CLEARWATERS: Mapping the Hudson Estuary's submerged lands

by John W. Ladd, PhD; Robin E. Bell, PhD; Elizabeth A. Blair, PhD; Henry Bokuniewicz, PhD; Suzanne Carbotte, Phd; Robert M. Cerrato, PhD; Steven Chillrud, PhD; Vicki Lynn Ferrini; Roger D. Flood, PhD; Nicole P. Maher; Cecilia M. G. McHugh, PhD; Frank O. Nitsche, PhD; William B. F. Ryan, PhD; David L. Strayer, PhD; JoAnn Thissen; Roelof Versteeg, PhD

In 1998 the NYS Department of Environmental Conservation began mapping the submerged lands of the Hudson River Estuary. Our work was conducted through the Hudson River Estuary Program and the Hudson River National Estuarine Research Reserve. The project's initial phase in 1998 and 1999 mapped 40 mi of the river (about one-third of the area of the estuary) using a suite of geophysical tools such as multibeam swath mapping, sidescan sonar, and subbottom profiling.

The geophysical data have been supplemented with sediment profile imagery (SPI) and sediment sampling (cores and grabs). In 2001 we mapped another third of the estuary and conducted pilot studies of temporal change and analyzed invertebrate habitat. Data products include acoustic images and interpretive maps. The maps show anthropogenic deposits, recently deposited fine-grained sediments, sediment grain size, bedforms, and river bottom morphology.

This project is the first-ever comprehensive mapping of the Hudson River Estuary. The resulting data have great utility in management of this unique natural resource. They will be used to understand and manage the habitat of species of interest in the estuary as well as to understand and manage sediment and contaminant transport. In addition, the images and maps should prove useful in understanding change over time in the estuary and in enforcing the laws that govern human activity in the estuary.

Land management tools

It is axiomatic that land managers must be able to see the land that they are managing. For decades land managers have relied on aerial photography and interpretive maps of land features to do their job. The agricultural sector routinely uses such tools to understand the distribution of soil types and the status of grazing lands. Regional planners use them to understand the distribution of land uses.

Before the benthic mapping project, no images or interpretive maps of the submerged lands of the Hudson River Estuary were available. The turbidity of the river makes aerial photography useless when water depths exceed a few feet. Even in shallower water, aerial photography is unable to distinguish bottom types.

This is not to say that we have no idea of the estuary floor. Numerous sediment sampling programs have been directed primarily at understanding contaminant distribution in the estuary. Without imaging of the estuary floor, however, the results of the sampling can often be misinterpreted.

To appreciate how we developed an understanding of the estuary floor before the benthic mapping project, consider how we would understand the dry land in the Hudson Valley if we were blind and only could take samples from a blimp above a cloud cover. We might get a sample of a roof or a road or a field or a forest. Using good statistical sampling techniques we might infer the proportions of the land covered by roofs, roads, fields, and forests. We probably would not be able to tell that roads are generally linear features and that roofs cluster along roads. It would be extremely difficult to determine whether the amount of forest was changing over time. If we could determine change, it would be even harder to understand why it was changing.

Using other sampling techniques we might discover that there are deer and people on the land, but we would have trouble learning that people live under roofs and that deer live in the forests and fields. It is unlikely that we would discover that people spend most of their time under roofs. Our sampling procedures would probably only sample people while they were not under roofs.

This is exactly how we had to learn about the submerged lands in the estuary before the benthic mapping project.

The reality of the world is that we are not blind and are not sampling the land surface from above a cloud cover. We can see and accurately map the relationships of roofs, roads, forests, and fields. We can see change over time, and we can see how deer and people use the land. This reality does not extend to the submerged lands of the Hudson River Estuary.

Water, though often opaque to light, conducts sound very well, and we can use sophisticated sonar systems to create acoustic analogs of aerial photographs. This is the essence of the benthic mapping project. We are using sound to make an image of the river bottom in the same way that we have used aerial photography for many decades to make images of the land surface.

Subbottom acoustic file showing deposition

Not only are we developing images of the estuary floor, but we are also conducting sampling to understand what these images mean. Extensive previous sampling in parts of the estuary did not necessarily yield the kind of information required to understand acoustic images.

To make acoustic images, we use sonar systems that send sound toward the river bottom and measure the energy reflected from the bottom. We record the time it takes for sound to travel to the bottom, the time to travel back, and the strength of the reflected sound energy.

The strength of reflected energy is controlled by physical properties of the river bottom including the distribution of grain size in the sediments that compose the river bottom, the cohesiveness of the sediments, the roughness of the river bottom at various scales, and any plants or animals living on the river bottom. Our sampling program is designed to measure these variables so that we can understand our acoustic images and make interpretive maps of parameters of interest.

Our sampling program is designed to allow us to interpret the acoustic images in terms of grain size and the timing of erosional and depostional events as well as other variables of interest. For instance, it is known that contaminants adhere preferentially to fine-grained sediments. By mapping fine-grained sediments, we are making rather good maps of contaminant distribution. We know that sturgeon build nests from gravel. When we map the distribution of gravel beds, we are mapping potential breeding habitat for sturgeon. When we map areas of erosion or areas of deposition, we are mapping how the river is likely to change over time and the pathways for sediment transport.

Habitat

Many species of interest depend for food on the invertebrate animals that live on or in the river bottom. These species include sturgeon, tomcod, white perch, young-of-the-year striped bass, herring, and shad. These fish eat a variety of insects and crustaceans that live at least a part of their lives in or on the sediments.

The distribution of this fish food is controlled by environmental factors including the proportions of mud and sand, sediment organic content, micro-organism content, sediment stability, and water movement above the sediment. If we can map these variables, we are a step closer to mapping areas of importance for maintaining fish stocks in the estuary.

Various fish species also depend on the configuration of the river bottom for refuge. Smaller fish often retreat to the shallows to avoid larger predators. In winter when fish are less active, they may seek deep areas away from the river currents where they avoid being swept from the estuary with little or no expenditure of energy. Our detailed bathymetric mapping is helping us to find and, eventually, to protect these refuges.

Sidescan mosaic of Newburgh Bay

Measurement techniques

We employ several remote sensing techniques to image the estuary floor. They include sidescan sonar, multibeam swath sonar, subbottom profiling sonar, and ground-penetrating radar. Sidescan sonar and multibeam swath systems produce map views of the estuary floor. Subbottom profiling sonar and ground-penetrating radar produce images in the vertical plane (profiles) of the sediment structures beneath the estuary floor. As usual there are tradeoffs with all systems.

The sidescan system permits us to image the riverbed in shallow (less than 15 ft) as well as deep water. Our sidescan system does not, however, provide a quantitative measure of water depth. The multibeam swath sonar system permits us to image the river bottom as well as to provide quantitative water depth measurements for every square meter of riverbed, but it is only feasible for use in water depths greater than about 15 ft. Since about 50% or the estuary from Yonkers to Catskill is less than 15 ft deep, the multibeam system is limited in aerial coverage compared to the sidescan system.

The subbottom profiling systems are most useful for discovering areas of the river that have experienced recent erosion or deposition. The acoustic profiling system is useful in all water depths but does not work well where the river bottom is hard, nor does it work well where sediments contain appreciable gas, which often results from the decomposition of organic matter. The ground-penetrating radar is not hindered by hard bottom or the presence of gas but does not work in the more saline parts of the estuary or in water depths greater than about 15 ft.

Development of sediment profile images

To get a closer look at small pieces of real estate, we have deployed a special camera system to acquire sediment profile images (SPI). These photographs let us see a profile or cross-section of the upper few inches of the sediments, and they enable us to determine the type and number of animals living in the sediments and on the sediment surface. The SPI images also enable us to see the depth of the oxygenated layer of sediments that supports bottom-dwelling animals, the distribution of burrows, gas pockets, sediment structure, and sediment surface roughness. These observations are crucial in helping us to interpret our acoustic images.

Sediment cores

We take sediment cores and sediment grabs to obtain quantitative measures of grain size, sediment composition, and sediment structure Again there are tradeoffs. The sediment cores preserve the structure of the sediments and, to some extent, the history of deposition at a site, and they allow us to measure composition and grain size. Grab samples, on the other hand, only allow measurement of composition and grain size. The trade-off is that the grab samples are cheaper to acquire and process.

Recently, we began a pilot study in which we are collecting sediment samples from different bottom types to determine associated invertebrate animal populations. Biologists working together with geophysicists, geologists, and geochemists are attempting to use our acoustic images to map benthic habitats.

Results to date

Mapping the bottom of the Hudson estuary is an ongoing project, and we have just begun to assimilate the various data sets. We have developed sediment classification maps for a third of the estuary including the Tappan Zee, Newburgh Bay, the reach from Kingston to Saugerties, and the reach from the City of Hudson to New Baltimore. These reaches of the estuary were chosen for the initial phase of mapping because they represent different physical environments in the estuary:

Prior to the current mapping project these generalizations pretty much summarized our knowledge of the river.

Our sediment classification map of the Tappan Zee reveals, not unexpectedly, the deep main channel floored with coarse-grained sediment and broad shallow banks dominated by finer-grained sediment. Surprisingly, there is very little recent sedimentation in this stretch of the river. Also surprisingly, we have mapped large patches of dead oyster reef in places buried by a few meters of sediment. The radio carbon dating of these reefs is still in progress, but we have dates ranging from several hundreds of years to ten thousand years. One of the unsolved mysteries of the lower Hudson Estuary is what controls the existence of living oyster reefs.

Reef and grain-size mapping

In Newburgh Bay we have mapped a deep channel containing coarse-grained sediments with fields of sediment waves flanked by finer-grained margin deposits that also contain fields of sediment waves. Ribbons of sediment extend over a mile down-stream, south of the abutments of the Newburgh-Beacon bridge. These sediment ribbons were deposited in the lee of the bridge abutments. In Newburgh Bay we also see lobes of sediment deposited at the mouths of Moodna Creek and Wappinger Creek. These sediment lobes have been spread laterally north and south of the creek mouths by tidal currents.

In the Kingston to Saugerties reach, we again see deep coarse-grained sediments that form sediment waves that are, in places, over a meter high. Finer-grained sediments have accumulated on marginal flats that are dominated by submerged aquatic vegetation in many places.

In the reach north of the City of Hudson, the estuary is dominated by the channel that is floored with sandy sediments. In most places, these sediments form extensive wave fields. Margin deposits are relatively minor compared to farther south.

Throughout the estuary we see evidence of human activity. In many of the sediment samples, we find coal and cinders from the days of coal-burning engines. We also find shards and even fields of debris and evidence of dumping in the form of doughnut-shaped features that we think formed when a load of material was dumped on the river bottom. We can see scars left from laying pipelines and cables across the river. In the Tappan Zee there is a large debris field off Hook Mountain where, presumably, loads of quarry rock were dumped inadvertently when the Palisades were being quarried in the 19th century. In Newburgh Bay we see fields of doughnut-shaped features particularly in the vicinity of the Roseton power plant. Near Kingston just north of Rondout Creek, we see the scar of a buried pipeline crossing the river. North of the City of Hudson are numerous debris fields on either side of the channel.

Benthic mapping of the Hudson Estuary is a new tool for environmental analysis. The detail and variety of data scientists now have promise to lead to new insights about the river's dynamics, its life forms, even its past and future. It is a potent tool that will assist in managing this important resource.

Suggested reading

Boyle, R.H., 1969, The Hudson River: A Natural and Unnatural History, New York: Norton & Co.

Limburg, K.E., M.A. Moran, W.H. McDowell, 1986, The Hudson River Ecosystem, New York: Springer-Verlag, 331 pages.

Stanne, S.P., R.G. Panetta, B.E. Forist, 1996, The Hudson: An Illustrated Guide to the Living River, New Brunswick, NJ: Rutgers University Press.

Strayer, D.L., N.F. Caraco, J.J. Cole, S. Findlay, M.L. Pace, 1999, Transformation of freshwater ecosystems by bivalves, BioScience, 49, 19-27.

Benthic mapping online (Long Island Sound)

John W. Ladd, PhD (corresponding author), after receiving his doctorate in marine geology and geophysics from Columbia University in 1974, worked as a research scientist at the University of Texas and Columbia University conducting studies in marine geology using the tools of reflection seismology. Since 1997 he has served as habitat restoration coordinator and benthic habitat coordinator for NYSDEC helping manage the agency's wetland restoration and benthic mapping projects on the Hudson. Phone 914-923-1108. Click here for additional information (opens new browser window).

Elizabeth A. Blair, PhD, is a biologist who has managed the Hudson River National Estuarine Research Reserve program since 1985, including a variety of Hudson River research, habitat inventory, education, outreach, land management, and stewardship programs.

Henry Bokuniewicz, PhD, is a professor of oceanography specializing in marine and coastal geophysics. He is also director of Stony Brook University's COAST Institute.

Suzanne Carbotte, PhD, is a marine geoscientist at Lamont-Doherty Earth Observatory. Her current research involves the use of sound sources to study sedimentary processes in the Hudson River estuary.

Robert M. Cerrato is a benthic ecologist interested in population and community dynamics. He is an associate professor at the Marine Sciences Research Center, State University of New York at Stony Brook.

Steven Chillrud, PhD, has worked on Hudson River sediments and associated contaminant issues since 1992. He currently is an associate research scientist at Lamont-Doherty Earth Observatory of Columbia University.

Vicki Lynn Ferrini is pursuing her PhD at the Marine Sciences Research Center at Stony Brook University. Her current research focuses on the use of multibeam sonar as a tool for understanding nearshore sedimentary environments.

Roger D. Flood, PhD, is a professor of oceanography at Stony Brook University with research interests that include sediment dynamics and high-resolution imaging of coastal environments.

Nicole P. Maher is pursuing her PhD at the Marine Sciences Research Center at Stony Brook University. Her current research focuses on the development of a method to identify and map benthic habitats based on multivariate environmental and biotic data.

Cecilia M. G. McHugh, PhD, is an Associate Professor at the School of Earth and Environmental Sciences, Queens College and an Adjunct Research Associate at Lamont-Doherty Earth Observatory of Columbia University. One area of her research is sediment transport and deposition in the Hudson system and on the adjacent continental margin.

William B. F. Ryan, PhD, is Doherty Senior Scholar at Columbia University with research interests in ocean margins, estuaries, and rivers.

JoAnn Thissen is a sedimentologist interested in sediment transport and deposition. Formerly at the Marine Sciences Research Center, SUNY at Stony Brook she is presently at the Liberty Science Center.

Roelof Versteeg, PhD, is a near surface geophysicist who pioneered the use of GPR in the Hudson river. He currently is working on imaging of processes in the near surface at the Idaho National Engineering and Environmental Laboratory.