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Monitoring Our River’s Improving Health

The Hudson River has been on a remarkable journey. Once very polluted, water quality has vastly improved — the result of 40 years of environmental regulations and advocacy. This is great news for both River wildlife and the large community of boaters that rely on the Hudson River for recreation.

Hudson River Park is one of several organizations that monitors River health. At two locations, Pier 25 and Pier 84, the Park manages weather and water quality stations that continuously measure environmental conditions. Scientists and educators use this information to learn and teach about the environment.

The Park’s River Project has been working with experts to develop a dashboard that makes the collected environmental data accessible to broader audiences. Last year, we were thrilled to receive the “Innovate for Social Impact” award from Cantina, a highly regarded design innovation agency that — among other things — works with organizations to design user-friendly experiences involving complex technology. Through this partnership, and with technical support from Dr. Wade McGillis of Columbia University, we are proud to launch our new water quality model that uses actual and historic data to predict, in close to real time, occasions when water quality is degraded due to pollution events. The dashboard will support the Park’s stewardship and environmental education efforts while also helping non-motorized boaters make informed decisions when water quality conditions are poor.

We invite you to explore our new dashboard to learn more about physical and environmental factors affecting our local water environment. Thank you to all our partners, especially Cantina, Wade McGillis at Columbia University, NYS DEC, USGS and NYC Water Trail Association, for helping to make continuous water quality data more accessible and relevant to the public at large.

How much rain has fallen?

What is this measurement?

Rainfall is the measurement of wet weather accumulation. Hudson River Park tracks rainfall in 24-hour periods at two locations: Pier 25 and Pier 84. Rainfall is measured using an acoustic sensor that distinguishes the unique sounds of precipitation. Because snowfall is quieter than rainfall, the measuring devices are less accurate for snow than rain.

Why is it important?

Hudson River Park and others track rainfall in part because of how it relates to NYC’s Combined Sewer Overflow (CSO) system. During heavy rainfalls, some untreated wastewater is released into local waterways because our aged sewer infrastructure system cannot accommodate the greater volumes. Sewage discharges are a major source of harmful bacteria, pollutants and plastic debris in our waters. Tracking rainfall also helps us track the potential for flooding and other risks associated with climate change.

Heavy rainfall can overwhelm NYC’s wastewater treatment plants, adding sewage to our local waters.

How much rain and over what period of time are the two factors that influence the extent to which untreated rainwater and sewage may enter our local waterways through overwhelmed CSOs. In other words, the intensity of the storm matters. This means that when there are multiple CSO events paired with a lower monthly rainfall, we can infer that those individual events were less intense, with less rainfall per event.

In February 2021, New York City experienced one of the snowiest months on record, with almost 26 inches. Unfortunately, our sensors didn’ t capture this precipitation, because the sound of snow falling is harder for our acoustic sensors to measure than rain. Therefore, snow accumulation was underreported. Stay tuned while we explore other ways to better capture this information.

“Days with High CSO Risk” is based on a science model that uses a combination of actual and predictive information to estimate dates when bacteria levels may be negatively affected by CSOs.

How healthy is the River for humans?

What is this measurement?

Like most other urban areas around the world, much of New York City relies on a combined sewer system. Pipes normally carry water from our homes, businesses and street drains to sewage treatment plants. On days with heavy rain events, this network becomes overwhelmed, resulting in some sewage being diverted into our local waterways. During these precipitation events, known as combined sewer overflows (CSOs), bacteria and plastics are released into local waterways. Using a predictive model developed by scientists and data visualization experts, this chart shows days when CSO events are expected to have occurred.

Why is it important?

CSOs are the largest and most significant source of pollutants in the Hudson River. By releasing bacteria into waterways, CSOs affect water quality for both humans and wildlife. Providing information on CSO events in close to real time can help people make informed decisions about conservation measures and exposure to potentially contaminated water. Fortunately, water quality in the Hudson River is mostly very good!

Hudson River Park’s water quality supported indirect contact for recreation … out of the past … days.

The NYC Department of Environmental Protection is the local governmental agency with official responsibility for monitoring and providing information on water quality, including CSOs.

The water quality calendar displayed below is not an official report. It’s a science model that uses a combination of actual and predictive information to estimate dates when bacteria levels may be negatively affected by CSOs.

You’ll see that overall, harmful bacterial levels in Hudson River Park’s Sanctuary are mostly low. New York City is increasing the types and amount of its green infrastructure as one way to mitigate CSO effects. Additional green spaces like Hudson River Park also help advance this goal.

How healthy is the water for fish?

What is this measurement?

Dissolved oxygen is the amount of oxygen in waterways. Oxygen is created by plankton and aquatic plants conducting photosynthesis in the water. Some oxygen also dissolves into the water from the atmosphere. Within the lower Hudson River, a healthy level of oxygen is considered to be above 5mg/L.

Why is it important?

Almost all living things need oxygen to survive, including fish, plankton and other wildlife. Hudson River Park’s Sanctuary waters normally have an adequate level of dissolved oxygen for local organisms. If levels become too low for too long, River wildlife can become stressed.

When oxygen levels in water fall too low, fish can be adversely affected.

In the Hudson River, dissolved oxygen levels in the winter are higher because the river water is cold. Cold water holds more oxygen than warm water. This is because cold water molecules are more tightly packed and less energetic than warm water molecules, effectively trapping oxygen molecules.

Dissolved oxygen is typically lower in the summer because of warmer water temperatures. When there are low dissolved oxygen levels, you may see more sensitive fish species swimming at the surface where there is typically more oxygen exchange. During extended periods of warm water and low dissolved oxygen levels, fish die-offs can happen.

How cloudy is the water?

What is this measurement?

Turbidity is a measurement of water clarity. It tells us how much light passes through water. Suspended particles like algae and muddy sediment can make even clean water look murky. That’s why the Hudson River’s water around New York City typically looks brown, not blue. The Hudson River Sanctuary is a naturally turbid environment because of its muddy bottom.

Why is it important?

Turbidity is a visual cue that helps indicate the extent of large storm events and erosion on our River environment. The Hudson River’s daily tides and swift currents keep particles suspended throughout the water column year-round. After a big storm, the River’s turbidity increases, making the water look less clear, until the particles either settle or are flushed into the Harbor.

The lower Hudson River is a naturally turbid environment because of its muddy bottom and tidal conditions.

What is the relationship between turbidity and rainfall? Big rain events, including those well north of New York City, can create major impacts on turbidity. An influx of rainfall causes water to move more quickly — a factor known as the flow rate. Water turbidity increases because higher flow rates cause increased suspension of sediment from the bottom of the River.

Is too much turbidity ever bad? In its own right, no. However, high levels of suspended sediment can damage habitat. For example, in 2011, Hurricane Irene’s deluge of rain and flooding up river caused immense amounts of sediment to be transported down river, destroying a lot of the Hudson River Estuary’s remaining submerged aquatic vegetation.

Hudson River wildlife is well adapted to the River’s turbid conditions. Resident fish have evolved retinas and sensory nerves called lateral lines that run the lengths of their bodies to detect movement around them, even in low visibility conditions.

What is the current water temperature?

What is this measurement?

Temperature is a measure of heat and is recorded using a thermometer. Compared to other waterways, the Hudson River has one of the largest temperature ranges in the United States. During the late summer, temperatures can exceed 80°F, while during the winter, they can go below 32 degrees F. Some years, the River can even freeze over.

Why is it important?

Temperature has a direct relationship to many other river health factors, including dissolved oxygen, salinity and pH. Beyond those relationships, temperature is also an important variable when studying climate change in the Hudson River. This is because the overall warming of our waters influences which fish populations are migrating and living in the Estuary.

Each year, the Hudson River experiences a sixty degree change in temperature depending on the season.

Because of seasonal temperature differences, some fish populations within Hudson River Park vary significantly depending on the season. During the late summer, when temperatures peak in the low 80° F, tropical strays can be commonly sighted. These tropical fish, which include spotfin butterfly fish, jack crevalles and banded rudderfish, visit temporarily and then head to the Gulf Stream for warmer waters further south along the Atlantic coast.

With climate change warming our waters, fish species composition is also changing. During colder waters, species like the Atlantic tomcod, also known as the “frost fish,” should be visiting the Hudson River Estuary in abundance. With warming water temperatures, the tomcod’s presence has decreased in the last 30 years in the Hudson River. Today, the tomcod’s southern migration range is shifting north towards Cape Cod.

How salty is the water?

What is this measurement?

Salinity is a measurement of how much salt is dissolved into water. The Hudson River Sanctuary is an estuary where fresh water from the greater Hudson River Watershed to the north and salt water from the Atlantic Ocean to the south meet and mix to create brackish water. Tides, currents and seasonal weather patterns all influence salinity levels in the lower Estuary.

Why is it important?

Salinity impacts which fish species use the River. The Hudson River and other estuaries are among the world’s most important productive ecosystems, second only to rainforests, because of the variety of life they support. Estuaries support a wide range of fish species because the water is brackish. Learn more about native Hudson River Park fish species by visiting the Park’s Virtual Wetlab.

Tides and seasonal precipitation both impact salinity levels in the Hudson River Estuary.

Salinity is lower in the early spring because of snow melt north of New York City. Melted snow from fresh water systems, including the portion of the Hudson River north of Troy, makes its way south and decreases the River’s salinity in the Park. On the other hand, during summer months, increased air temperature, drought and surface evaporation lower water levels, causing the overall salinity to increase.

The Hudson River’s tidal cycle has an impact on salinity. With each of the two daily high tides, salty Atlantic Ocean water floods into the River and increases the salinity. Around six hours later, during an ebbing low tide, the salinity decreases when fresh water arrives from the north.

What is the River’s pH?

What is this measurement?

pH is a measure of how acidic or basic a liquid substance is on a scale of zero to 14. Acidic solutions have a lower pH, while basic solutions have a higher pH. Hudson River pH is relatively steady and is typically just above neutral, at approximately 7.8 on the pH scale.

Why is it important?

Worldwide, there is an important connection between climate change and pH in our waterways. The rampant burning of fossil fuels releases excess carbon dioxide into our atmosphere. Our waterways absorb this carbon dioxide, causing acidification – a reduction of pH. Acidification is a major health issue for our shellfish communities (like oysters).

Even small changes in pH can have detrimental impacts on River health.

In the Hudson River, pH is protected by a layer of rich calcium carbonate rock, or limestone, located beneath our muddy sediment – sometimes hundreds of feet beneath. This carbonate river bottom buffers against ocean acidification impacts and acts as an antacid, like Alka Seltzer.

Acidification matters to the Hudson River because even slight decreases in pH can hinder shell production for some of our most valuable shellfish and filter feeders including the Eastern oyster. Changes of even less than one on the pH scale can have a devastating effect, underscoring the importance of maintaining our current pH levels.

What is the current tide level?

What is this measurement?

Tides are the rise and fall in water height created by the gravitational pull between Earth and our moon. In NYC, we experience two high tides and two low tides daily. High tide cycles influence the entire lower Estuary, pulling salt water from the Atlantic Ocean all the way to the Troy Dam 150 miles north of NYC.

Why is it important?

The tide cycle helps move nutrients within the water system. Tides also “flush” the system, helping to dissipate pollutants that enter our waters. Tidal conditions influence migrating fish populations that travel up and down the Hudson River. Boaters must also understand the tides for safety. You can find evidence of changing tides on the Park’s piles and bulkhead.

The Hudson River Estuary experiences two high tides and two low tides daily with a 5 foot range between them.

The Hudson River was called Mahicantuck, or the “river that flows two ways,” by the native Lenape people, in recognition of the River’s impressive tidal exchange.

A flood tide brings salt water from the Atlantic Ocean into the Hudson River, while an ebb tide pulls water back south towards New York Harbor. Many migratory fish follow the flood and ebb tides when swimming up or down river.

Dashboard strategy, design and development by our partner Cantina