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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. It's a different story on the Connecticut River, and no coincidence that Dutch explorers dubbed it the Fresh River; so much water flows out through it that the river's waters are fresh little more than a mile inland 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.

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.

On Day Three, we collected water samples as we came into the Connecticut River, starting while we were still a mile out in Long Island Sound. We made a special effort to collect samples as we collected the tidal bore formed by the river's mouth, thinking it might be possible to find a notable drop in salinity from one side to the other, but in fact the salinity levels remained steady on both sides of the bore. By the time we reached the dock at Old Lyme, however, the water was completely fresh.

Students use a refractometer (pictured here) 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 d'Halve Maen in 1609 would have had simply gone by taste.

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