Hudson River and mixes with freshwater to create brackish water, the third type of water in the river. This mixing of freshwater and saltwater, along with the tides, is what makes the southern 153 miles of the Hudson River an estuary. Estuaries like the Hudson River are highly productive and diverse ecosystems.

Salinity, or the amount of salt that is in water, can be measured in units called parts per thousand (ppt). One part per thousand represents a proportion of salt to water equal to one gram of salt in one liter of water. Consequently, two parts per thou- sand equal two grams of salt in every liter of water. Freshwater has a salinity scale from 0 -0.5 ppt. The salinity of saltwater in the open ocean is between 33 -35 ppt. Brackish water corresponds to the intermediate range between fresh and ocean water.

Freshwater dominates the northern part of the Hudson River. The transition from fresh to brackish water is called the salt front, and occurs where the salinity reaches 0. I ppt. Typically, during the summer and early fall, the salt front is located around the cities of Newburgh and Beacon. Precipitation increases the volume of freshwater in the river, diluting the salt concentration and pushing the salt front further downstream. During spring runoff or major storms, the salt front sometimes reaches south of the Tappan Zee Bridge. Drought and heat will decrease the volume of freshwater in the river, pushing the salt front further north. The Hudson River in Yonkers is usually brackish. However, large amounts of precipitation can push the salt front so far south that we can have freshwater in Yonkers. In fact, this occurred after the major storms we experienced earlier this past summer. You can see this for yourself by looking at the salinity data online for June 30th through July 3rd. Keep an eye on the salinity of the river to watch it change throughout the seasons. Can you guess what the salinity is today?