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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 "salt" or "fresh" water. It's just as important for sailors (of any era), who need a fresh water supply to survive.

To measure the salinity gradient as we travel downriver, our students collect water samples at a series of predetermined locations, which our students find using landmarks (such as buoys and river markers) and GPS coordinates.

On the Half Moon, we collect 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 by the entire student crew as part of our Deck Log duties. We started this Voyage of Discovery well into the freshwater portion of the estuary, and will continue collecting samples until we reach the river's mouth at New York Harbor.

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 to simply go by taste.

In addition to our Deck Log samples, on this Voyage of Discovery Bennett and Jonathan have chosen to study salinity for their student presentation. As we traveled down the Hudson, the crew took readings at every minute of latitude, the equivalent of one nautical mile. During our data layover at Grave's End Bay, Bennett and Jonathan also took readings every hour to track the tidal cycle.

Our results on this voyage were typical for our experiences; salt was first detected in the river off Beacon/Newburgh, rising steadily from there as we continued toward the ocean.

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