Exploring the Hudson—from the bottom up

Cecilia McHugh turns the clock back 6,000 years to see into the climatic future.

Cecilia McHugh knows the Hudson River better than Henry Hudson ever did.

McHugh, a member of The Graduate Center's doctoral faculty in Earth and Environmental Sciences and a professor at Queens College, is one of the metro area's few experts on the river's shape, sedimentary composition, and ecological history.

The valley, McHugh explains, was scoured out and formed by great glaciers that rumbled south from the polar regions approximately 20,000 years ago, obliterating everything in their path. At one time, half of North America was submerged beneath a mile-thick sheet of ice. When the glaciers finally retreated, perhaps 15,000 years ago, they left behind a series of high lakes; later, when water began pouring south from the Great Lakes region as more ice melted, the water level rose, breaching natural dams and forming one continuous river that mostly hides this history of ice and lakes.

It's McHugh's job to bring that history back to the surface--and to try to use it to understand natural cycles of precipitation and drought in the region.

Sampling the bottom of a bathtub

During the summer months, McHugh heads out onto the river in the rented R/V Lionel Walford, a shallow-draft research vessel berthed in New Jersey. On board: a handful of graduate and undergraduate students, a series of heavy-duty gravity corers, and enough food to last a full day of trawling the river bottom.

Cruising the channel, McHugh and her students first make a pass using SONAR geophysical measurements--supplied by Robin Bell and others at Columbia University's Lamont-Doherty Earth Observatory, in collaboration with Queens College and the New York State Department of Environmental Conservation--as a kind of 'road map' to the river bed. Using the map, McHugh carefully plots out sampling locations.

"The sonar images let you see the bottom without the water," she says. "This allows us to decide where the channel is, and where the best places are to take core samples."

Armed with this detailed map of the bottom's nooks and crannies, McHugh and her team return to the same stretch of river for another long day's work. This time, they scoop up the mud piled and spread on the river's bottom. Back in her Queens College laboratory, McHugh analyzes the sediment for density, magnetic properties, and grain-size variability--which give her clues to the sedimentation processes of the river.

"To obtain a whole climatic record for the valley, we need to first understand the sedimentation," explains McHugh. "It's very important to figure out where to take core samples."

That's because sediments are not evenly distributed across the bottom of the river. Though the Hudson isn't a heavily sedimented river like, say, the Mississippi, it still filled relatively quickly with sediments. Think of it as a narrow bathtub: once sediments have filled up the groove cut into the river bottom, any extra volume will push out to Staten Island--or out to sea--rather than being laid down on top of older mud and sand. As a result, some stretches of river contain continuous historical records of sediment, but other stretches don't.

Surprises and insights

Peering back 6,000 years into the river's past, there are bound to be a few surprises. And, indeed, McHugh's sediment research has turned up some unexpected insights into the environmental history and health of the river. She comes across plenty of magnetite, for instance--a mineral produced as a by-product when coal is burned at high temperatures. It's a souvenir from the coal-burning heydays of the Industrial Revolution. And she still comes across radioactive cesium-137 isotopes produced during worldwide nuclear testing in the 1960s, as well as some inadvertently released from the riverside Indian Point nuclear reactor facility in Westchester County in the 1970s.

All these events were recorded in the river's sediments and provide clues about its age--and its future. One of two things is likely to happen next: as sediments continue to fill the Hudson's 'bathtub,' islands and marshes will form (as they are now forming upriver, today north of Kingston); someday in the distant future, there may be lakes once more in the valley, and no river. The second possibility is that sea levels may continue to rise as a result of global warming, expanding the space for sediments on the river bottom--and keeping the river rolling freely.

After building a historical profile of the sediment from the cores, McHugh examines the mud's composition itself, dissolving away much of it with laboratory chemicals to leave only diatoms--a tiny plant, of which there are three major types: marine, freshwater, and brackish-water. The prevalence of one or another type indicates the saltiness of the water at the moment in question, and by mapping salty areas versus fresh areas--salty water means a dry year, because the ocean would have flowed farther upriver to fill in the lower water level--then matching the data with similar profiles being developed for Chesapeake Bay, McHugh can determine river volumes from year to year or decade to decade. And that indicates periods of drought, normal precipitation, or unusually heavy rain.

In some years, she found, the Northeast was so dry that salty

waters rolled all the way north to Newburgh, more than 60 miles upriver from the Statue of Liberty,

"There is a definite cyclicity to the region's climate," explains McHugh. "Every one thousand years, there is a drought period of roughly 300 years, and for every 100 years, there seems to be a drought cycle of thirty years. We seem to be just coming out of one--the 1960s were very dry, and there was a smaller drought in the 1970s."

When she completes the study, in conjunction with researchers from Lamont-Doherty, Queens College, and Hunter College, the resulting data and comprehensive maps of the river will be released to the public. Environmental groups are already lining up to download the massive new archive of climate data, pollution hotspots, and other ecological processes McHugh's work will help uncover.

"This all started with a grant from CUNY in 1998," points out McHugh, whose work is also funded by the Hudson River Foundation and the National Science Foundation. "We obtained our first ship time and did some initial coring with that grant. We're quite thankful for that. This is a perfect example of you can get something started with very little money, and then build upon it."