Understanding Hudson River Basics

The Hudson is really two rivers: a freshwater river that starts in the Adirondacks and flows south to Troy, New York where it spills over the Federal Dam and a tidal estuary between Troy and New York City flowing both north and south, and rising and falling with the ocean’s tides. From Mount Marcy in the Adirondacks to the Federal Dam, the river’s elevation drops from over 4,000 feet to about 4 feet. From Troy to the Battery in NYC, a distance of over 150 miles, the river is barely above sea level. The Hudson River Estuary is also considered a fjord, a drowned river valley cut by a glacier. Some 20,000 years ago, a miles worth of ice lay on top of the Hudson Valley. The massive glacier cut through the mountains and eventually left a pile of debris as it retreated became Long Island. The fjord nature of the river is most evident in the Hudson Highlands near West Point. The estuary is where the Clearwater sails and where amazing diversities of life and habitat are found.

Read The State of the Hudson!  This is an illustrated, comprehensive report published in 2015 by the New York Department of Environmental Conservation’s Hudson River Estuary Program, covering a variety of current issues on the river.

Below is detailed information that is often covered in our educational stations during onboard educational programs.


Over 200 species of fish live in the Hudson watershed. Some fish are native to the Hudson like the white perch, Morone americana, while others, like the common carp, Cyprinus carpio, are invasive, unwelcome and sometimes destructive guests. The Hudson estuary is a great example of biodiversity. Giants like the Atlantic sturgeon, Acipenser oxyrinchus, patrol the river bottom eating shellfish and sometimes leaping into the air, much to the astonishment of onlookers. Others are tiny, like the three-spined sticklebacks, Gasterosteus aculeatus, who make nests by gluing together bits of vegetation.

The saltwater-freshwater nature of the estuary can prove challenging for fish, but some species can take advantage of this, spending part of their lives in the Hudson and another part in the ocean. Striped bass, American shad, and sturgeon spend much of their adult lives in the ocean, returning to the Hudson to lay their eggs (spawn). This strategy (used also by salmon, though there are no salmon in the Hudson) is called anadromous. American eels do the reverse, spending most of their lives in the freshwater part of the Hudson before returning to the ocean to spawn. This is called catadromous.

You can tell a lot about a fish by its adaptations. Adaptations are tools for survival. For a fish, these include the shape and size of its mouth, coloration, ability to process salt, lateral line, and fin structure. The lateral line is a distinct line along the side of some fish. Inside the line are cells with tiny hairs that are sensitive to movement in the water, sort of like built in motion detectors. Look for this feature at the fish station.

When you visit the fish station, try to figure out what the fish’s niche is. A niche is the role a species plays in its ecosystem. Some fish are generalists, like carp, while others are very specialized, like a seahorse. Some fish are apex predators at the top of the food chain (striped bass). Other species swim in schools, like killifish, and are often eaten by larger fish. Still others are scavengers, like catfish.

Hold the anchovies! Yes, the Hudson River has anchovies, though not the same ones found on pizza. Bay anchovies, Anchoa mitchilli, have eyes out of proportion to their heads, and a big overbite. They can open their mouths wide to strain the water for plankton. Plankton is anything living in the water that cannot swim faster than the current. Most plankton is small, although some jellyfish can reach dozens of feet long. Plankton is either phytoplankton (like a plant, photosynthetic) or zooplankton (like an animal). Plankton is a crucial part of the Hudson food chain.

Striped bass, Morone saxatilis, are predators that rule the top of the fish food chain in the Hudson, but they are only temporary visitors, spending most of their lives out in the ocean. American shad, Alosa sapadissima, also only visit the Hudson to spawn. Shad “roe” is considered a local delicacy, but shad numbers have dropped so low in the Hudson that fishing for them is no longer allowed.

What’s a hogchoker, Trinectes maculatus? We call them the “sole of the Hudson”. This little flatfish is the unofficial mascot of Clearwater. What is this fish’s niche? What are its adaptations? How did they get that bizarre name? Ask onboard.

Imagine a species of fish that has been around since the time of the dinosaurs, with bony ‘scutes’ instead of scales, and a mouth that looks like a vacuum hose. That’s the Atlantic sturgeon, the largest fish in the Hudson. Sturgeon eggs are valuable as caviar, an expensive delicacy. Once called “Albany beef” because they were so plentiful, but now fishing for sturgeon is no longer allowed. Why not?

Clearwater and many other river organizations are constantly monitoring and educating the public on our fish populations. Read current information on Hudson River fish research on Riverkeeper’s website.


Invertebrates Station

crabAnimals without backbones- in the Hudson, these can be tiny, Cyclops-like copepods or large blue crabs. The world of invertebrates is vast and diverse and they can serve as indicator species, telling us about the health of the ecosystem in which they live by their presence or absence. There are the mollusks, like oysters and clams. Crustaceans include crayfish, crabs, barnacles, and shrimps. The freshwater parts of the river are home to many insect larvae, which may eventually transform into adult mosquitoes, mayflies, midges, dragonflies, or beetles. Then there are shell-less worms, including leeches and clamworms.

At the Hudson River life station, the invertebrate you are most likely to encounter is the humble amphipod (Gammarus), often called a “scud”. Scuds look like tiny shrimp and scavenge along the river bottom, eating detritus, dead plant and animal material. A tremendous amount of energy enters the Hudson from dead plants and leaves washing in from the tributaries. Scuds do a great job of cleaning up the bottom of the river and they provide food for a host of fish species.

Copepods are probably the most numerous animal group on the planet. They are vital to river and ocean food chains, providing food to larval fish that are just getting started in life. They’re very small, but you can see them with the naked eye.

Are there any zebra mussels, Dreissena polymorpha, in your sample? These invasive mollusks got into the Hudson River accidentally when free-swimming larvae escaped from the ballast water of ships from Eurasia. Once in the Hudson, zebra mussels reproduced at a fast rate. They attach themselves to any hard surface, including boats, pipes, docks, and even the shells of native clams, by secreting byssal threads. Zebra mussels also filter the water, removing plankton and oxygen from the water. You will only find zebra mussels in freshwater. They are intolerant of salt.

The Hudson is known for its blue crabs Callinectes (“beautiful swimmer”) sapidus (“savory”). These crustaceans move into freshwater parts of the river during the summer. Their ten legs have different jobs: the first pair are the dangerous claws; the middle three pair are walking legs; the back legs are flattened, serving as paddles and making this crab an effective swimmer.

Chinese mitten crabs, Eriocheir sinensis, are a recent arrival in the Hudson. They are potentially an invasive species (any animal or plant that has arrived from another place and has become destructive in the Hudson). Mitten crabs are known for the hair-like tufts on their claws that resemble mittens. The exact purpose of these “mittens” is unknown. These crabs lack the swimming feet and are designed more for walking and burrowing.

History Station


History Station Video 

2009 marked the 400th anniversary of Henry Hudson’s voyage to the river that bears his name, but we know that the Hudson’s history dates back long before it was called the Hudson River. The sloop Clearwater is like a time machine in many ways. The boat is a replica of cargo vessels that sailed the river from the 1700s through the 1800s. They were the tractor-trailers of their day, and the Hudson was the highway that served as the lifeline for communities like Albany, Hudson, Kingston, Poughkeepsie, Newburgh and New York. Hudson River sloops like the Clearwater carried just about anything that needed to be brought up and downriver, from bricks, stone, and hay, to live animals, mail, and paying passengers.

Down below in the sloop’s main cabin you will get a taste of what it was like to live on a tall ship today and you’ll work through a timeline of Hudson history that stretches from the time of the Native Americans through the Dutch settlers, the English and the Hudson Valley of today.

Why were beaver pelts once used as the equivalent of money? How was whaling a big part of the Hudson River’s history? Were there whales in the Hudson River? What part of the river once was blocked by a giant chain and why? Why did the river sloops fade from history? What replaced them? Why did the Native Americans call the Hudson River “Muheakannatuck”?

From the Hudson River School painters to the beginning of the modern environmental movement, the Hudson has helped shape America’s relationship with its lands and waters.

The Hudson River has changed dramatically since Henry Hudson first sailed upriver in 1609. How will the river change over the next 20 years?

Navigation Station

Whether it is biking to work, walking to school, or giving directions to a friend, we use some form of navigation every day. When you come aboard Clearwater, you will find that our captain and crew do not rely on a GPS or digital chart plotter to navigate the ship. Instead, students, volunteers and crew alike use paper charts of the river, compasses, visual aids and simple math for river navigation. Lighthouses, floating buoys, bridges and even church steeples are navigational aids for determining the location of the vessel as well as direction and speed of travel.

Because the Hudson is a tidal estuary, the ever-changing current is often the most important influence on the boat’s progress along the river. On a three-hour sail, Clearwater might leave the dock with the tide going out, pulling us towards the ocean, only to return to the dock with the water flowing in the opposite direction. At times the strength of the current is so great that even with our sails full the boat moves backwards. When you are near the river, tossing a stick in the water is a great way to observe the direction and speed of the current. The amount of rainfall can affect the currents but they are primarily determined by the sun and moon’s gravitational pull, creating high and low tides. The tides can be predicted far into the future with great accuracy and these are published for mariners to consult in tide tables.

Wind in the sails propels Clearwater across the surface of the water. The speed and direction of this mighty force is constantly changing and affecting how fast and in which direction the boat will move. There are many ways to observe the wind’s activity; such as a wind speed indicator, surface water texture, a flag flying or the feel of it across your skin. The wind direction and strength will help the captain determine Clearwater’s course and when to make maneuvers such as, tacks or gybes.

Determining the water depth is also essential for sailing safely on the Hudson River. This can be done with a number of tools such as a sonar depth sounder or lead line. Clearwater was specifically designed for this river, so it goes only six and a half feet below the surface of the water with our centerboard up. With such a shallow draft, much of the Hudson is accessible to us, however, if we navigate into shallower water, the vessel would be resting on the bottom.

Water Quality Station

Water qualityWater Quality Station Video 

At the Water Quality Station, we discuss the state of the water and what affects it, and test some parameters such as dissolved oxygen, pH, salinity, and turbidity.




Every breath we take has contains about 21% oxygen. That’s a huge amount compared to what’s available to fish in the water. Even though water is made up of oxygen (H2O), that oxygen is not useful for breathing. Instead fish and other aquatic organisms must rely on oxygen that’s dissolved in the water. Oxygen in the water is measured in parts per million (or milligrams per liter). Four parts per million (4ppm) is considered the minimum amount of oxygen to sustain fish life. Some species, like trout, need much more while others, like carp, can survive at very low levels.

On Clearwater, we test the oxygen levels with a titration. You will add drops of an indicator solution to a prepared sample of Hudson River water and look for a color change. Once the sample turns from yellow to clear, the test is over. The total number of drops used to turn the sample clear represents the amount of dissolved oxygen (DO) in parts per million.

Just because the water may have 4 or more ppm of dissolved oxygen, that doesn’t mean it is healthy. What if the sample you tested had 5ppm but should have much more, maybe 9ppm? If the water isn’t holding all the oxygen it should be, that might be an indication of pollution, such as sewage. For more context, we take the water’s temperature. We know how much oxygen the water should hold at a given temperature. Colder water can hold more oxygen than warm water. Using a special chart and our DO and temperature data, we can see if the water is close to saturation, or if it should be holding more.

Sometimes sewage spills into the Hudson. That provides food for bacteria and the bacteria multiply, using up lots of oxygen thus lowering the levels in the water.


We will test the acidity/alkalinity level of the Hudson using a simple pH test. The lower the number, the more acid a sample is. Seven on the pH scale is neutral. Rainwater is normally slightly acid at around 5.5, but acid rain can be much more acid (around 4). Each point on this scale represents 10 times more acidity, so a pH of 5 is ten times more acid than a pH of 6 and a hundred times more acid than 7. This is called a logarithmic scale. What was the pH of your sample? The cool thing about the Hudson River is that the pH is always between 7 and 8. The limestone bedrock in the Hudson’s watershed creates a natural buffer, neutralizing any acid rain.


Even freshwater streams have some small amounts of salt dissolved from the land. Since the Hudson is an estuary, saltwater moves far upriver, creating something called the salt front, the leading edge of the salt upriver, where the measured amount of ocean salt (100 mg/l) can be detected in the river. You couldn’t taste this amount of salt in the water, but it’s an indication of the ocean’s presence. Saltwater is denser than freshwater and this tends to inhibit mixing. During the spring, with all of the snowmelt from the mountains, huge volumes of freshwater push the salt front down toward the ocean. By late summer, much less water flows into the Hudson, so the ocean saltwater gets a chance to push north. To find out where the salt front is today, click this link:

When finding the level of salinity onboard the boat you will be measuring in parts per thousand (ppt). You can observe the salinity at home by using Clearwater’s Hudson River Environmental Conditions Observation System sensor at ( salinity is measured in Practical Salinity Units. For our purposes the two can be used interchangeably.

This all proves a challenge to fish. Salt can determine whether or not a fish can live in a certain area. Some fish are not very salt tolerant and need freshwater. Some need to be in fairly salty water. But in the Hudson, there are fish that can adapt to varying levels of salinity. Eels, white perch, banded killifish, and hogchokers are all in that category. Some fish, like shad and striped bass, spend most of their lives in saltwater and only come upriver to spawn. This is called anadromous. Eels, on the other hand, spend most of their adult lives in the river, returning to the ocean to spawn. This is called catadromous.


This is a measure of total suspended solids (TSS), or, stuff suspended in the water. The Hudson is naturally turbid, or murkey. Much of this material washes in from the tributaries. It is mostly fine clay particles, so fine that the water keeps them in suspension. Too much turbidity is an indication of soil erosion. Hudson River fish generally have a high tolerance to turbidity. We can measure it using a Secchi disk or a turbidity tube. The more turbid the water, the harder it is for light to pass through.


In 2010, Clearwater will be equipped with sophisticated water quality monitoring equipment and be part of a river-wide network that measures water quality. The network is called HRECOS (Hudson River Environmental Conditions Observing System) and provides real time data, relayed back to the Internet, where anyone can access it. Visit HRECOS here:

Sail Physics Station

sail physicsWhat better way to learn about sail physics than by looking up at one of the largest mainsails in America?

Sails harness the energy of the wind . When a sail catches wind it acts just like an airplane wing, creating lift. This is a function of the sail and an airplane wing both being foils.

When air passes over a foil it must travel over a greater distance, at a greater speed creating lower air pressure on one side and over a shorter distance, at a slower speed creating high air pressure on the other.

Molecules in a high-pressure area like to move directly to a lower pressure situation. By walking to the leeward or rounder of the sail you can feel the air moving faster than on the windward or hollow side where it will feel less windy. This difference in pressure on either side of the sail is necessary for the Clearwater to travel across the surface of the water.

Below the surface of the water the keel, rudder, and centerboard become very important as the sails begin to catch wind and push the boat through the water. Without water flowing past these fins, Clearwater would slide sideways through the water. When water passes over the large surface area of these fins, they redirect the sideways motion of the boat into a more forward course, allowing us to sail closer to the direction the bow points. If you watch very carefully, you will notice our leeway; Clearwater still travels slightly sideways through the water.

In order to slow a sailboat, the wind must be spilled from the sails making the foil disappear. You will notice when the crew set or drop the mainsail, our captain keeps Clearwater head to wind in order to maintain air passing evenly over both sides of the sail. In this case, the sail is essentially a flag and will not propel the boat forward.

Another time you may see Clearwater head to wind is as the boat is turning through a tack or gybe. After the captain has determined the general direction in which he/she wants the vessel to travel, a number of zigzags might be necessary to arrive at the chosen destination. A tack is when the bow of the boat passes through the wind, a gybe is when the stern passes through the wind. A Hudson River Gybe is a dramatic event that you may be lucky enough to witness while aboard Clearwater.

Explore these two websites for more information on the physics of sailing:

Geology Station

Geology Station Video

The geologic history of the Hudson River and the Hudson River Valley is not only important in understanding the topography or the regions and the formation of the river as an estuary, but also in the development of the region and its invaluable place in history.

Rocks are classified based on their mineral and chemical composition, by their texture, and by the processes that created them. All rocks are made up of minerals. Minerals are a naturally occurring solid that have a specific chemical composition, ordered atomic structure, and specific physical properties. The Rock Cycle portrays the origin and formation processes of the three main types of rock: igneous, sedimentary, and metamorphic. We have examples of each type of rock found on the banks of the Hudson River for you to examine.

Hudson River Sloops carried rocks and minerals up and down the river. Iron ore, limestone, trap rock, emery, granite, and clay –  all natural resources found in the Hudson Valley. Bricks were made from glacial till deposits represented a huge industry in the Valley, we still find pieces of the red clay blocks near abandoned brickworks along the banks of the river.

Glaciers played a large part in the formation of the Hudson River Valley.  As glaciers moved across the landscape they picked up silt, sand, rocks, and boulders and later set them down in deposits of glacial till.

During the glacial era, a large amount of water was trapped in the massive ice sheets. Sea level was more than three hundred feet lower than it is today so the river flowed miles farther across the continental shelf before reaching the sea. As the mile high glaciers began to melt, Glacial Lake Iroquois was formed which eventually burst its ice dam and flooded the Hudson Valley. The new path of the river allowed seawater to enter the mouth of the Hudson making it a tidal estuary and burying the Hudson Canyon under water.

Simple Machines Station

mech advantage

Simple Machines are tools sailors’ use aboard Clearwater to help make our work easier. We use many simple machines every day to steer the ship, raise the sails, open buckets, cut wood, fix planks, and lift heavy loads. There are six different simple machines: a lever, a pulley, a screw, an inclined plane, a wheel and axle, and a wedge.

Even with these simple tools, you don’t get something for nothing. With each advantage these tools provide, we must give something up in return. For instance, Clearwater’s tiller is very long making it easier to turn the boat than if we had a short tiller. But what the crew gives up is a great deal of deck space. A longer lever makes a load feel lighter, but you have to push the end of the lever much farther than your load is ultimately transported. This is a dynamic example of the Mechanical Advantage formula: work = force X distance. Try it! When the captain gives you a tiller command, try pushing from the very end of the tiller and then from halfway down the tiller.

Mechanical Advantage is the factor by which a simple machine magnifies the force applied, essentially making you stronger! If you are able to lift 50 pounds to your waist without breaking a sweat and then use a lever that has a mechanical advantage of 2, you will be able to lift 100 pounds to your waist without breaking a sweat. It will take the same amount of work to lift twice as much weight with this simple machine. However, the end of the lever, just like our tiller, will move twice the distance of your waist to the ground.

Another great example of a simple machine used aboard Clearwater are the pulleys. Sailors call this useful tool a block. Blocks give us a tremendous advantage when raising the sails. When you come aboard, keep an eye out for the largest block and the smallest block. What you gain and give up by using a pulley is the same as with our tiller. We can haul up our 3,000 pound mainsail with a class of fourth graders, but it takes a long rope, or halyard. Imagine how long the rope would have to be for one lonely sailor to lift the sail alone, at some point team work outweighs mechanical advantage.

Simple Machines are tools for helping sailors work smarter rather than harder. How do you use simple machines in your life? Do you see opportunities for using your salty skills and making your work more efficient?