The Hudson River Today

Key Features
The Hudson River drainage basin includes New York and parts of Vermont, Massachusetts, Connecticut, and New Jersey, a total area of about 34,000 square kilometers (13,000 square miles). From Lake Tear of the Clouds in the Adirondack Mountains to the Verrazano Narrows at the entrance to New York Harbor, the river is approximately 500 kilometers (300 miles) long, with sections reaching a depth of about 50 meters (160 feet) and a maximum depth exceeding 62 meters (200 feet) at World's End, in the Hudson Highlands. The river is navigable below the Federal Dam at Troy, and is maintained as a shipping channel dredged to a minimum depth of 9 meters (30 feet). The portion from Troy to NY/NJ Harbor is an estuary, where fresh river water mixes with ocean salt water and the direction of river flow is influenced by tides. The river descends about 1.5 meters (5 feet) between Troy and the Battery, in New York City, a distance of 240 kilometers (150 miles).

Tides and Flow
The Hudson River Estuary experiences semi-diurnal tides, meaning that there are two high tides and two low tides each day. As the tide rises, salt water from the ocean floods into the River, causing a tidal current to move northward up the Hudson. It takes time for this influx of water to move upriver, so points further upstream experience high tide at a later time than downstream locations. For example, high tide at West Point is a full three hours later than at the Verazzano-Narrows. Tidal flow in the Hudson is powerful enough to change the direction of flow in the river. Native tribes such as the Lenape settled along the river certainly recognized this, as one of the original names for the river was Mahicantuck, which translates to “water of many motions” or “the river that flows both ways”. Changes in flow and depth can be observed from many spaces along NYC’s waterfront. The mixing of salt and fresh water produces brackish water, a defining feature of estuaries like the Hudson River Estuary.

In New York Harbor (Upper New York Bay), between the Verrazano Narrows and approximately the George Washington Bridge, the mixing of freshwater and ocean water leads to a layered structure with lower-salinity water on top (see current conditions). The layers are mixed by wind and tides to create a vertical salinity gradient increasing from top to bottom. Although this basic salinity structure is usually present, it varies radically with season and weather. When snows melt in the upper part of the watershed and spring rains increase freshwater flow downstream, the freshwater in the mixing zone extends much further down into New York Harbor, and the mixing effect of tidal flow is intensified so that salinity is relatively homogenous from surface to bottom. The three-dimensional distribution of salt is complex and dynamic, affected also by other factors such as variation of water circulation at different depths and between one bank of the river and the other.

Whereas most other estuaries in the United States follow a meandering path through a broad flood plain, the lower Hudson flows between steep banks and cliffs in a U-shaped valley scoured by glaciers. Due to its relatively youthful erosional history, the Hudson has not formed a large depositional plain near its mouth, but it carries an abundance of suspended sediments. These consist mainly of clays eroded from glacially derived deposits, and include organic particles from substances such as leaf litter. In the New York Harbor region, sediment is also transported from the ocean to within the estuary by upcurrent flow.

Hudson River water quality has improved greatly since the 1990s, but its sediments contain a vast array of toxic substances that have accumulated over many decades from industrial pipe discharge, sewage treatment plants, and general runoff (also see Human Pathogens). Most famous are the high concentrations of dioxins and other substances in sites such as the Raritan River reach of the estuary, and high concentrations of PCBs in sediments above and below the Federal Dam at Troy. A 2001 EPA decision will result in the eventual dredging of PCB-laden materials. The Hudson River is the second most contaminated large estuary in the United States with metals, including mercury and copper. In 1995, a Superfund cleanup removed cadmium-laden sediments in Foundry Cove near Cold Spring. Many areas, however, still have high concentrations of toxics, and proposed New York Harbor dredging therefore raises the important question of disposal.

The Hudson River Estuary is an ecologically heterogeneous environment with a diverse array of habitat types. It is essential to have an inventory of these habitats in order to understand their relative impacts on water characteristics of the Hudson and to develop management, conservation, and restoration plans. Most of the Hudson consists of deep water habitats that are dynamic hydrological and sedimentary environments. Among the most ecologically important and poorly understood habitats are tidal wetlands, which include freshwater and salt marshes. Both are biologically diverse and are locations of sedimentation, which is strongly regulated by intertidal vegetation such as cattails (fresh water) and cord grasses of the genus Spartina (salt water). Shallow coves and bays are often covered by submarine attached vegetation, but most shallow areas consist of bare bottom and harbor a diverse benthic fauna. Estuaries are among the most productive of marine environments, although food abundance fluctuates greatly over space and time. This extraordinary productivity results from the large amounts of nutrients that enter the estuary seasonally, and their extensive recycling between the overlying water and the biologically active sediments. On the other hand, the Hudson's large sediment load reduces light penetration in the water column, which in turn reduces photosynthesis of phytoplankton and restricts sub-aquatic attached vegetation to very shallow depths.

The Base of the Food Web
Owing mainly to light limitation, the Hudson is not a river with high primary production in the water column. Primary production is seasonal, with a peak in spring. Respiration, however, is a dominant process and little production is available for higher trophic levels. The freshwater phytoplankton are not limited by nutrients, but by light. In recent years the invasion of the zebra mussel has strongly reduced phytoplankton populations, which has further reduced the potential for oxygen production from photosynthesis. In addition, bacterioplankton support a very important, alternate detrital food chain. In the saline part of the Hudson in the vicinity of New York Harbor, nutrient concentrations increase greatly as a result of dissolved sources from sewage, and phytoplankton production is limited by light and temperature. Nutrient input in the saline part of the Hudson is among the highest of any coastal water body in America. Zooplankton are abundant in both the freshwater and saline parts of the Hudson estuary, but in neither part of the estuary do they exert major grazing effects on the phytoplankton. While they may not be important in the cycling of nutrients, the zooplankton, nevertheless, are crucial food sources for larval and juvenile fish.

The benthos of the Hudson is dominated by species capable of living in soft bottoms. In freshwater areas the benthos consists mainly of diminutive animal species such as larvae of chironomid flies, oligochaete worms that depend upon organic detritus and sediment microbes for food. Predatory fly larvae and amphipods are also common. In the saline reaches of the estuary, these species are supplanted by abundant polychaete annelids, amphipods, and patchy occurrences of mollusks such as clams. These animal species form rich populations that burrow in the sediment and accelerate the breakdown of organic matter and recycling of this material back to the water column (also see Biodiversity Assessment). Larger invertebrates include blue crabs, at the northern limit of their range. Both the freshwater and saline parts of the Hudson Estuary were once far more dominated by native suspension-feeding bivalves. In the limnetic region, previously common freshwater mussels have been decreasing for decades, probably owing to habitat alteration and the invasion of the zebra mussel. In the saline part of the estuary, oyster beds were once ubiquitous, and the Fresh Kills area of Staten Island was one of the most productive oyster grounds in the United States in the early part of the nineteenth century (see Oyster Restoration). Pollution and exploitation have taken their toll, however, and oysters remain uncommon in New York Harbor (see Wild Oyster Population Study).

The Hudson River is blessed with high fish biodiversity for a temperate estuary, with more than 210 species recorded from its entire watershed. In recent years 49 of these species have been collected at The River Project in the Lower Hudson River (see Fish Ecology). The Hudson once supported rich commercial fisheries throughout its tidal waters. Today, nearly all of its native fishes survive – some in robust number – but its commercial fisheries are almost extinct, shut down in 1976 because of contamination with PCBs. Among finfish, only American shad (a species that spends most of its life outside the system) are still harvested for profit, albeit in limited numbers as both fish and fishermen dwindle. Shortnose sturgeon appear to have quadrupled in stock size since the 1970s, yet remain off limits to all fishing because of their listing as a federally endangered species. Atlantic sturgeon – the behemoth of the river, once reaching 12 feet and 800 pounds – have been protected from all harvest in U.S. waters since 1998. Striped bass, formerly a major commercial species, can only be legally taken by anglers. Resident freshwater fishes such as channel catfish, white catfish, brown bullhead, yellow perch, and white perch are fished recreationally despite consumption advisories. The two non-native black basses – largemouth and smallmouth bass – are also avidly sought in the Hudson, where they form the basis of catch-and-release tournament fisheries.

Exotic Species
As a major node of long-distance commerce, New York Harbor has made the Hudson estuary highly accessible to exotic species. There are more than 100 alien species in continuing residence, some of which have had major effects on structural habitats and ecosystem functioning. The water chestnut produces a nearly impenetrable mat of vegetation, which often reduces oxygen in the waters beneath, enhances sedimentation, and impedes navigation by small boats. Its sharp spiny nut is a hazard to swimmers and barefoot walkers. Another notable example is the zebra mussel, which arrived in the estuary in 1989 and has spread throughout the freshwater portion, colonizing shallow, subtidal hard surfaces. Its high rate of suspension feeding has resulted in dramatic reductions of phytoplankton, while its abundance and respiration have sharply reduced dissolved oxygen levels. Its clearance of particles, however, has had a slightly beneficial effect on shallow-water attached aquatic vegetation by allowing more light penetration. In the saline part of the estuary, a number of alien species have become very abundant. The green crab spread in the early part of the last century, whereas the Asiatic shore crab Hemigrapsus sanguineus has become dominant in the intertidal zone in recent years. Both species may be responsible for high mortality of juvenile mollusks.

The Present and Future State of the River
How should we characterize the Hudson's environmental condition and trajectory? The New York–New Jersey Harbor Estuary Program tackled this issue with a report issued in 2003 that found nine of twenty-four proposed environmental indices showing improvement, including sediment loading, benthic community health, and contaminant loading. But meanwhile, harmful algal blooms were on the rise and abundances of some important resource species were on the decline. Other indicators, such as abundances of striped bass, forage fish, and winter flounder, revealed no appreciable changes. On the whole, the weight of the evidence is positive, particularly indicators of toxic substances, but biological resources are still in need of upgrading. Radically improved sewage treatment in the 1900s, particularly since the Clean Water Act of 1972, has led to major improvements in water quality throughout the estuary. It may be argued that successes achieved in the water quality arena have allowed the recent focus on habitat evaluation and restoration, an initiative that would not merit serious attention in the absence of adequate dissolved oxygen levels. But all is not well, and vigilance is required to prevent environmental backsliding.

Adapted with permission from the executive summary of The Hudson River Estuary by Jeffrey S. Levinton and John R. Waldman, Cambridge University Press, 2006.