Explore the River:


Cary Institute scientists have been monitoring portions of the Hudson River since 1986.

Long before Englishman Henry Hudson explored it in 1609, this majestic river was well traveled by Native Americans. Located in New York State, the Hudson was America's most celebrated and commercially important waterway for 200 years, until the Mississippi Valley was settled. Its beauty has inspired countless writers and artists, including the painters of a movement called the "Hudson River School." The Hudson River flows south about 500 kilometers (km) (310 miles) from the Adirondack Mountains to New York Harbor.

The tidal freshwater portion of the river extends about 163 km (100 miles) from the Troy Dam just north of Albany to Haverstraw Bay, not far from New York City. The difference between high and low tide can be as much as 1. 6 m (5.2 feet), and when the tide floods in from the Atlantic Ocean it actually causes the river to reverse direction and flow North! Much of its currents do not come from downhill flow but from these twice-daily tides. As a result, water makes its way down the river quite slowly, taking about 126 days to travel from Troy to the Battery in New York City.

With an average depth of 9.4 meters (31 feet), the water is high in nutrients, fairly turbulent (or well-mixed) because of the tides, and quite cloudy (containing a lot of sediment) from all the mixing.

The lower portion of the Hudson River extends from just south of Newburgh, NY to the southern tip of Manhattan. It is an estuary, where salty seawater mixes with fresh. Some organisms, like the zebra mussel, can't survive in salty or brackish water. They live only in the upper, freshwater portion of the river, which is the focus of this site.

Zebra mussels have changed the fish community of the freshwater Hudson (illustration by Jan C. Porinchak).

The Zebra Mussel Invasion

Zebra mussels were first spotted in the Hudson River in May, 1991, and probably introduced unintentionally. The larvae are only visible through a microscope and can travel in very small amounts of water. The adults can "hitchhike" on marine equipment like boat trailers and outboard motors, and spread when they spawn in the new water body. Zebra mussels had invaded the Great Lakes in 1988 (probably traveling from Europe in the ballast water carried by ships), and Cary scientists reasoned that a Hudson River invasion was likely. Within 16 months of their introduction, the mussels had spread throughout the freshwater tidal Hudson ecosystem.

Scientists at the Cary Institute of Ecosystem Studies in Millbrook, NY, have been monitoring the Hudson's tidal freshwater ecosystem since 1986, several years before zebra mussels appeared. That was fortunate, because it's rare to have data about a river or lake from before and after an invasion. In this case, scientists have been able to document the effects on the river's ecosystem by comparing their unique before-and-after datasets.

Zebra mussels pump water through their gills to filter out particles of food (primarily phytoplankton).

Zebra mussels can live for four to five years, and can grow to about 4 cm (1.57 inches) in length. Females can lay over one million eggs in a spawning season, and populations can reach astonishing levels of 10,000 mussels per square meter — or higher. They attach themselves firmly to hard surfaces on the riverbed with thread-like strands called byssal threads, which makes them hard to dislodge. They also attach to pipes, the bottoms of boats, and even to native mussels, which cannot then survive.

Zebra mussels are filter feeders: they pump water through their gills to filter out particles of food (primarily phytoplankton). A large population of zebra mussels can filter a volume of water equal to all of the water in the Hudson River estuary every 1-4 days, removing so much plankton and particles that they upset existing food webs. The zebra mussel invasion caused large changes in the fish community of the tidal freshwater Hudson, the near extinction of native pearly mussels, and additional changes to the river's physical and biological characteristics.

Zebra mussel populations can reach astonishing levels of 10,000 mussels per square meter — or higher.

Humans are also affected. Zebra mussels clog water pipes, and power plants and businesses that use river or lake water have had to spend millions of dollars on removing them. The mussels can also damage engines, docks, and equipment, and their shells wash up in large numbers on beaches.

Ecosystems are highly complex, however, and can respond to an invasive species in unexpected ways. Although zebra mussels have few natural predators in North America, populations in the Hudson have declined somewhat in recent years. Scientists are working to figure out why. Can you help them? Can you find other ways in which the river has changed since 1992, when the zebra mussel arrived?