Particles suspended in the river (such as silt, sediment, or plant material) determine the water's clarity (also called turbidity). Simply put, the higher the turbidity, the murkier the water. High turbidity can diffuse sunlight as it penetrates the water's surface, possibilty even affecting the local flora and fauna.

On the Half Moon, we use a Secchi disk to measure the Hudson River's turbity, although this device would have been an anachronism in 1609. Hudson's voyages predate the invention of the Secchi disk by more than two centuries; in the 17th century, sailors would simply keep an eye on objects floating beneath the water's surface -- an inexact, "rule of thumb" approach.

Mouse over to use
the Secchi disk.

A Secchi disk works like a lead line, except that instead of flinging a lead line overboard, its user simply lowers the disk into the water.

Once the Secchi disk itself sinks out of view, the user checks the markings on the line to see how far down into the water the disk went before it faded from sight. The user takes a second reading when they pull the disk back up, then compares the results.

On this Voyage of Discovery, Jena and Natalie initially chose to measure turbidity in New York Harbor for their student presentation, but ultimately opted to switch topics after a morning of static results (to are to be expected in these conditions). Whenever the ship was stationary, we also checked turbidity levels for the Deck Log.

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