Earth Institute News
posted 01/04/01 2:OO P.M. EST
Mud Yields Ghosts of
"It's clear that there are storms that go through and erode sediments," Dr. Bell said. "Stuff moves."
A G.E. spokesman, Mark L. Behan, said that the upper Hudson is in many ways a different river from the one being studied by Dr. Bell's group. The area proposed for dredging - a chain of PCB hot spots extending from Troy to the company's old factories in Hudson Falls and Fort Edward - is marked by an interconnected system of dams and locks and canals that collectively make the waters far more placid than they are to the south.
Recent studies by both the government and G.E. have suggested that floods are more likely to add new sediment to that part of the river than take it away, Mr. Behan said. The upper Hudson has largely been tamed.
Here to the south, especially on a blustery day, the Hudson still feels like a wild place, even though New York City is only a few miles away and the great span of the Tappan Zee Bridge dominates the horizon. Mergansers, graceful ducklike birds that fish these waters, float quietly by, unperturbed by the 39-degree temperature of the water. Fish of various sizes drift under the boat.
Other wonders of the river, like the salt wedge, are invisible. The salt wedge is the layer of ocean water that moves upriver with the tide. Because it is denser than fresh water, the wedge slides underneath the river's surface as it plows north, adding its own layer to the "cake." But the salt layer also creates a band of turbulence that can distort a sonar image, so it too must be understood so scientists can read the map results.
This day's work mostly consists of measuring the depth and strength of the salt current. Jay Ardai, an engineer-technician who hangs over the stern in an orange survival suit, dips various instruments into the water while Dr. Bell records the results. At the wheel, the Fletcher's captain, John Lipscomb, keeps track of the boat on a global positioning screen so that he can maneuver it for the next test.
The core drilling samples, like the salt wedge studies, are another way to provide backup verification, or "ground truth," as it is called, for the sonar images. But the cores also allow Dr. Bell to create a timeline of the river's history.
Scientists analyzing the samples at Columbia's Lamont-Doherty Earth Observatory in Palisades, N.Y., have identified a band of radioactive cesium in the river mud. They say that the cesium marks the period of above-ground nuclear testing in the 1950's and 1960's, a snapshot of the earth's atmosphere taken by the river at that one moment in time and then sealed on the bottom.
Dr. Bell sees a giant historical tapestry in all this. She became enthralled with geology when she learned about plate tectonics as a teenager and never quite got over it, she said. Examining a core sample at the Lamont-Doherty campus, she gently traces a finger down through the geologic record: a shell deposited in the river around the time of Jesus, a band of gravel, a compressed and preserved twig.
Now, standing on deck of the Fletcher - named for a conservationist who worked for many years with Riverkeeper, the boat's owner - Dr. Bell marvels at the ancient echoes of these waters. Since the Palisades on the river's western bank in Rockland County rose up in a volcanic eruption 180 million years ago, she said, the Hudson has been a timeless constant in the region. There are even different kinds of rocks on each side of the river, she says.
Daniel Wolff, a writer from Nyack who has come out on several mapping trips as a volunteer, stood near the stern, watching birds with his binoculars.
"They get so absorbed in looking at their computer screens that they don't notice the nature around them," he said, nodding toward the scientists.
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Columbia Hudson Research
Norman Y. Lono for The New York Times
Jay Ardai, a Lamont engineer on the Fletcher, measuring the Hudson River's depth, temperature and conductivity.