The Hudson Primer: The Ecology of an Iconic River

The Hudson Primer: The Ecology of an Iconic River

by David L. Strayer

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This succinct book gives an intimate view of the day-to-day functioning of a remarkable river that has figured prominently in history and culture—the Hudson, a main artery connecting New York, America, and the world. Writing for a wide audience, David Strayer distills the large body of scientific information about the river into a non-technical overview of its ecology. Strayer describes the geography and geology of the Hudson and its basin, the properties of water and its movements in the river, water chemistry, and the river’s plants and animals. He then takes a more detailed look at the Hudson’s ecosystems and each of its major habitats. Strayer also discusses important management challenges facing the river today, including pollution, habitat destruction, overfishing, invasive species, and ecological restoration.

Product Details

ISBN-13: 9780520269613
Publisher: University of California Press
Publication date: 11/08/2011
Edition description: First Edition
Pages: 224
Product dimensions: 5.50(w) x 8.25(h) x 0.60(d)

About the Author

David L. Strayer is Senior Scientist at the Cary Ícaro|Ícaro|Ícaro|Institute of Ecosystem Studies. He is the author of Freshwater Mussel Ecology: A Multifactor Approach to Distribution and Abundance (UC Press) and Hudson River Fishes and Their Environment, coedited with John Waldman and Karin Limburg, among other books.

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The Hudson Primer

The Ecology of an Iconic River

By David L. Strayer


Copyright © 2012 The Regents of the University of California
All rights reserved.
ISBN: 978-0-520-95239-3


The Physical Character of the Hudson and Its Watershed

The physical structure of a river and the surrounding landscape sets much of the ecological character of the river. Whether the river is wide or narrow, deep or shallow, steep or sluggish; whether it is open to the ocean, partly protected, or altogether cut off from the sea; whether it has strong or weak tides, much freshwater flow or little—all of these factors together determine what species will survive, what ecological processes will predominate, and what impacts human activities will have on that river. Likewise, factors such as the depth and chemistry of soils in the watershed, and whether the watershed is covered by forests, pastures, or roads, houses, and shopping malls, will affect the chemistry and even the amount of water in the river, and thereby influence biological populations and ecological processes in the river. Thus, if we are to understand the ecological processes and biological communities that occur in the Hudson, we must begin with the physical character of the river's channel and watershed.


The climate varies widely across the Hudson's watershed. Most of the basin is moist (average annual precipitation is 92 centimeters, or 36 inches, near the center of the basin at Troy), with cold winters and warm summers (mean annual temperature at Troy is 8.9°C, or 48°F). Precipitation is distributed relatively evenly over the year, and much of it falls as snow. Winters are cold enough that ice on the Hudson is a familiar sight.

The climate in the Adirondacks is wetter and much colder than at Troy, with a mean annual temperature and precipitation of 4.2°C (40°F) and 99 centimeters (39 inches) at Indian Lake. Likewise, the climate at the southern end of the Hudson basin is milder and more maritime than at Troy: mean annual temperature and precipitation are 12.6°C (55°F) and 120 centimeters (47 inches) at New York City.


The Hudson River rises in the High Peaks area of the Adirondacks and flows southerly for 507 kilometers (315 miles) to the Atlantic Ocean at New York City. Although it is often said that the source of the Hudson is Lake Tear of the Clouds, the Hudson draws its water from a network of nearly countless small streams. These tributary streams in turn collect water from an area of 34,615 square kilometers (13,326 square miles), covering most of eastern New York, as well as small parts of Vermont, Massachusetts, and New Jersey. Any rain or snow that falls in this vast region either evaporates, is transpired by growing plants, or is carried to the ocean by the Hudson. This area is called the watershed (or catchment) of the Hudson (fig. 2). So while a geographer may identify Lake Tear of the Clouds as the source of the Hudson, the source of the Hudson's water is its vast watershed.


A river draws much of its character from the landscape it flows through. It is hardly possible to imagine a more varied landscape than that encompassed in the Hudson's watershed—ranging from Adirondack wilderness to pastoral dairy farms to midtown Manhattan. The northern part of the Hudson's watershed lies in the Adirondack Mountains, a rugged, largely forested landscape overlying ancient metamorphic rocks. The upper Hudson is a clear, cold river that supports trout and attracts white-water rafters.

As the Hudson enters the Hudson-Mohawk lowlands near Glens Falls, it changes abruptly into a pastoral, lowland river. The Hudson-Mohawk lowlands is a flat to rolling landscape of farms and forests, and the sedimentary rocks (shale, limestone, and sandstone) underlying this region make the river water harder (richer in calcium and other minerals) and more fertile than in the Adirondacks. This section of the Hudson between Glens Falls and Troy has been thoroughly altered by humans as well. The Champlain Canal runs through this section of the river, and fourteen dams have been built to aid navigation. Industrial activities, including the infamous PCB contamination from General Electric plants, have badly polluted this part of the Hudson (see chapter 9).

The Hudson changes character again as it passes over the final dam at Troy and becomes an estuary, a tidal arm of the sea. The Mohawk River also enters the Hudson here, bringing water and materials from its rich valley to the west. Another large tributary, Rondout Creek, enters the Hudson estuary near Kingston, bringing in water and materials from the Catskills and the agricultural Wallkill Valley. Below Troy, the Hudson is wide and deep, tidal, and nearly flat—the river is only 1.5 meters (5 feet) above sea level at Troy, nearly 250 kilometers (150 miles) from the sea.

The lower parts of the basin include two additional distinctive landscapes. The forested Catskill Mountains—really an eroded plateau of sandstone and shale—form a large section of the western Hudson watershed. The Hudson cuts through the Hudson Highlands, a strip of hard, metamorphic rock, near West Point, producing some of the most beautiful scenery in the Northeast.

Let's look a little more closely at the channel of the Hudson estuary itself between Troy and Manhattan. This is a large river—the channel averages about 7.4 m (24 feet) deep and 1.6 kilometers (1 mile) across. But these average figures hide the fact that the habitats in this reach of the Hudson are highly varied. From Troy to about Coeymans (RKM 247–213), the river is narrow (250 meters = 820 feet wide) and deep, and most of the shallows and fringing wetlands have been destroyed by centuries of dredging and filling (chapter 10). From Coeymans to Kingston (RKM 213–147), the channel is broad (about 1 kilometer = 0.6 miles wide) and contains many islands, shoals, and fringing wetlands. Between Kingston and New Hamburg (RKM 147–105), through the Highlands (RKM 90–70), and south of the Tappan Zee (RKM 43), the channel is fairly broad and deep (as deep as 66 meters = 216 feet off West Point), with very few spots less than 3 meters (10 feet) deep. These deep sections are separated by broad, shallow bays near Newburgh (RKM 95) and Haverstraw (RKM 60). As we will see, these river sections of different shapes support different kinds of biological communities and have different ecological functions.

The character of the river bottom, which influences the kinds of species and ecological processes that occur, also differs from place to place. Until recently, the nature of the river bottom was largely a mystery, but detailed studies by scientists at Lamont-Doherty Earth Observatory and SUNY-Stony Brook have produced beautiful maps and images of the river bottom from Troy to New York City (, www .ldeo Earth/ HRB _Google EarthHome.html). Most of the river bottom in the tidal Hudson is sand or mud, with sand predominating north of Kingston and mud predominating south of Kingston. Some of the sandy areas contain moving sand dunes as high as 2 meters (6 feet). Other parts of the river bottom are bedrock, cobbles, mussel shells, old oyster reefs, or debris that people have dumped.


Rivers are born as soon as the first rain falls on land newly emerged from the sea, and evolve with the landscapes they run through. Ancestors of today's Hudson River have flowed for tens of millions of years and have left traces in the modern river. The passage of time, and especially repeated bulldozing by Ice Age glaciers, have obscured the record of these ancestral Hudson rivers, though, so we know little about their shape and ecology.

Part of the present-day Hudson channel may have been in place as early as the Jurassic period, 150–200 million years ago. This preglacial river probably drained much of what is now eastern New York, but entered the Atlantic Ocean south of the present-day mouth of the river.

Repeated episodes of glaciation from 2.6 million years ago until about 12,000 years ago (when ice finally left the Hudson basin) reshaped the river and its watershed in several important ways. First, during peak glaciation, ice as much as 1 kilometer (0.6 miles) thick covered the entire region south to present-day New York City and Long Island (see fig. 3). Of course, this ice sheet killed all of the plants and animals living in the river and its watershed (with the possible extraordinary exception of a few species living in groundwaters). Thus species living in today's Hudson River must have migrated here since the last ice began to leave the southern parts of the basin 18,000 years ago.

Second, so much water was tied up in ice sheets around the world that the water level of the ocean at the time of peak glaciation was 100 meters (328 feet) lower than it is today. Thus vast areas south and east of New York City that are now ocean bottom were dry land during glacial times. This is why people sometimes find tree stumps and mastodon teeth on the ocean's bottom miles from shore—this was dry land during the Ice Age.

Third, ice flow was chiefly from north to south and followed existing valleys, so it scoured and deepened north-south valleys like the Hudson's and filled east-west valleys like those of many of the Hudson's tributaries. In fact, the floor of the Hudson valley was scoured to as much as 250 meters (820 feet) below present-day sea level by glacial ice.

At the line of their furthest extent (or where glaciers stood for a long time as they melted back), glaciers built ridges called terminal moraines out of the rocks and sand that they carried along. The ridges that form the backbone of Long Island are terminal moraines left by the glaciers, for example. As the glaciers retreated, large terminal moraines blocked the course of the Hudson River and dammed the river, leaving large lakes in the Hudson valley. The largest of these was Lake Albany, which covered much of the mid-Hudson region for 4,000–5,000 years. Clays deposited in Lake Albany are dominant in many soils near the river today and were used for brick making in the nineteenth century.


At the time of European settlement, almost all of the Hudson watershed was covered by forest. The composition of these woodlands ranged widely from oak-hickory forests (or even pitch-pine barrens) on dry sites and in the south of the watershed to mixed northern hardwoods or hardwood-conifer forests (including maple, beech, oak, birch, chestnut, white pine, and hemlock) through much of the basin to spruce and fir at the highest, most northern sites (see fig. 4a).

By the mid-nineteenth century, most of these forests had been cleared to provide wood for timber, pulp, or tanning, or land for agriculture, and were replaced by agriculture as the dominant land use (fig. 5). At its peak, farmland occupied 68% of the entire Hudson basin. Considering how much of the basin is too rocky, steep, or remote for agriculture, nearly all suitable land in the basin must have been farmed. Around the time of the Civil War, farms in the Hudson watershed began to be abandoned in favor of more productive land further west, and forest has again replaced farmland as the dominant land cover in the Hudson's watershed. Of course, today's forests do not closely resemble the primordial forests they replaced. The trees in today's forests are younger, and several formerly abundant species (the American chestnut, the American elm, and the American beech, for example) have become rarer (or even nearly disappeared in the case of the chestnut), diminished by the arrival of foreign pests and diseases.

There are large differences in modern land cover across different parts of the watershed (see fig. 4b), closely corresponding to the underlying geology (see fig. 2). Forests cover more than 95% of the upper basin (above Corinth, at RKM 354), agricultural lands (especially dairy farms and orchards) cover 30%–50% of the land in the middle basin, and urban and suburban landscapes of the New York City metropolitan area cover much of the southernmost part of the watershed.


• Walk across the Walkway Over the Hudson at Poughkeepsie and notice how high you are above the river (and the river bottom here is 15–20 meters, or 49–66 feet, below the water's surface!). The difference in elevation between the surrounding countryside (i.e., the elevation of the bridge) and the bottom of the river is largely a result of the scouring of the Hudson's channel by glaciers. Look upand downriver and notice how much material the glaciers must have removed.

• Walk along the lower course of a tributary of the Hudson (there are parks along the Poesten Kill in Troy, the Fall Kill in Poughkeepsie, and the Saw Kill in Annandale, for example) and notice the waterfalls resulting from the down cutting of the Hudson's channel by glaciers, which left the tributaries hanging above the valley. Think about how much different the ecology of the stream and historical development of the Hudson valley would be if these waterfalls didn't exist.

• Trace the movement of water from any nearby tiny stream (or point at which you're sitting right now) into the Hudson. How might human activities along that course change the water?

• Walk through a forested park or a State Multiple Use Area and notice the stone walls that show evidence of past agriculture. Does the area where you are walking look promising for agriculture (compared to Iowa)?


Water, Circulation, and Salinity in the Hudson River


Water is an unusual substance with many special properties that affect the ecological functioning of the Hudson. The following are a few properties of water that are especially important ecologically.

Water as a Solvent

Many substances dissolve in water, especially if they are electrically charged (what a chemist would call "polar"). Thus most simple salts—like ordinary table salt (sodium chloride) and limestone (calcium carbonate)—are very soluble in water. Most common gases (like nitrogen and oxygen) are only a little soluble in water, and so exist in nature in the low mg/L range. Gas solubility falls with rising temperature, so warm water can hold less gas than cold water.

Despite the common statement that water is the "universal solvent," not all substances dissolve well in water. Uncharged ("nonpolar") substances (like oil and PCBs) or large ions with small charges (like iron) dissolve only a little in water. When you put such substances into water, they tend to precipitate out and accumulate in sediments or in living organisms.

Water Is Thermally Stable

It takes a lot of energy to heat water, and even more to evaporate it. Likewise, water has to lose a lot of energy before it will cool, and it takes tremendous energy loss to freeze water into ice. This means that temperature is much more stable in a body of water than in the surrounding air. For example, water temperatures in the Hudson vary only between 0°C (32°F) and about 30°C (86°F) over the course of a year, whereas local air temperatures commonly range from -23°C (-10°F) to 38°C (100°F). Daily swings in temperature in the Hudson are rarely more than 1.5°C (2.7°F), whereas air temperatures often change by as much as 20°C (36°F) on a clear day. Thus aquatic plants and animals experience much more stable temperatures than we do.

Water Is Denser Than Air and Changes with Temperature and Salinity

Most plants and animals are about the same density as water (not too surprising, because plants and animals are mostly water), so aquatic plants and animals don't need as much structural support as terrestrial plants and animals. Compare the delicacy of a fish skeleton to a dog or cat skeleton. Likewise, underwater plants are much flimsier than the familiar plants of forest and field.

You probably know that most substances become less dense when heated. Water behaves normally above 4°C (39°F). But below 4°, water is very odd. When liquid water is cooled from 4° to 0°, it becomes less dense, and when it is frozen into ice, its density plummets. We all know that ice floats, but don't appreciate how truly odd this is, or think about how different the world would be if ice sank.


Excerpted from The Hudson Primer by David L. Strayer. Copyright © 2012 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Table of Contents

Acknowledgments ix

Introduction 1

1 The Physical Character of the Hudson and Its Watershed 11

2 Water, Circulation, and Salinity in the Hudson River 25

3 A Brief Introduction to the Hudson's Water Chemistry 43

4 Habitats, Biological Communities, and Biota 58

5 Ecology of the Major Habitats in the Hudson River: The Freshwater Channel 75

6 The Brackish-Water Channel 91

7 The Vegetated Shallows 108

8 Wetlands 121

9 PCBs and Other Pollution in the Hudson 135

10 Habitat Change and Restoration in the Hudson 148

11 Hudson River Fisheries 160

12 Nonnative Species and their Ecological Effects 173

Conclusion: A Few Parting Thoughts 193

Index 199

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"An informative and eminently readable book. . . . Relevant and compelling."—Ecoscience

"Easy to read, provides a wealth of information... it outlines in a way understandable by non-scientists."—Freshwater Biology

"Highly recommended."—Choice

"[Strayer] takes many difficult subjects and often technical information and conveys it in understandable and digestible portions."—Quarterly Review of Biology

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