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River Science: Salinity

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.

Salinity levels have an immediate effect on the surrounding ecology, since many aquatic creatures can survive only in either marine, brackish, or fresh environments. It's just as important for sailors (of any era), who need a fresh water supply to survive.

To measure the salinity gradient, our students collect water samples at regular intervals. On the Half Moon, we collect surface water samples the old-fashioned way: by simply heaving a tethered canvas bucket overboard and hauling it back on board.

Students use a refractometer to measure salinity levels using refraction: the degree to which light bends as it passes through different materials. Saltier water bends light farther than fresher water, so a trained refractometer user can check salinity levels (measured in ppt, or parts per thousand) with just a glance.

Sailors on board the original d'Halve Maen in 1609 would have had to simply judge the water's salinity using their sense of taste.

On this voyage, the research team of Anthony, Dominique, Elisa, and Jazmine studied salinity for their presentation project, collecting samples as the ship traveled from Gravesend Bay to the Palisades. Their results were unusual compared to the results of past voyages; usually, the Hudson River reaches 0 ppt (completely fresh water) near Newburgh Bay, but in this case, the water turned completely fresh shortly after entering the Hudson River. Why? Rain! Runoff from Hurricane Irene and subsequent recent storms is still coursing through the Hudson River watershed.

The team recorded their most significant change in salinity while the ship was anchored at Bay Ridge Flats. The team took water samples as a tidal boll passed under the ship a visually distinctive swell of water formed as the incoming seawater pushes its way into the outflowing river water. Within a span of 20 minutes, the team recorded a drop in salinity of 6 ppt quite a dramatic difference!

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