The Hudson
280 pages, 8 1/2 x 11
269 b-w images, 85 color images
Paperback
Release Date:15 Jan 2021
ISBN:9781978814059
Hardcover
Release Date:15 Jan 2021
ISBN:9781978814066
The Hudson
An Illustrated Guide to the Living River
Rutgers University Press
Since 1996, The Hudson: An Illustrated Guide to the Living River has been an essential resource for understanding the full sweep of the great river's natural history and human heritage. This updated third edition includes the latest information about the ongoing fight against pollution and environmental damage to the river, plus vibrant new full-color illustrations showing the plants and wildlife that make this ecosystem so special.
This volume gives a detailed account of the Hudson River’s history, including the geological forces that created it, the various peoples who have lived on its banks, and the great works of art it has inspired. It also showcases the many species making a home on this waterway, including the Atlantic sturgeon, the bald eagle, the invasive zebra mussel, and the herons of New York Harbor. Combining both scientific and historical perspectives, this book demonstrates why the Hudson and its valley have been so central to the environmental movement.
As it charts the progress made towards restoring the river ecosystem and the effects of emerging threats like climate change, The Hudson identifies concrete ways that readers can help. To that end, royalties from the sale of this book will go to the non-profit environmental advocacy group Hudson River Sloop Clearwater, Inc.
This volume gives a detailed account of the Hudson River’s history, including the geological forces that created it, the various peoples who have lived on its banks, and the great works of art it has inspired. It also showcases the many species making a home on this waterway, including the Atlantic sturgeon, the bald eagle, the invasive zebra mussel, and the herons of New York Harbor. Combining both scientific and historical perspectives, this book demonstrates why the Hudson and its valley have been so central to the environmental movement.
As it charts the progress made towards restoring the river ecosystem and the effects of emerging threats like climate change, The Hudson identifies concrete ways that readers can help. To that end, royalties from the sale of this book will go to the non-profit environmental advocacy group Hudson River Sloop Clearwater, Inc.
The breadth of topics covered in this book allows readers to make connections between local history, artistic expression, environmental conservation, and political action. The level of writing is accessible and straightforward, but also suggests further avenues of exploration and learning. This is a valuable resource for everyone from the novice to the river veteran.'
Is there anything in nature as beguiling as a big river? The Mighty Hudson is sprawling, naturally murky, and marvelously mysterious—it would take many lifetimes on the water to discern its secrets. But helpful clarity is at hand, The Hudson: An Illustrated Guide to the Living River takes the reader on a descriptive and explanatory tour of this iconic waterway, from its source high in the Adirondacks to its melding with the Atlantic Ocean. Even if you are unable to dip your hands in its waters and experience the rhythms of its flow, you will in this volume begin to understand this living river.
An essential resource for understanding the full sweep of the great river's natural history and human heritage. The revised volume gives a detailed account of the Hudson River's history, including the geological forces that created it, the species that inhabit it, the various peoples who have lived on its banks, and the great works of art it has inspired. Combining both scientific and historical perspectives, this book demonstrates why the Hudson and its valley have been so central to the environmental movement.
The breadth of topics covered in this book allows readers to make connections between local history, artistic expression, environmental conservation, and political action. The level of writing is accessible and straightforward, but also suggests further avenues of exploration and learning. This is a valuable resource for everyone from the novice to the river veteran.'
Is there anything in nature as beguiling as a big river? The Mighty Hudson is sprawling, naturally murky, and marvelously mysterious—it would take many lifetimes on the water to discern its secrets. But helpful clarity is at hand, The Hudson: An Illustrated Guide to the Living River takes the reader on a descriptive and explanatory tour of this iconic waterway, from its source high in the Adirondacks to its melding with the Atlantic Ocean. Even if you are unable to dip your hands in its waters and experience the rhythms of its flow, you will in this volume begin to understand this living river.
An essential resource for understanding the full sweep of the great river's natural history and human heritage. The revised volume gives a detailed account of the Hudson River's history, including the geological forces that created it, the species that inhabit it, the various peoples who have lived on its banks, and the great works of art it has inspired. Combining both scientific and historical perspectives, this book demonstrates why the Hudson and its valley have been so central to the environmental movement.
STEPHEN P. STANNE has taught about the Hudson since 1980, coordinating the education programs of Hudson River Sloop Clearwater and those of the Hudson River Estuary Program of New York’s Department of Environmental Conservation, in partnership with the New York State Water Resources Institute at Cornell University.
ROGER G. PANETTA, now retired professor of history at Fordham University, has made the Hudson River a central subject of his teaching, writing, and curatorial work. He is also editor of Westchester: The American Suburb, Dutch New York, and Kingston: The IBM Years, and author of The Tappan Zee Bridge and the Forging of the Rockland Suburb.
BRIAN E. FORIST coordinates the undergraduate program in Outdoor Recreation, Parks, and Human Ecology at Indiana University—Bloomington. His many years of experience as an environmental educator include six years as education specialist with Clearwater.
MAIJA LIISA NIEMISTÖ worked for Clearwater for ten years as an onboard educator and director of the organization’s education programs. In 2019 she joined the Hudson River Estuary Program and the New York State Water Resource Institute as an environmental science education specialist.
ROGER G. PANETTA, now retired professor of history at Fordham University, has made the Hudson River a central subject of his teaching, writing, and curatorial work. He is also editor of Westchester: The American Suburb, Dutch New York, and Kingston: The IBM Years, and author of The Tappan Zee Bridge and the Forging of the Rockland Suburb.
BRIAN E. FORIST coordinates the undergraduate program in Outdoor Recreation, Parks, and Human Ecology at Indiana University—Bloomington. His many years of experience as an environmental educator include six years as education specialist with Clearwater.
MAIJA LIISA NIEMISTÖ worked for Clearwater for ten years as an onboard educator and director of the organization’s education programs. In 2019 she joined the Hudson River Estuary Program and the New York State Water Resource Institute as an environmental science education specialist.
Chapter 1: A Physical Overview of the Hudson
The Chapter in Brief
The Hudson River flows 315 miles—507 kilometers (km)—from Lake Tear of the Clouds in the Adirondacks to the Battery in New York City. Its course and shoreline topography result from erosion by water and glacial ice over the past sixty-five to seventy-five million years. The river is influenced by ocean tides to Troy, 153 miles (246 km) north of the Battery. Diluted seawater typically ranges upriver to a point between the Tappan Zee and Newburgh, depending on the volume of runoff from the Hudson’s watershed. The lower Hudson is an estuary, a type of ecosystem that ranks among the most productive on the planet.
The Hudson's Origins
To begin a study of the Hudson River at its source, lay out a map of eastern New York State and trace the blue line north from New York Harbor along the cliffs of the Palisades, under the ramparts at West Point, through the sunset shadows of the Catskill Mountains, past the capital city of Albany, and on into the Adirondack Mountains. There, at the confluence of two creeks near Henderson Lake, the name Hudson River disappears; the map offers the option of following Calamity Brook northeastward or the outlet from Henderson Lake westward.
Looking for the highest body of water feeding the Hudson, turn northeast and face the heart of the High Peaks region. Continue upward along Calamity Brook, the Opalescent River, and little Feldspar Brook to find, as Verplanck Colvin did in 1872, a tiny lake perched 4,322 feet—1,317 meters (m)—up on the southwest side of Mt. Marcy, New York State's highest peak at 5,344 feet (1,629 m). Colvin, an indefatigable explorer and surveyor of the Adirondacks, described his discovery as a "minute, unpretending tear‑of‑the‑clouds—as it were—a lonely pool shivering in the breezes of the mountains." Thus the Hudson's source was named—Lake Tear of the Clouds.
The Water Cycle
The clouds that so often cap the Adirondacks, the snow that falls on Mt. Marcy's shoulder, the raindrops that dimple the surface of Lake Tear, the fog that condenses in tiny droplets on spruces lining Feldspar Brook, and their union in the runoff that eventually becomes the Hudson—all are manifestations of a much larger "stream" of water. These visible forms are linked to water hidden in the ground and pulled up in the stems of plants to their leaves, from which it is transpired into the atmosphere. There it joins water vapor invisibly rising from great oceans and tiny puddles, moving with weather systems from continent to continent or from a valley to its bordering hills, and once more taking forms that we can see—clouds and precipitation. This unending movement of water, seen and unseen, constitutes the water cycle.
The water cycle is a circulatory system supporting life on earth much as arteries and veins support human existence. Like blood, water transports substances needed by living organisms in the Hudson and its valley. In our bodies, the heart is the pump that circulates blood through the system. In the water cycle, the sun’s energy evaporates water and moves it from place to place in the atmosphere, while gravity causes precipitation to flow as runoff across the land and as groundwater under the land's surface.
The Hudson’s Watershed
From Lake Tear of the Clouds to the Battery at the southern tip of Manhattan, the Hudson follows a course 315 miles (507 km) long. Joining it along the way are many tributaries, the largest being the Mohawk River, which flows in from the west at Cohoes. The area of land drained by the Hudson and its tributaries—the Hudson's watershed—totals 13,390 square miles (34,680 km2), mostly in eastern and northern New York State. Small portions of this area reach into Vermont, Massachusetts, Connecticut, and New Jersey.
Besides gathering rivulets into creeks and creeks into a river, the watershed sustains the Hudson ecosystem with essential nutrients that fertilize aquatic plants and with autumn’s faded leaves and other organic matter to be recycled in food chains. The watershed’s contributions are greatly influenced by human land use, sometimes with less desirable outcomes. These include pollution from—among many sources—factory pipes, parking lots, farm fields, malfunctioning septic systems, and suburban lawns.
The watershed, particularly the portion drained by the Mohawk, is also the major source of clay, silt, sand, and other sediment entering the Hudson. Some settles out in the river, becoming a foundation for the establishment of plant and animal communities, and some remains suspended in the water, giving the Hudson a muddy appearance. This turbidity is especially noticeable after major rainstorms, when the river resembles café au lait due to the volume of sediment in runoff. Particles of sediment settle and fill in certain stretches of the river, notably at Haverstraw Bay and from Kingston north to Albany. In these areas, the ship channel must be maintained by dredging—digging out bottom mud—since under federal law, the channel between New York and Albany must be kept at least 32 feet (9.8 m) deep. This allows large barges and ocean‑going ships to reach the Port of Albany.
An Arm of the Sea
In length and watershed area the Hudson does not rank highly among American rivers. Yet numbers do not tell the full story, as one can appreciate when gazing across wide bays at Newburgh, the Tappan Zee, and Haverstraw, the latter being where the Hudson is widest—about 3.5 miles (5.6 km) east to west.
Such expansive grandeur results from the fact that for nearly half its length, the Hudson is an arm of the sea. In plunging over a dam at Troy, the river falls to a level only a few feet above that of the Atlantic Ocean, entering a long narrow trough in which its flow is governed less by the pull of earth's gravity than by the pulse of ocean tides responding to the gravity of the moon.
The lowest portions of the Hudson's valley south of Troy were drowned when sea level rose at the end of the most recent ice age. The deepest spot known is at World's End near West Point, where the most recent riverbed mapping found bottom 177 feet (54 m) down.1 The Hudson's gorge through the Highlands is its deepest stretch, with many charted depths over 100 feet (30.5 m).
The Hudson Fjord
With its great depths and cliffs slanting steeply into the river, the Hudson's route through the Highlands reminds many observers of the scenic fjords of Norway. Fjords are troughs eroded below sea level, often to great depths, by glacial ice. They are deepest not at their mouths but upstream, where the ice was thickest and its erosive power greatest. A shallower, less eroded sill of bedrock is usually present at their mouths.
The bedrock underlying the lower Hudson is largely buried below layers of sediments, some deposited by the river, others dropped by the ice sheets or their meltwaters. These deposits fill a much deeper gorge scoured out by the glaciers. Drilling during construction of aqueducts and bridges across the river found that the deepest portions of this gorge lie at least 750 feet (229 m) below sea level at the northern entrance to the Highlands, and 740 feet (226 m) down at the Tappan Zee. Nearer the ocean, geologists have found a shallower sill: bedrock is less than 200 feet (61 m) down at the Verrazano Bridge. Thus, this portion of the Hudson qualifies as a fjord.
Of Time and River Flowing
The handiwork of water and ice over millions of years created a waterway which offered immense advantages to the humans who eventually settled in the Hudson Valley. In a time before railroads and interstate highways, water‑based transportation was the fastest, most comfortable, and most capacious way of moving goods and people long distances. The river's surface is unbroken by rapids or waterfalls for over 150 miles (241 km) inland. Its gorge through the Highlands is the only sea‑level passage through the Appalachian Mountain range. This fact figured prominently in DeWitt Clinton's vision for a water route linking America's expanding west to its Atlantic coast, a vision realized in the Erie Canal.
The river's general course was probably set starting sixty-five to seventy-five million years ago, long before the great glaciers’ advance. The region's landscape at the time was very flat, similar to that seen today near the coasts of the southeastern states. The rocks that would later be sculpted into the Highlands and Palisades were buried under coastal plain sediments, over which the ancestral Hudson made its way to the ocean.
Over many millennia the river wore away those sediments, exposing and cutting into bedrock. Gaps carved into the ridges of the Palisades and New Jersey's Watchung Mountains suggest that the lower Hudson followed a course different from its present route between the Tappan Zee and the Atlantic. It crossed the Palisades at the Sparkill Gap, located a few miles south of the Tappan Zee Bridge, and continued southwest across the Watchungs near Paterson, New Jersey. The river then paralleled the Watchungs south for about 15 miles (24 km) before turning eastward and re-crossing them on its way to the ocean at what is today Delaware Bay.
During the early part of the Ice Age some 2.5 million years ago the Hudson was diverted and flowed south across Queens to reach the Atlantic. Its present course east of the Palisades was eroded by the last southward push of the Pleistocene’s continental glaciers.
Shaped by the Ice Sheets
At the peak of the final episode of glaciation, the metropolitan New York area was buried under as much as 3,000 feet (914 m) of ice. So much water was frozen in ice sheets worldwide that the Atlantic Ocean was about 400 feet (122 m) lower than it is today.
Debris eroded by the ice sheets was piled up at the limit of their southward advance. At this terminus the rate of melting equaled the speed of advance; like Cinderella, left without conveyance as her carriage turned back into a pumpkin, the debris was dropped, forming rows of low hills, as ice turned to water. A ridge of such deposits, called a terminal moraine, constitutes the backbone of Long Island and Brooklyn. This ridge extended to Staten Island, damming meltwater from the ice sheets to form Glacial Lake Albany.2 Its outlet to the sea went through Hell Gate and Long Island Sound until about 16,000 years ago, when huge volumes of water flooded into Lake Albany from another glacial lake in the Wallkill River valley. This event breached the moraine to create the Narrows between Brooklyn and Staten Island.
With sea level much lower at that time, the Hudson flowed an additional 120 miles (193 km) across a wide coastal plain to the Atlantic Ocean. A submerged valley running across the continental shelf—a feature not seen off other East Coast rivers—marks its route. It was deepened by torrential flows of glacial meltwaters from the prehistoric Great Lakes and Lake Champlain, their St. Lawrence River outlet blocked by the ice sheets. These flows carried great loads of sediment—the remains of rock and soil pulverized by the moving ice. Reaching the edge of the continental shelf, these surges of meltwater and accompanying turbidity currents (underwater landslides generated by the buildup of sediment deposits) carved an even deeper gorge—the Hudson Submarine Canyon—southeast of New York Harbor.
Further north, rivers swollen by meltwater carried gravel and sand into Lake Albany and other glacial lakes that pooled behind moraines, glacial debris, and melting ice masses in the region's valleys. As they entered the lakes and their currents slowed, the rivers dropped this material to form deltas. Much of Croton Point is such a delta, built up by the ancestral Croton River.3 In the still waters of the lake, tiny particles of rock flour—soil and rock ground up by the glaciers—gradually settled to the bottom, forming deep beds of clay. These beds later supplied the raw material for the brick industry that flourished along the Hudson.
As the post-glacial thaw continued, sea level rose. About 12,000 years ago, the ocean reached the Narrows and seawater pushed into the Hudson. Sea level rise tapered off globally some 6,000 years ago and over most of the last 2,000 years remained fairly stable. However, it started upwards again in the late 1800s, an increase linked to climate change.
Rising Seas, Rising River
Today’s rising sea levels are mainly the result of global warming due to surging levels of atmospheric carbon dioxide (CO2) from burning fossil fuels. Carbon dioxide traps heat generated by sunlight, keeping it from radiating back into space. This phenomenon, known as the greenhouse effect, has elevated sea level in two ways. Oceans have warmed, and as water warms it expands. This thermal expansion accounts for about one third of sea level rise over the last century. Meltwater from shrinking land-based glaciers accounts for the other two thirds.
Globally, sea level has risen eight inches since 1880, and more along the U.S. Atlantic Coast. Data from the National Oceanic and Atmospheric Administration’s tide gauge at the Battery reveal that rising sea level has pushed the Hudson up 12 inches over the last hundred years, and since the start of this century the rate of rise has been increasing. New York State predicts that water levels in the lower Hudson may be 2 to 6 feet (0.6 to 1.8 m) higher by 2100, depending on what happens to CO2 emissions and the rate of melting in the world’s two largest reservoirs of land-based ice, Greenland and Antarctica.
A River That Flows Two Ways
The breach of the terminal moraine at the Narrows and rising sea levels opened the Hudson to the ocean’s influence. The most visible evidence of the Atlantic Ocean’s sway over the river are high and low tides and their accompanying tidal currents.
Tides occur in patterns set by a celestial dance involving the earth, the moon, and, to a lesser extent, the sun. The most obvious of these patterns is the daily tidal cycle along the Atlantic coast, in which two high tides and two low tides occur over roughly twenty-four hours. A simple overview of how tides work will be helpful in understanding the Hudson.
The Pull of the Moon . . .
The moon is large enough and near enough to exert considerable gravitational pull on the earth. In the oceans, this attraction literally causes water to bulge out towards the moon. This bulge remains positioned under the moon (slightly behind it due to inertia and friction) as the earth spins on its axis. Thus, while beachcombers on the Atlantic coast watch the moon rise, they are being inexorably carried into a mound of water, evident in the rising tide lapping around their feet.
The water in this bulge is also pulled horizontally as the earth rotates under the moon. As the tidal bulge moves into New York Harbor and past the Battery on Manhattan's southern tip, strollers gazing out at the Statue of Liberty might notice not only that the water is rising along pilings lining the shore but that the Hudson's current is pushing northward, in from the sea towards the mountains.
Hours later, after the beachcombers and strollers have gone to bed, the Atlantic coast has passed under the moon and reached the backside of the bulge. The tide is now falling, and the current at the Battery reverses and starts flowing toward the sea once more. The native peoples of the valley have a descriptive name for the river: Muhheakantuck, often loosely translated as “river that flows two ways."4
A second tidal bulge forms at a point on the earth opposite the moon. Between the two bulges ocean levels are lower, resulting in low tides. Thus, in the twenty-four hours it takes the earth to spin around its axis, a given point on the Atlantic coast will usually experience two high tides and two low tides, one following the other roughly every six hours.5
. . . and of the Sun
In addition to this daily rhythm, tides vary cyclically over the twenty-eight day lunar month—the time the moon takes to circle once around the earth. The lunar month is marked by the phases of the moon. More extreme tides (higher highs and lower lows) occur when the moon is in its new or full phase; these are the spring tides. During the moon's first and last quarter, the range between high and low tide heights is minimal; these are the neap tides.
Spring and neap tides reflect the interaction of the sun's gravitational attraction with the moon's. One might expect the sun to have greater tidal influence because it is so much bigger than the moon. However, gravitational attraction decreases with distance. Since the sun is much further away from the earth, its effect in raising tides is only about half that of the moon's.
Up and Down, Back and Forth
The rise and fall of ocean tides affects the river all the way to the dam at Troy, 153 river miles (246 km) north of the Battery. In fact, the Hudson's maximum tidal range (the difference in level between average high and low tides) of 4.7 feet (1.4 m) is observed at Troy, due to the crest of the tidal wave being forced upward as it reaches this shallow and narrow section of the river. Tidal range is least along the mid‑Hudson, averaging only 2.7 feet (0.8 m) at West Point.
Like high and low tides, reversals in current direction follow roughly a six‑hour schedule. The current draining the river south towards the ocean is called the ebb; that pushing north from the ocean is called the flood.
The velocity of the Hudson's currents varies depending on the strength of tidal forces at a given time, location along the river, the volume of runoff entering the estuary, and weather conditions. Currents are swiftest near the George Washington Bridge (average flood 1.9 mph; ebb 2.6 mph) and further north around Catskill (flood 1.9 mph; ebb 2.4 mph).
An adventure like Huckleberry Finn's raft trip down the Mississippi would be quite a different experience on the Hudson below Troy. Instead of progressing steadily downstream, a rafter on the Hudson might admire the scenery while drifting southward on an ebb current for about six hours, then view the same scenery again as the flood current pushed the raft back upstream for the next six hours, and endure it yet again as the ebb current took over once more. The ebb current is generally stronger than the flood; thus our Hudson River rafter would eventually reach New York Harbor.
How long would the trip take? That depends on the flushing rate – the time it takes water entering the estuary at Troy to reach the harbor. The rate varies greatly, depending on freshwater runoff. In very dry summers with minimal runoff, the raft’s net movement downriver might only be 1.5 miles (2.4 km) per day. At that rate, it would take 102 days to float from the head of tide at Troy to New York Harbor. On the other hand, a major rainstorm can cause heavy runoff that presses against and shortens the duration of the flood current while strengthening the ebb. This speeds up the flushing rate, perhaps to 5 miles (8 km) per day, reducing the rafter’s trip to about 30 days.
At any given time different parts of the river will be experiencing different tides. Since the Hudson's tides are generated in the ocean, there is an increasing lag in the timing of a specific event as one moves away from the sea. The crest of a high tide which occurs at the Battery at 12:00 noon will not reach Poughkeepsie until about 4:30 P.M., and Albany around 9:00. The accompanying table shows lag times based on high and low tides at the Battery. Tide predictions are available for numerous locations along the river.
Storm Surge and Blowout Tides
These published tide predictions take into account the relative positions of the earth, moon, and sun, coupled to knowledge of how geography influences water levels and currents in a particular waterbody. These predictions cannot account for the effects of daily weather as they are made years in advance. For example, extreme rain and accompanying runoff will sometimes suppress the flood current in northern reaches of the estuary.6
Winds can have major impacts on currents and water levels. Strong easterly winds off the Atlantic, associated with nor’easters and hurricanes, can create storm surge—a bulge of ocean water pushing towards the coast and into the estuary. The record storm surge here occurred during Hurricane Sandy in 2012; water levels at the Battery were 9 feet (2.7 m) higher than expected. Given that the surge arrived at high tide—already a higher-than-usual spring tide—the resulting flooding was disastrous, submerging 55 square miles (142 km2) of New York City. It drowned subway and railroad tunnels and was responsible for most of the 43 deaths that occurred in the five boroughs. The surge then continued up the Hudson, causing severe damage in low-lying areas of communities many miles north of Manhattan.
At the opposite extreme are blowout tides—extremely low water levels—caused by strong and persistent northerly or westerly winds. Such winds can depress sea level by pushing water away from Atlantic coast; these lower levels then translate up the river.
The Hudson Estuary
Tides and storm surge are not the only ocean influence in the Hudson. Swimmers escaping summer's heat with a dip in the river at Croton, or sailors hit with a faceful of spray as they tack across the Tappan Zee might have their tastebuds surprised by the tang of salt water. They learn by experience that the lower Hudson is an estuary, a semi‑enclosed coastal body of water freely connected to the sea, in which salty seawater is mixed and diluted with fresh water running off the land.
The Salt Front
The fresh water of the upper Hudson estuary contains some salt. Nearing the ocean the river’s salinity rises above that background level at the salt front, the leading edge of seawater entering the river. While the term suggests a sharp line of demarcation, seawater at the front is greatly diluted and only slightly saltier than fresh water; the difference is not apparent to the eye or to most taste buds.
Ecologists studying estuaries defining water as fresh until salinity reaches 500 parts per million (ppm), also expressed as 0.5 parts per thousand (ppt).7 By contrast, in the open ocean salinity is approximately 35 ppt. These figures refer to total salinity—all the dissolved chemicals in seawater. Sodium chloride—common table salt—accounts for about 78 percent of the mix; magnesium chloride another 11 percent or so.
The Hudson estuary supplies drinking water for a number of communities, making the salt front’s location a matter of interest to them. In studies of salt front movements, the United States Geological Survey defined the front’s location as the point where chloride concentration reaches 100 ppm.8 The Survey measured chloride rather than total salinity because public health laws specify a limit of 250 ppm of chloride in drinking water supplies, mainly for reasons of taste. However, the sodium in seawater is also a concern; individuals on low-sodium diets must pay attention as the salt front nears drinking water intakes.
The salt front's position is chiefly controlled by the volume of freshwater runoff from the watershed, which tends to follow a seasonal pattern. Rainfall and melting snow and ice in spring send runoff surging against the salty ocean water, pushing it well downriver. As freshwater flow slackens during summer, salt water penetrates further upriver, only to be driven seaward again as rainfall increases in late autumn. In winter, extended periods of subfreezing weather may lock fresh water up as ice and snow, reducing runoff and allowing the salt front to creep upriver.
So how far up the Hudson does the salt front travel? In a year with typical amounts and patterns of precipitation, spring finds the salt front in the Tappan Zee. By late summer, salt water often reaches Newburgh Bay, 60 miles (97 km) north of the Battery. Dry spells reduce freshwater runoff, allowing the front to penetrate further north. In severe droughts it may push past Poughkeepsie to Hyde Park, over 80 miles (129 km) upriver. On the other hand, runoff from major rainstorms can push the front well below the Tappan Zee; after Tropical Storm Irene in 2011 the river ran fresh all the way to New York Harbor.
Salinity From Top to Bottom
One would expect salinity in estuaries to increase from the salt front toward the ocean. Less obviously, salinity often increases from surface to bottom at a given place in an estuary. Salt water is denser than fresh water; it spreads upriver underneath fresh water flowing seaward at the surface—a phenomenon called estuarine circulation. In an idealized model of an estuary, with a small, steady input of fresh water and no obstructions in the channel, gravity would draw the denser salt water upriver almost to the point where the river bottom rises to sea level. It is this circulation pattern—not tidal action—that moves the salt front up the Hudson.
In the Hudson estuary, such layering—called stratification—occurs in partial fashion according to runoff conditions, geographic location, and tide cycles. During times of high freshwater flow, salinity at the bottom is, on average, about 20 percent greater than at the surface. In low flow periods, the average difference is reduced to about 10 percent.
However, these averages mask much variation. Through the Highlands salinity is fairly uniform from top to bottom. The river's twisting course through the mountains and bottom irregularities like reefs and deep holes promote turbulence that disrupts the smooth flow of the layers and mixes them together. The Hudson's waters generally remain well‑mixed into Haverstraw Bay. From the southern end of the Tappan Zee to New York Harbor the channel straightens out, promoting stratification. In this stretch of the river salinities at the bottom can be three or four times greater than those near the surface.
The cycle of spring and neap tides also plays a role in stratification. The swifter currents typical of spring tides disrupt the layers, mixing the fresh and salt water layers. The slower currents during neap tides allow stratification to occur.
Salinity and Productivity
In many estuaries, the layering of outgoing fresh water over intruding salt water contributes to high biological productivity. The richest estuaries rank among the most productive ecosystems on the planet, matching tropical rain forests and coral reefs. They are typically more productive than either the freshwater systems draining into them or the oceans beyond their mouths. Estuaries frequently generate or collect more nutrients and organic material than they can use, a surplus eventually exported to nearby coastal waters.
In some estuaries, stratification of fresh and salt water promotes high productivity through creation of a nutrient trap. The landward movement of seawater underneath fresh water tends to keep nutrients in an estuary, and they do not sink readily into the denser lower layer. In addition, friction between the layers creates turbulence that keeps nutrients suspended in well‑lit surface waters. There algae and other tiny organisms can readily take advantage of these vital substances and grow in great abundance.
However, research suggests that this trapping phenomenon does not play an important role in promoting productivity in the Hudson; its most productive regions are not located where stratification of salt water and fresh water is greatest. Nonetheless, the Hudson estuary is clearly a productive ecosystem; the huge rafts of waterfowl floating on its surface and tremendous schools of striped bass that annually appear in its waters are eloquent evidence of this. Factors accounting for this wealth of life and the ways in which this biological productivity is defined and measured are the subject of the next chapter.
Notes
1. All depths in this book are referenced to average low tide level, also called mean low water.
2. Successive stages of Glacial Lake Albany, sometimes called Glacial Lake Hudson, occupied different portions of the Hudson Valley from New York City to Glens Falls.
3. Other deltas formed in Glacial Lake Albany near the present‑day sites of Newburgh, Kingston, Red Hook, Hudson, Kinderhook, Albany, and Schenectady.
4. Muhheakantuck is spelled in various ways including Muhheakunnuk, Mahicannittuck and Mahicantuck.
5. Because the moon is in motion, revolving around the earth, a complete tidal cycle actually takes twenty-four hours and fifty minutes. Imagine yourself on the spinning earth's surface, checking your watch as you pass directly under the moon. As you wait for the earth to whirl you around full circle, the moon is not standing still. It is moving ahead, towards the east as you view things, so that you will need more than twenty four hours to catch up. An additional fifty minutes is needed to put you directly under the moon again. For this reason, the timing of a given tidal event will fall back by fifty minutes each day, on average. For example, if low tide on Monday morning is at 9:00, low tide Tuesday morning would be at 9:50.
6. The flood tidal current should not be confused with flooding due to rain. Destructive floods of the latter sort do occur in the Hudson above the Troy dam; their effects are dampened by tidal action further to the south. Except in the highest floods, river levels south of Catskill fall within ranges determined by ocean tides.
7. Parts per million are often used to describe a chemical's concentration in a given substance such as water. Imagine one part per million as one letter of a million printed in a book. There are about 500,000 printed in this book; thus the letter “v” in volume would be one per million in a book twice the length of this one. Concentrations are also commonly expressed as milligrams per liter (mg/L); with very dilute solutions, the units are practically equivalent.
8. Water’s salt content is also expressed as specific conductivity, a measure of how well water conducts electricity. Conductivity increases as salinity goes up.
The Chapter in Brief
The Hudson River flows 315 miles—507 kilometers (km)—from Lake Tear of the Clouds in the Adirondacks to the Battery in New York City. Its course and shoreline topography result from erosion by water and glacial ice over the past sixty-five to seventy-five million years. The river is influenced by ocean tides to Troy, 153 miles (246 km) north of the Battery. Diluted seawater typically ranges upriver to a point between the Tappan Zee and Newburgh, depending on the volume of runoff from the Hudson’s watershed. The lower Hudson is an estuary, a type of ecosystem that ranks among the most productive on the planet.
The Hudson's Origins
To begin a study of the Hudson River at its source, lay out a map of eastern New York State and trace the blue line north from New York Harbor along the cliffs of the Palisades, under the ramparts at West Point, through the sunset shadows of the Catskill Mountains, past the capital city of Albany, and on into the Adirondack Mountains. There, at the confluence of two creeks near Henderson Lake, the name Hudson River disappears; the map offers the option of following Calamity Brook northeastward or the outlet from Henderson Lake westward.
Looking for the highest body of water feeding the Hudson, turn northeast and face the heart of the High Peaks region. Continue upward along Calamity Brook, the Opalescent River, and little Feldspar Brook to find, as Verplanck Colvin did in 1872, a tiny lake perched 4,322 feet—1,317 meters (m)—up on the southwest side of Mt. Marcy, New York State's highest peak at 5,344 feet (1,629 m). Colvin, an indefatigable explorer and surveyor of the Adirondacks, described his discovery as a "minute, unpretending tear‑of‑the‑clouds—as it were—a lonely pool shivering in the breezes of the mountains." Thus the Hudson's source was named—Lake Tear of the Clouds.
The Water Cycle
The clouds that so often cap the Adirondacks, the snow that falls on Mt. Marcy's shoulder, the raindrops that dimple the surface of Lake Tear, the fog that condenses in tiny droplets on spruces lining Feldspar Brook, and their union in the runoff that eventually becomes the Hudson—all are manifestations of a much larger "stream" of water. These visible forms are linked to water hidden in the ground and pulled up in the stems of plants to their leaves, from which it is transpired into the atmosphere. There it joins water vapor invisibly rising from great oceans and tiny puddles, moving with weather systems from continent to continent or from a valley to its bordering hills, and once more taking forms that we can see—clouds and precipitation. This unending movement of water, seen and unseen, constitutes the water cycle.
The water cycle is a circulatory system supporting life on earth much as arteries and veins support human existence. Like blood, water transports substances needed by living organisms in the Hudson and its valley. In our bodies, the heart is the pump that circulates blood through the system. In the water cycle, the sun’s energy evaporates water and moves it from place to place in the atmosphere, while gravity causes precipitation to flow as runoff across the land and as groundwater under the land's surface.
The Hudson’s Watershed
From Lake Tear of the Clouds to the Battery at the southern tip of Manhattan, the Hudson follows a course 315 miles (507 km) long. Joining it along the way are many tributaries, the largest being the Mohawk River, which flows in from the west at Cohoes. The area of land drained by the Hudson and its tributaries—the Hudson's watershed—totals 13,390 square miles (34,680 km2), mostly in eastern and northern New York State. Small portions of this area reach into Vermont, Massachusetts, Connecticut, and New Jersey.
Besides gathering rivulets into creeks and creeks into a river, the watershed sustains the Hudson ecosystem with essential nutrients that fertilize aquatic plants and with autumn’s faded leaves and other organic matter to be recycled in food chains. The watershed’s contributions are greatly influenced by human land use, sometimes with less desirable outcomes. These include pollution from—among many sources—factory pipes, parking lots, farm fields, malfunctioning septic systems, and suburban lawns.
The watershed, particularly the portion drained by the Mohawk, is also the major source of clay, silt, sand, and other sediment entering the Hudson. Some settles out in the river, becoming a foundation for the establishment of plant and animal communities, and some remains suspended in the water, giving the Hudson a muddy appearance. This turbidity is especially noticeable after major rainstorms, when the river resembles café au lait due to the volume of sediment in runoff. Particles of sediment settle and fill in certain stretches of the river, notably at Haverstraw Bay and from Kingston north to Albany. In these areas, the ship channel must be maintained by dredging—digging out bottom mud—since under federal law, the channel between New York and Albany must be kept at least 32 feet (9.8 m) deep. This allows large barges and ocean‑going ships to reach the Port of Albany.
An Arm of the Sea
In length and watershed area the Hudson does not rank highly among American rivers. Yet numbers do not tell the full story, as one can appreciate when gazing across wide bays at Newburgh, the Tappan Zee, and Haverstraw, the latter being where the Hudson is widest—about 3.5 miles (5.6 km) east to west.
Such expansive grandeur results from the fact that for nearly half its length, the Hudson is an arm of the sea. In plunging over a dam at Troy, the river falls to a level only a few feet above that of the Atlantic Ocean, entering a long narrow trough in which its flow is governed less by the pull of earth's gravity than by the pulse of ocean tides responding to the gravity of the moon.
The lowest portions of the Hudson's valley south of Troy were drowned when sea level rose at the end of the most recent ice age. The deepest spot known is at World's End near West Point, where the most recent riverbed mapping found bottom 177 feet (54 m) down.1 The Hudson's gorge through the Highlands is its deepest stretch, with many charted depths over 100 feet (30.5 m).
The Hudson Fjord
With its great depths and cliffs slanting steeply into the river, the Hudson's route through the Highlands reminds many observers of the scenic fjords of Norway. Fjords are troughs eroded below sea level, often to great depths, by glacial ice. They are deepest not at their mouths but upstream, where the ice was thickest and its erosive power greatest. A shallower, less eroded sill of bedrock is usually present at their mouths.
The bedrock underlying the lower Hudson is largely buried below layers of sediments, some deposited by the river, others dropped by the ice sheets or their meltwaters. These deposits fill a much deeper gorge scoured out by the glaciers. Drilling during construction of aqueducts and bridges across the river found that the deepest portions of this gorge lie at least 750 feet (229 m) below sea level at the northern entrance to the Highlands, and 740 feet (226 m) down at the Tappan Zee. Nearer the ocean, geologists have found a shallower sill: bedrock is less than 200 feet (61 m) down at the Verrazano Bridge. Thus, this portion of the Hudson qualifies as a fjord.
Of Time and River Flowing
The handiwork of water and ice over millions of years created a waterway which offered immense advantages to the humans who eventually settled in the Hudson Valley. In a time before railroads and interstate highways, water‑based transportation was the fastest, most comfortable, and most capacious way of moving goods and people long distances. The river's surface is unbroken by rapids or waterfalls for over 150 miles (241 km) inland. Its gorge through the Highlands is the only sea‑level passage through the Appalachian Mountain range. This fact figured prominently in DeWitt Clinton's vision for a water route linking America's expanding west to its Atlantic coast, a vision realized in the Erie Canal.
The river's general course was probably set starting sixty-five to seventy-five million years ago, long before the great glaciers’ advance. The region's landscape at the time was very flat, similar to that seen today near the coasts of the southeastern states. The rocks that would later be sculpted into the Highlands and Palisades were buried under coastal plain sediments, over which the ancestral Hudson made its way to the ocean.
Over many millennia the river wore away those sediments, exposing and cutting into bedrock. Gaps carved into the ridges of the Palisades and New Jersey's Watchung Mountains suggest that the lower Hudson followed a course different from its present route between the Tappan Zee and the Atlantic. It crossed the Palisades at the Sparkill Gap, located a few miles south of the Tappan Zee Bridge, and continued southwest across the Watchungs near Paterson, New Jersey. The river then paralleled the Watchungs south for about 15 miles (24 km) before turning eastward and re-crossing them on its way to the ocean at what is today Delaware Bay.
During the early part of the Ice Age some 2.5 million years ago the Hudson was diverted and flowed south across Queens to reach the Atlantic. Its present course east of the Palisades was eroded by the last southward push of the Pleistocene’s continental glaciers.
Shaped by the Ice Sheets
At the peak of the final episode of glaciation, the metropolitan New York area was buried under as much as 3,000 feet (914 m) of ice. So much water was frozen in ice sheets worldwide that the Atlantic Ocean was about 400 feet (122 m) lower than it is today.
Debris eroded by the ice sheets was piled up at the limit of their southward advance. At this terminus the rate of melting equaled the speed of advance; like Cinderella, left without conveyance as her carriage turned back into a pumpkin, the debris was dropped, forming rows of low hills, as ice turned to water. A ridge of such deposits, called a terminal moraine, constitutes the backbone of Long Island and Brooklyn. This ridge extended to Staten Island, damming meltwater from the ice sheets to form Glacial Lake Albany.2 Its outlet to the sea went through Hell Gate and Long Island Sound until about 16,000 years ago, when huge volumes of water flooded into Lake Albany from another glacial lake in the Wallkill River valley. This event breached the moraine to create the Narrows between Brooklyn and Staten Island.
With sea level much lower at that time, the Hudson flowed an additional 120 miles (193 km) across a wide coastal plain to the Atlantic Ocean. A submerged valley running across the continental shelf—a feature not seen off other East Coast rivers—marks its route. It was deepened by torrential flows of glacial meltwaters from the prehistoric Great Lakes and Lake Champlain, their St. Lawrence River outlet blocked by the ice sheets. These flows carried great loads of sediment—the remains of rock and soil pulverized by the moving ice. Reaching the edge of the continental shelf, these surges of meltwater and accompanying turbidity currents (underwater landslides generated by the buildup of sediment deposits) carved an even deeper gorge—the Hudson Submarine Canyon—southeast of New York Harbor.
Further north, rivers swollen by meltwater carried gravel and sand into Lake Albany and other glacial lakes that pooled behind moraines, glacial debris, and melting ice masses in the region's valleys. As they entered the lakes and their currents slowed, the rivers dropped this material to form deltas. Much of Croton Point is such a delta, built up by the ancestral Croton River.3 In the still waters of the lake, tiny particles of rock flour—soil and rock ground up by the glaciers—gradually settled to the bottom, forming deep beds of clay. These beds later supplied the raw material for the brick industry that flourished along the Hudson.
As the post-glacial thaw continued, sea level rose. About 12,000 years ago, the ocean reached the Narrows and seawater pushed into the Hudson. Sea level rise tapered off globally some 6,000 years ago and over most of the last 2,000 years remained fairly stable. However, it started upwards again in the late 1800s, an increase linked to climate change.
Rising Seas, Rising River
Today’s rising sea levels are mainly the result of global warming due to surging levels of atmospheric carbon dioxide (CO2) from burning fossil fuels. Carbon dioxide traps heat generated by sunlight, keeping it from radiating back into space. This phenomenon, known as the greenhouse effect, has elevated sea level in two ways. Oceans have warmed, and as water warms it expands. This thermal expansion accounts for about one third of sea level rise over the last century. Meltwater from shrinking land-based glaciers accounts for the other two thirds.
Globally, sea level has risen eight inches since 1880, and more along the U.S. Atlantic Coast. Data from the National Oceanic and Atmospheric Administration’s tide gauge at the Battery reveal that rising sea level has pushed the Hudson up 12 inches over the last hundred years, and since the start of this century the rate of rise has been increasing. New York State predicts that water levels in the lower Hudson may be 2 to 6 feet (0.6 to 1.8 m) higher by 2100, depending on what happens to CO2 emissions and the rate of melting in the world’s two largest reservoirs of land-based ice, Greenland and Antarctica.
A River That Flows Two Ways
The breach of the terminal moraine at the Narrows and rising sea levels opened the Hudson to the ocean’s influence. The most visible evidence of the Atlantic Ocean’s sway over the river are high and low tides and their accompanying tidal currents.
Tides occur in patterns set by a celestial dance involving the earth, the moon, and, to a lesser extent, the sun. The most obvious of these patterns is the daily tidal cycle along the Atlantic coast, in which two high tides and two low tides occur over roughly twenty-four hours. A simple overview of how tides work will be helpful in understanding the Hudson.
The Pull of the Moon . . .
The moon is large enough and near enough to exert considerable gravitational pull on the earth. In the oceans, this attraction literally causes water to bulge out towards the moon. This bulge remains positioned under the moon (slightly behind it due to inertia and friction) as the earth spins on its axis. Thus, while beachcombers on the Atlantic coast watch the moon rise, they are being inexorably carried into a mound of water, evident in the rising tide lapping around their feet.
The water in this bulge is also pulled horizontally as the earth rotates under the moon. As the tidal bulge moves into New York Harbor and past the Battery on Manhattan's southern tip, strollers gazing out at the Statue of Liberty might notice not only that the water is rising along pilings lining the shore but that the Hudson's current is pushing northward, in from the sea towards the mountains.
Hours later, after the beachcombers and strollers have gone to bed, the Atlantic coast has passed under the moon and reached the backside of the bulge. The tide is now falling, and the current at the Battery reverses and starts flowing toward the sea once more. The native peoples of the valley have a descriptive name for the river: Muhheakantuck, often loosely translated as “river that flows two ways."4
A second tidal bulge forms at a point on the earth opposite the moon. Between the two bulges ocean levels are lower, resulting in low tides. Thus, in the twenty-four hours it takes the earth to spin around its axis, a given point on the Atlantic coast will usually experience two high tides and two low tides, one following the other roughly every six hours.5
. . . and of the Sun
In addition to this daily rhythm, tides vary cyclically over the twenty-eight day lunar month—the time the moon takes to circle once around the earth. The lunar month is marked by the phases of the moon. More extreme tides (higher highs and lower lows) occur when the moon is in its new or full phase; these are the spring tides. During the moon's first and last quarter, the range between high and low tide heights is minimal; these are the neap tides.
Spring and neap tides reflect the interaction of the sun's gravitational attraction with the moon's. One might expect the sun to have greater tidal influence because it is so much bigger than the moon. However, gravitational attraction decreases with distance. Since the sun is much further away from the earth, its effect in raising tides is only about half that of the moon's.
Up and Down, Back and Forth
The rise and fall of ocean tides affects the river all the way to the dam at Troy, 153 river miles (246 km) north of the Battery. In fact, the Hudson's maximum tidal range (the difference in level between average high and low tides) of 4.7 feet (1.4 m) is observed at Troy, due to the crest of the tidal wave being forced upward as it reaches this shallow and narrow section of the river. Tidal range is least along the mid‑Hudson, averaging only 2.7 feet (0.8 m) at West Point.
Like high and low tides, reversals in current direction follow roughly a six‑hour schedule. The current draining the river south towards the ocean is called the ebb; that pushing north from the ocean is called the flood.
The velocity of the Hudson's currents varies depending on the strength of tidal forces at a given time, location along the river, the volume of runoff entering the estuary, and weather conditions. Currents are swiftest near the George Washington Bridge (average flood 1.9 mph; ebb 2.6 mph) and further north around Catskill (flood 1.9 mph; ebb 2.4 mph).
An adventure like Huckleberry Finn's raft trip down the Mississippi would be quite a different experience on the Hudson below Troy. Instead of progressing steadily downstream, a rafter on the Hudson might admire the scenery while drifting southward on an ebb current for about six hours, then view the same scenery again as the flood current pushed the raft back upstream for the next six hours, and endure it yet again as the ebb current took over once more. The ebb current is generally stronger than the flood; thus our Hudson River rafter would eventually reach New York Harbor.
How long would the trip take? That depends on the flushing rate – the time it takes water entering the estuary at Troy to reach the harbor. The rate varies greatly, depending on freshwater runoff. In very dry summers with minimal runoff, the raft’s net movement downriver might only be 1.5 miles (2.4 km) per day. At that rate, it would take 102 days to float from the head of tide at Troy to New York Harbor. On the other hand, a major rainstorm can cause heavy runoff that presses against and shortens the duration of the flood current while strengthening the ebb. This speeds up the flushing rate, perhaps to 5 miles (8 km) per day, reducing the rafter’s trip to about 30 days.
At any given time different parts of the river will be experiencing different tides. Since the Hudson's tides are generated in the ocean, there is an increasing lag in the timing of a specific event as one moves away from the sea. The crest of a high tide which occurs at the Battery at 12:00 noon will not reach Poughkeepsie until about 4:30 P.M., and Albany around 9:00. The accompanying table shows lag times based on high and low tides at the Battery. Tide predictions are available for numerous locations along the river.
Storm Surge and Blowout Tides
These published tide predictions take into account the relative positions of the earth, moon, and sun, coupled to knowledge of how geography influences water levels and currents in a particular waterbody. These predictions cannot account for the effects of daily weather as they are made years in advance. For example, extreme rain and accompanying runoff will sometimes suppress the flood current in northern reaches of the estuary.6
Winds can have major impacts on currents and water levels. Strong easterly winds off the Atlantic, associated with nor’easters and hurricanes, can create storm surge—a bulge of ocean water pushing towards the coast and into the estuary. The record storm surge here occurred during Hurricane Sandy in 2012; water levels at the Battery were 9 feet (2.7 m) higher than expected. Given that the surge arrived at high tide—already a higher-than-usual spring tide—the resulting flooding was disastrous, submerging 55 square miles (142 km2) of New York City. It drowned subway and railroad tunnels and was responsible for most of the 43 deaths that occurred in the five boroughs. The surge then continued up the Hudson, causing severe damage in low-lying areas of communities many miles north of Manhattan.
At the opposite extreme are blowout tides—extremely low water levels—caused by strong and persistent northerly or westerly winds. Such winds can depress sea level by pushing water away from Atlantic coast; these lower levels then translate up the river.
The Hudson Estuary
Tides and storm surge are not the only ocean influence in the Hudson. Swimmers escaping summer's heat with a dip in the river at Croton, or sailors hit with a faceful of spray as they tack across the Tappan Zee might have their tastebuds surprised by the tang of salt water. They learn by experience that the lower Hudson is an estuary, a semi‑enclosed coastal body of water freely connected to the sea, in which salty seawater is mixed and diluted with fresh water running off the land.
The Salt Front
The fresh water of the upper Hudson estuary contains some salt. Nearing the ocean the river’s salinity rises above that background level at the salt front, the leading edge of seawater entering the river. While the term suggests a sharp line of demarcation, seawater at the front is greatly diluted and only slightly saltier than fresh water; the difference is not apparent to the eye or to most taste buds.
Ecologists studying estuaries defining water as fresh until salinity reaches 500 parts per million (ppm), also expressed as 0.5 parts per thousand (ppt).7 By contrast, in the open ocean salinity is approximately 35 ppt. These figures refer to total salinity—all the dissolved chemicals in seawater. Sodium chloride—common table salt—accounts for about 78 percent of the mix; magnesium chloride another 11 percent or so.
The Hudson estuary supplies drinking water for a number of communities, making the salt front’s location a matter of interest to them. In studies of salt front movements, the United States Geological Survey defined the front’s location as the point where chloride concentration reaches 100 ppm.8 The Survey measured chloride rather than total salinity because public health laws specify a limit of 250 ppm of chloride in drinking water supplies, mainly for reasons of taste. However, the sodium in seawater is also a concern; individuals on low-sodium diets must pay attention as the salt front nears drinking water intakes.
The salt front's position is chiefly controlled by the volume of freshwater runoff from the watershed, which tends to follow a seasonal pattern. Rainfall and melting snow and ice in spring send runoff surging against the salty ocean water, pushing it well downriver. As freshwater flow slackens during summer, salt water penetrates further upriver, only to be driven seaward again as rainfall increases in late autumn. In winter, extended periods of subfreezing weather may lock fresh water up as ice and snow, reducing runoff and allowing the salt front to creep upriver.
So how far up the Hudson does the salt front travel? In a year with typical amounts and patterns of precipitation, spring finds the salt front in the Tappan Zee. By late summer, salt water often reaches Newburgh Bay, 60 miles (97 km) north of the Battery. Dry spells reduce freshwater runoff, allowing the front to penetrate further north. In severe droughts it may push past Poughkeepsie to Hyde Park, over 80 miles (129 km) upriver. On the other hand, runoff from major rainstorms can push the front well below the Tappan Zee; after Tropical Storm Irene in 2011 the river ran fresh all the way to New York Harbor.
Salinity From Top to Bottom
One would expect salinity in estuaries to increase from the salt front toward the ocean. Less obviously, salinity often increases from surface to bottom at a given place in an estuary. Salt water is denser than fresh water; it spreads upriver underneath fresh water flowing seaward at the surface—a phenomenon called estuarine circulation. In an idealized model of an estuary, with a small, steady input of fresh water and no obstructions in the channel, gravity would draw the denser salt water upriver almost to the point where the river bottom rises to sea level. It is this circulation pattern—not tidal action—that moves the salt front up the Hudson.
In the Hudson estuary, such layering—called stratification—occurs in partial fashion according to runoff conditions, geographic location, and tide cycles. During times of high freshwater flow, salinity at the bottom is, on average, about 20 percent greater than at the surface. In low flow periods, the average difference is reduced to about 10 percent.
However, these averages mask much variation. Through the Highlands salinity is fairly uniform from top to bottom. The river's twisting course through the mountains and bottom irregularities like reefs and deep holes promote turbulence that disrupts the smooth flow of the layers and mixes them together. The Hudson's waters generally remain well‑mixed into Haverstraw Bay. From the southern end of the Tappan Zee to New York Harbor the channel straightens out, promoting stratification. In this stretch of the river salinities at the bottom can be three or four times greater than those near the surface.
The cycle of spring and neap tides also plays a role in stratification. The swifter currents typical of spring tides disrupt the layers, mixing the fresh and salt water layers. The slower currents during neap tides allow stratification to occur.
Salinity and Productivity
In many estuaries, the layering of outgoing fresh water over intruding salt water contributes to high biological productivity. The richest estuaries rank among the most productive ecosystems on the planet, matching tropical rain forests and coral reefs. They are typically more productive than either the freshwater systems draining into them or the oceans beyond their mouths. Estuaries frequently generate or collect more nutrients and organic material than they can use, a surplus eventually exported to nearby coastal waters.
In some estuaries, stratification of fresh and salt water promotes high productivity through creation of a nutrient trap. The landward movement of seawater underneath fresh water tends to keep nutrients in an estuary, and they do not sink readily into the denser lower layer. In addition, friction between the layers creates turbulence that keeps nutrients suspended in well‑lit surface waters. There algae and other tiny organisms can readily take advantage of these vital substances and grow in great abundance.
However, research suggests that this trapping phenomenon does not play an important role in promoting productivity in the Hudson; its most productive regions are not located where stratification of salt water and fresh water is greatest. Nonetheless, the Hudson estuary is clearly a productive ecosystem; the huge rafts of waterfowl floating on its surface and tremendous schools of striped bass that annually appear in its waters are eloquent evidence of this. Factors accounting for this wealth of life and the ways in which this biological productivity is defined and measured are the subject of the next chapter.
Notes
1. All depths in this book are referenced to average low tide level, also called mean low water.
2. Successive stages of Glacial Lake Albany, sometimes called Glacial Lake Hudson, occupied different portions of the Hudson Valley from New York City to Glens Falls.
3. Other deltas formed in Glacial Lake Albany near the present‑day sites of Newburgh, Kingston, Red Hook, Hudson, Kinderhook, Albany, and Schenectady.
4. Muhheakantuck is spelled in various ways including Muhheakunnuk, Mahicannittuck and Mahicantuck.
5. Because the moon is in motion, revolving around the earth, a complete tidal cycle actually takes twenty-four hours and fifty minutes. Imagine yourself on the spinning earth's surface, checking your watch as you pass directly under the moon. As you wait for the earth to whirl you around full circle, the moon is not standing still. It is moving ahead, towards the east as you view things, so that you will need more than twenty four hours to catch up. An additional fifty minutes is needed to put you directly under the moon again. For this reason, the timing of a given tidal event will fall back by fifty minutes each day, on average. For example, if low tide on Monday morning is at 9:00, low tide Tuesday morning would be at 9:50.
6. The flood tidal current should not be confused with flooding due to rain. Destructive floods of the latter sort do occur in the Hudson above the Troy dam; their effects are dampened by tidal action further to the south. Except in the highest floods, river levels south of Catskill fall within ranges determined by ocean tides.
7. Parts per million are often used to describe a chemical's concentration in a given substance such as water. Imagine one part per million as one letter of a million printed in a book. There are about 500,000 printed in this book; thus the letter “v” in volume would be one per million in a book twice the length of this one. Concentrations are also commonly expressed as milligrams per liter (mg/L); with very dilute solutions, the units are practically equivalent.
8. Water’s salt content is also expressed as specific conductivity, a measure of how well water conducts electricity. Conductivity increases as salinity goes up.
Contents
Preface
Chapter 1: A Physical Overview of the Hudson
Chapter 2: Energy Flow and Nutrient Cycles in the Hudson
Chapter 3: The Hudson's Habitats and Plant Communities
Chapter 4: The Hudson's Invertebrate Animals
Chapter 5: The Hudson's Fishes
Chapter 6: The Hudson's Birds and Beasts
Chapter 7: Exploration, Colonization, and Revolution
Chapter 8: The Romantic River
Chapter 9: Industrialization and the Transformation of the Landscape
Chapter 10: Conservation and Environmentalism
Chapter 11: Resolving River Conflicts
Chapter 12: Is the Hudson Getting Cleaner?
Chapter 13: Climate Change and the Hudson
Afterword
Sources and Suggested Readings
Glossary
Acknowledgments
Index
About the Authors
Preface
Chapter 1: A Physical Overview of the Hudson
Chapter 2: Energy Flow and Nutrient Cycles in the Hudson
Chapter 3: The Hudson's Habitats and Plant Communities
Chapter 4: The Hudson's Invertebrate Animals
Chapter 5: The Hudson's Fishes
Chapter 6: The Hudson's Birds and Beasts
Chapter 7: Exploration, Colonization, and Revolution
Chapter 8: The Romantic River
Chapter 9: Industrialization and the Transformation of the Landscape
Chapter 10: Conservation and Environmentalism
Chapter 11: Resolving River Conflicts
Chapter 12: Is the Hudson Getting Cleaner?
Chapter 13: Climate Change and the Hudson
Afterword
Sources and Suggested Readings
Glossary
Acknowledgments
Index
About the Authors
Related Titles
