1. Estuary Essentials: The Influence of the Sea
Steve Stanne, Education Coordinator, Hudson River Estuary Program
Of Time and Rivers Flowing conference; the Wallace Center
December 1, 2016

2. Change in elevation of river surface: 1.5 meters (5 feet)
The Hudson estuary is influenced by events in its watershed and in the Atlantic Ocean. Scott Cuppett’s presentation covers the watershed influences; this presentation covers the influence of the ocean. Events out on the Atlantic can impact the entire estuary, from Manhattan Click past the Palisades, Click Haverstraw Bay, Click the Highlands, Click the Mid-Hudson Bridge and Poughkeepsie, Click the Catskills, and Click on past Albany to the dam at Troy. Click From Troy south the river’s surface drops 1.5 meters (about 5 feet).
Change in elevation of river surface: 1.5 meters (5 feet)

3. Average difference in elevation ~13”
Stations ~66 miles apart
Average difference in elevation ~13”
This graph compares daily average water elevations at HRECOS stations in Albany and Poughkeepsie over the year previous to this conference. Click The two sites are about 66 miles apart. Over the course of the year the average difference in elevation Click was about 13”. Water levels vary greatly from day to day, and at Albany there appear to be times of year when levels are higher or lower than average. However, Click at Poughkeepsie over the course of the year, there is no up or down trend because the Hudson here is at sea level.
Water elevation at Poughkeepsie fairly constant – sea level

4. Average change in elevation ~4.4”
During dry spells there is little runoff from the watershed. With low volumes of water entering the estuary at Troy, the Hudson drops close to sea level at Albany, Click only about 4 inches higher on average than it is at Poughkeepsie.

5. Albany & Schodack ~8 miles apart Average change in elevation ~8”
During high flows, with greater volumes of fresh water coming over the dam at Troy, the water level in Albany is higher Click – more than 16 inches above the average level in Poughkeepsie. However, the river’s surface loses elevation rapidly as one moves south. At Schodack Island Click – only 8 miles south of Albany – Click it has already lost 8 inches of elevation, almost half of the difference.

6. Given that the estuary’s surface is so close to sea level, ocean tides roll up the Hudson all the way to the Troy dam. High tide (here at Kowawese in New Windsor- Orange County) is followed by Click low tide about 6 hours later.

7. Tides in the Hudson estuary
There are two high and two low tides on the Hudson estuary and New York harbor on most days. The tides shown here were observed in Poughkeepsie, about half way between New York Harbor and Albany. Click. How are tides in Albany different from tides in Poughkeepsie? A given tide occurs later in Albany than at Poughkeepsie, as it needs time to travel up the Hudson. Tides in Albany (and also in NY harbor) tend to be more extreme than tides in the mid-Hudson – highs are higher, lows lower.
How do tides at Albany differ from tides at Poughkeepsie?

8. Spring/neap tide cycle
6.1 ft
6.1 ft
2.0 ft
3.8 ft
This graph covers nearly a month, the length of the lunar cycle. It shows water level as depth; the twice daily high and low tides are visible. Tide heights vary quite a bit. So does the tidal range – the difference between high and low tide heights. Tidal range varies based on the lunar cycle. Neap tides are less extreme – they occur at first and last quarter moons because the gravitational pulls of the moon and sun are at right angles then. Spring tides are more extreme – they occur at new and full moons, when the moon and sun are in a line with earth, and their gravitational pulls are working together. Click 4x to see the moon phases and the range (in feet) between the high and low tides at the times of neap and spring neap tides.

9. Tides and water levels in the ocean off New York are influenced by ocean storms and other events. This is a satellite photo of Hurricane Sandy, which had major impacts along New York Harbor and the Hudson.

10. Sandy’s strong winds and low atmospheric pressure resulted in major storm surge flooding. This graph from NOAA’s tide gauge at the Battery, Manhattan’s Island’s southern tip, shows that water levels were almost 10 feet higher than the levels predicted based on standard astronomical factors. It also shows that the storm surge arrived right at high tide, making its impacts even more severe.

11. South Ferry subway station, NYC. Image credit: MTA
Blackout and flooding images by Hudson River Park Naturalist Keith Michael
Subway tunnels flooded, NY Harbor flowed into buildings along the waterfront, and lower Manhattan lost electrical power.

12. Sandy’s storm surge peak continued past Manhattan and on up the Hudson to Poughkeepsie and eventually to Albany.

13. Low-lying areas along the Hudson were hit by the flooding too; this yacht club in Newburgh (Orange County) was heavily damaged.

14. This graph from the NOAA station at the Battery shows the opposite effect from Sandy’s winds. The actual water levels (shown in red and called preliminary because they hadn’t yet been reviewed for accuracy) are lower than predicted. This is also caused by winds over the Atlantic Ocean.

15. This graph shows data recorded by a NOAA buoy located on the ocean off the mouth of New York Harbor. Notice how – starting on 11/20 – the wind blows steadily from the west – 270 degrees on a compass dial. Click On that same day the wind speed also quickly reached 35 knots – about 40 miles per hour. These wind conditions went on for two days. Blowing away from the coast, they pushed water out to sea, lowering water levels in New York Harbor and on up the Hudson as well.

16. These strong, sustained north and west winds…
These winds can cause a phenomenon known as a blowout tide, during which water levels are much lower than usual.
…can cause “blowout tides.”

17. Mahicantuck: River That Flows Both Ways
One of the native American names for the river was Mahicantuck (spelled various ways – Muhheahkantuk is an example). A popular translation of this name is “River that flows both ways.” The slide on the left shows the flood tidal current flowing upstream at the Walkway; Click the slide on the left shows the ebb current flowing downriver.

18. Tidal currents in the Hudson are not very swift – a few miles per hour at most, on average. However, swimmers and boaters in non-motorized craft need pay attention to the timing of these currents. For instance, passengers on the sloop Clearwater often ask why the sailboat seems to be staying in one place when it is sailing into the tidal current and beating (tacking) into the wind. Though the boat may be sailing briskly forward through the water, the water is carrying it in the opposite direction. Thus it is making no progress up or down the river.

19. 0 = slack water; negative # = flood current; positive # = ebb current
maximum ebb
slack before flood
slack
before ebb
maximum flood
0 = slack water; negative # = flood current; positive # = ebb current
Though this graph has the twice daily, up and down pattern of the rising and falling tide, it is not a plot of water elevation but rather of current velocity. At times when it intersects the 0 gridline, the water is not moving; it is paused between ebb and flood. This point in the tidal current cycle is called slack water Click – slack before ebb or slack before flood. As the plot moves into positive territory above the zero line, the current is picking up speed and flowing downriver – an ebb current. The point at which the plot peaks is the time of the swiftest flow downriver, Click called maximum ebb. The current then slows, and the plot descends back to the zero line Click – the slack before flood. As the flood current begins to push upriver, the plot descends into negative territory. The point at which the plot reaches its lowest point is the time of the swiftest flow upriver, Click called maximum flood. [These plots were make by downloading data from the U.S. Geological Survey station on the Hudson just south of Poughkeepsie into Excel. Contact Steve Stanne if you have questions about how to do this.]

20. 0 = slack water; negative # = flood current; positive # = ebb current
maximum ebb
slack before flood
slack
before ebb
maximum flood
0 = slack water; negative # = flood current; positive # = ebb current
One might expect that the times of slack water and high/low tides would match; Click that is, that the slack before ebb would occur at the time of high tide, and slack before flood at low tide, as shown in this plot of tides. However, that is not the case. Click On this day high tide occurs about two hours ahead of slack before ebb; likewise, low tide happens about two hours ahead of slack before flood. This lag is caused by friction between moving water and the bottom and banks of the river, and also by inertia. Imagine riding a bicycle down a hill, getting to the bottom – low tide - and immediately starting up another hill. The momentum of your downhill glide will carry you some distance up the next slope even if you don’t pedal.

21. In addition to the tides, salt water from the Atlantic Ocean pushes up the Hudson and is increasingly diluted by fresh water as it moves upriver. The upper reaches of the Hudson estuary are tidal and fresh, Click inhabited by zebra mussels, largemouth bass, and other freshwater organisms. The estuary’s lower reaches become saltier as one nears the ocean, Click and are inhabited by bluefish, barnacles, and other organisms of brackish and marine waters. Near the limit of seawater influence, one can find both zebra mussels and barnacles on the same stick or rock.

22. salt front = 100 mg/L chloride
U.S. Geological Survey
salt front = 100 mg/L chloride
(0.181 parts per thousand total salinity)
U.S. Environmental Protection Agency
drinking water standard for chloride = 250 mg/L
primarily a taste and odor standard rather than a health issue
BUT
Industrial uses and people on low-salt diets may require lower salt concentrations
Atlantic Ocean off New York
18,820 mg/L sodium chloride (34 parts per thousand total salinity)
The salt front is the leading edge of dilute seawater entering the Hudson. The USGS defines it as being where the chloride concentration reaches 100 mg/L. Click That is lower than the drinking water standard set by EPA, and much, much less than salinity in the Atlantic Ocean Click off the mouth of the Hudson.

23. Summer/early fall – salt front near Beacon-Newburgh Bridge
Spring – salt front near Tappan Zee Bridge
Generally speaking, the salt front is located in the Tappan Zee in spring, as rain and melting snow increase runoff from the watershed, pushing the salt front towards the ocean. In late summer and early fall, seasons that usually have less rain and higher rates of evaporation, the salt front pushes upriver to Newburgh Click. Again, this is a general description; there can be great variation in the timing and extent of the salt front’s movements.

24. What Brings the Salt Up the River?
What brings the salt front up the river? Most people would say the tides. Here is a plot of tides over 10 days at Piermont. There is little difference in the height of the tides over this period. Click However, there is a notable increase in salinity during that time. The plot does show that the salinity increases and decreases over the flood and ebb tide cycles – saltier water moving past the sensor on the flood current, and fresher water moving past the sensor on the ebb current. However, this sloshing isn’t what brings seawater up the river.

25. The last graph showed salinity changes in response to tides
The last graph showed salinity changes in response to tides. It is more accurate to say that the salinity changes with the direction of flow of tidal currents. This graph shows current velocity at Poughkeepsie; positive numbers indicate ebb current, negative numbers indicate flood current. Again, there is little change in the current velocity over this time period. Click Plotting salinity over current velocity, however, a slight increase in salinity is visible during this time. Adding a trend line Click makes this more visible. There must be some other reason behind increase in salinity as the salt front moves upriver.

26. That reason is the density of salt water as compared to fresh
That reason is the density of salt water as compared to fresh. At the Hudson’s mouth, there is a huge supply of salt water, top to bottom. Since it is denser than the fresh water coming down the river, this salt water tends to flow under the fresh water, pulled upriver by gravity. This sets up a phenomenon called estuarine circulation, in which dense salt water flows upriver under fresher water flowing downriver at the surface.

27. Turbulence due to irregularities and obstructions in the channel breaks down stratification.
Imagine an aquarium with a divider down the middle, top to bottom. On one side of the divider, the aquarium is filled with fresh water - on the other, with salt water. Using food coloring, color the salt water green. Then slowly lift the divider out. What would happen? The dense salt water would flow under the fresh water, forming a green layer at the bottom. Does such layering, called stratification, occur in the Hudson? Yes, but there isn’t always a clean line between the layers. Click Irregularities in the channel cause turbulence which mixes the salt and fresh water, breaking down the layers.

28. Hudson is usually flushed rapidly, but currents weaker at times of neap tides
Another factor that influences the degree of stratification is the spring/neap tidal cycle. The Hudson estuary is typically flushed fairly rapidly, meaning that currents carry water and all it contains – nutrients, plankton, fine sediments – through the system rather quickly. However, at times of neap tides – when high and low tides are less extreme – the tidal currents also slow down. This can be seen in this plot of current velocities at Poughkeepsie. Around the time of neap tides (Aug 10-12), the current slow to only about 1.5 feet per second. At time of spring tides earlier and later in the month, the current speeds increase to 2.5 to nearly 3.0 feet per second, almost twice as fast.

29. During neap tides, weaker currents allow stratification.
During spring tides, strong currents promote friction between layers, causing turbulence and mixing.
Neap and spring tides influence stratification because there is friction between the layers of salt and fresh water. That friction is minimal during the slower currents typical of neap tides, so the estuary is often more stratified then. The stronger currents that occur during spring tides promote friction between the layers. Click That causes turbulence which breaks down stratification, mixing the salt and fresh water layers.
During neap tides, weaker currents allow stratification.

30. These physical phenomena have major influences on the Hudson’s plants and animals.
The physical forces of the tides and estuarine circulation have major impacts on the plants and animals of the Hudson estuary. Plants that live in the intertidal zone, like spatterdock, must be able to survive being out of the water for part of the day and submerged Click for the rest. Plants’ abilities to cope with tidal flooding varies, so they are grouped into zones depending on their tolerance for flooding. Click Fish like the Atlantic silverside move up and down the Hudson with the salt front. Click And tiny blue crabs, born in salt water in New York Harbor, manage to move far upriver by sticking close to the bottom and riding the flow of dense salt water there into the Hudson.

31. For more information, contact
Steve Stanne, Education Coordinator
NYSDEC – Hudson River Estuary Program/New York Water Resources Institute – Cornell University
21 South Putt Corners Road
New Paltz, NY 12561