Lamont Team Discovers Oyster Beds, Wrecks and a Tsunami

On Sunday, October 25, at one of the Palisades Free Library's Sunday Symposia, Bill Ryan gave us fascinating glimpses of the floor of the Hudson River. Bill, a Doherty Senior Scholar and Adjunct Professor Emeritus at Lamont-Doherty Earth Observatory, described the findings of a recent project intended to increase our understanding of the Hudson habitat. Originating with the Riverkeeper, the project was funded by the Department of Environmental Conservation.

As part of the project, Lamont scientists and their instruments were involved in mapping the river floor and gathering information on the sediments below. They worked in the early spring season over a period of three or four years, going back and forth across the river in an area from the Verrezzano Bridge to the dam at Troy, then up and down the river. Sonar, used to look at the riverbed, showed them sand waves and wrecked ships, including 150 sunken barges. A multi-beam echo-sounder was used to establish the depth of the river and a sub-bottom profiler gave information about the sediment layers.

In the course of their work, the scientists identified a strange pattern of alternating reflective bands crossing the river. When, in the second year of the survey, they began to take samples from the bottom, they discovered that these bands were made of oyster shells. The oysters had established their beds in areas undisturbed by streams and small rivers entering the Hudson and bringing silt that might cover them. Most early travel in our area was by boat. But mooring a boat above an oyster bed was difficult, as opposed to setting an anchor in the sediment off stream entrances. So early settlements like Nyack and Sleepy Hollow were placed between oyster beds, near stream mouths.

Cores taken from below the river floor found a layer of dirt and leaves deep in the sediment that was later dated to 400 BC. The layer exists throughout the bodies of water in our area and is hypothesized to be the result of a large tsunami that would have washed right over Manhattan Island. Very large oyster shells were found in the sediment below this layer, some of them seven thousand years old; smaller ones appeared after the tsunami.

The river bottom continually changes its shape, affected by weather, tides, and alterations in the shoreline. New York City has filled in many areas to create more land for building. Debris taken from the foundations of the World Trade Center when it was being built during the 1960s was later used to extend the Manhattan shoreline for the construction of Battery Park City. This constriction of the Hudson River caused it to cut a deep channel that creates a potential threat to the security of the Lincoln and Holland Tunnels.

Dating of the bottom sediments has revealed that, in the area between Manhattan and the Newburgh bridge, 80% of the riverbed dates back to a time before Henry Hudson╩╝s arrival. The incoming and outgoing tide scours the river bottom, eventually sending most contamination out to sea. Oysters are returning to the Hudson as its waters become cleaner. Part of this is due to restrictions on pollution and part is due to water filtration by an invasive species, the zebra mussel, which produces problems for boaters and power plants but is actually helping to clean the river. Who would have dreamed of the history under the surface