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River Science: Salinity

Nadia collects water with a canvas bucket.

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.

Ms. Cowles oversees as Alanna applies a water sample to a refractometer.

To measure the salinity gradient as we travel through the marine environment of Long Island Sound and up into the fresh waters of the Connecticut River, 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.

These water samples are then collected in sample jars for analysis. We continue to collect samples regularly until the salinity gradient consistently drops to zero, which is to say, until the water is completely fresh.

Jose uses a refractometer.

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 with just a glance.

Sailors on board the original Half Moon in 1609 would have had simply gone by taste, as the crew of one of our Zodiac expeditions independently remarked upon after a particularly choppy return trip to the ship in Niantic Bay.

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