The Ecological Impacts of 200 Years of Hudson River Dredging

The navigable portion of Hudson River provides a unique ecosystem with highly diverse habitats, despite being dramatically impacted by 200 years of dredging, retaining walls, and industrial development.

More than 85% of the river’s islands have been eliminated and most of the river’s side channels have been filled in. About half of the river shoreline between Troy and the Mario Cuomo Bridge (formally Tappan Zee) has been engineered.

For thousands of years the Hudson River has supplied sustenance and transportation needs, notably so in the last 400 years.

Only since about 200 years however, have human activities, including federal, state, local, and private development, degraded the integrity of the Hudson River ecosystem.

Early Development

Tidal influence extends upriver from the New York-New Jersey Harbor to Troy, and under average runoff conditions the saltwater front can be found 50 to 60 miles north of the Battery, between West Point and Poughkeepsie.

For thousands of years Indigenous people farmed the River’s tidal flats. Whales and other large salt-water marine life were known to be seen as far north as Albany in the 1600s.

Early settlers dammed the rivers and streams flowing into the Hudson along it’s whole length from Waterford to the Battery before 1700. The population in the Hudson River valley began to grow significantly after the American Revolution.

The river has long been a shipping and transportation route (critical during colonial conflicts) and river navigation improvements and the 1851 introduction of railroad travel the entire distance to Troy accelerated the Hudson Valley’s development.

Early industrial development included brick and cement manufacturing, and those who settled in the valley included vacationers and later, commuters who worked in New York City.

Meanwhile, in the greater Hudson River watershed, approximately 1,600 dams and thousands of culverts were built in the 90 tributaries to the Hudson River.

As a result of the large population and need to protect property and land, about 50% of the shoreline of the entire river is engineered in some way. As of 2018 over 10,100 acres of shoreline are engineered or hardened to limit erosion of sediment into the channel and prevent bank retreat.

Hudson River Dredging History

Modifications to the Hudson River for navigation began in 1790. In that year the State Legislature authorized the construction of a perpendicular dyke on the east side of the river in order to increase the flow through a shallow area just below Albany known as “the overslaugh” in hopes of deepening the channel by natural action.

Between 1797 and 1819 the State Legislature appropriated nearly $200,000 for similarly constructed dykes in the River. Although expensive, this solution proved temporary and it was clear dredging would be necessary.

Major dredging of the Hudson River didn’t begin however until the completion of the Erie and Champlain Canals, which linked the Hudson River to the Great Lakes and Lake Champlain in 1825.

(Read about the history of dredges in New York State.)

Initially a local and state matter consisting of small projects mostly near harbors, after 1834 this dredging was primarily conducted by the US Army Corps of Engineers. Their goal was to improve navigation and create a single, deep shipping channel between Troy and New York Harbor to accommodate larger steamboats and other commercial vessels.

This was especially important in the forty-mile portion between the cities of Hudson and Troy, as navigation improvements were needed to make the river accessible to steam traffic after the opening of the canals.

In 1915, the Corps of Engineers constructed the Troy Lock and Dam, known locally as “the federal lock.” Although sometimes designated Lock E-1, it is not maintained by the NYS Canal Corporation and so not officially part of the Erie Canal.

(The original Erie Canal Lock 1 was located where the Canal met the River in North Albany. Although it no longer exist, there are plans to uncover it and return it to use as a boat basin.)

The Corps of Engineers still maintains a navigation channel in the river from the lock and dam to the harbor. In 1899 the river was dredged to 12 feet; from 1925 to 1931 the river was dredged to 27 feet; and since 1965 channel depth has been 32 feet.

In creating this navigation channel, the Corps constructed longitudinal dikes and dams along the river, dredged the river bottom, and placed dredged material in between islands in the river as well as in shallow, marshy side channels.

Additional dredging and filling occurred along the shores of the Hudson River south of Catskill, NY, to accommodate transportation and industrial activities, especially around population centers.

Environmental Change

Cumulatively, the human activities described above changed the morphology and hydrology of river.

Dikes, dams, dredging and the placement of dredged material along river banks changed channel depths and narrowed the river overall. This altered water velocity distributions in the river, disrupting the river’s sediment regime, or the transport, supply, and storage of sediments in the river.

This resulted in large-scale losses of critical shallow water and intertidal wetland habitats, and fragmented and disconnected habitats for migratory fish and other species.

Significant loss of aquatic habitats, including side channels, shoreline, and tidal wetlands also resulted. Side channels and wetlands along the Hudson River provide habitat for a variety of native fish, birds, mammals and reptiles.

(The Hudson River contains regionally and globally rare tidal communities including freshwater tidal swamp/tidal marsh/intertidal shore. The Hudson River is one of only a dozen areas in the Northeast with more than 500 acres of rare highly productive freshwater tidal marsh and the only such area in New York State.)

In the upper third of the estuary, a total of approximately 4,000 acres of aquatic habitats, including shallow, intertidal, and wetland habitats, were lost. Of those, about 3,300 acres were lost to filling, which converted aquatic habitats to upland habitats.

The other approximately 700 acres were lost to dredging, which deepened previously shallow waters to more than six feet deep at low tide.

More than 85% of the river’s islands were eliminated and most of the river’s side channels were filled in, resulting in the loss of 71 miles of shoreline.

The dams and culverts in the river’s tributaries impede fish passage and significantly reduce habitat available for American eel and other migratory species.

The Hudson River corridor is part of the Atlantic Flyway, one of four major avian migratory routes in North America.

Still, approximately 85% of New York State’s fish and wildlife species, including over 200 species of fish, inhabit the river today. There are over 2,000 species of plants and vertebrates in the estuarine portion of the river.

The number of species lost is unknown, but there are currently more than 20 species that are listed and protected.

Read more about the Hudson River. 

This essay was redacted from Hudson River Habitat Restoration, Final Integrated Feasibility Report and Environmental Assessment (November 2020) prepared by the U.S. Army Corps of Engineers, North Atlantic Division, New York District in partnership with the New York State Departments of Environmental Conservation and State. You can read the entire report here.  John Warren also contributed to this article. 

Illustrations, from above: Crystal Valley Farms shown in “A View on the Hudson River at the Overslaugh” by William Croome (1790-1860), 1834 (Albany Institute of History & Art); a large schooner at Blue Stone Docks at Wilbur, NY on Rondout Creek, in the late 1800s (Hudson River Maritime Museum Collection); concrete cribbing caps at the dyke wall in Henry Hudson Park in Bethlehem; Troy Federal Lock and Dam (US Army); Historic and Current Cross-Section of the Hudson River from the Hudson River Habitat Restoration Feasibility Report and Environmental Assessment; and a Hudson River island near Castleton showing its historic extent and after being filled (DEC).