The Friends of the Great Swamp
of southeastern New York State

The Great Swamp


Situated in a valley between two parallel ridges of the Hudson Highlands, the Great Swamp wetland drains 63,000 acres and spans two watersheds, divided at Pawling into a north and south flow.

The North and South flows of the waters of the Great Swamp

Learn More...

FrOGS Document Archive

The Great Swamp Fact Sheet

Animals of The Swamp



Threats to The Swamp

Trees and Plants

Primarily forested, the entire watershed area encompasses about 97 square miles. The Department of Interior named it a National Historic Landmark. It was declared a Critical Environmental Area by Dutchess and Putnam Counties, and an Important Bird Area by Audubon. It provides critical habitat for rare and endangered species of plants and animals, vital water filtration and flood control, and incredible scenic views for paddlers along its 14 miles of navigable waters.

The Great Swamp spans two watersheds, divided at Pawling into a north and south flow. To the north, it traverses the towns of Pawling and Dover via the Swamp River, then the Ten Mile River. It crosses into Connecticut, where it meets the Housatonic River and eventually flows into Long Island Sound.

The south flow traverses the towns of Patterson and Southeast via the East branch Croton River, then it flows into Westchester via the East Branch, then the East Branch Reservoir of New York City's Croton Reservoir System, making the Great Swamp important headwaters for New York City's water supply.

Located less than 70 miles from New York City, this vast and fragile swamp provides numerous benefits to residents of the Harlem Valley, including improved water quality, flood control, recreation, open space, and wildlife habitat.

Valuable Benefits of the Great Swamp

Improved Water Quality
Instead of rushing straight into a lake or river, the runoff from the watersheds of the Great Swamp flow into a wide wetland. As the water spreads out, it slows down, and the specialized wetland plants filter out pollutants while sediments have time to fall to the bottom of the swamp and out of the water supply.
Flood Control
The wide, fairly flat part of the Great Swamp (widest in Patterson, NY) is called a floodplain. Here, the wetland acts like a giant sponge to absorb rainwater and reduce what might otherwise be destructive flooding during rain events.
Recreational Opportunities
The Great Swamp offers recreational opportunities in every season, from hiking and snowshoeing to cross-country skiing, birding, fishing, kayaking and canoeing. Access to the Great Swamp with a boat can most easily be done at three specific places .
Open Space
Keeping large tracts of the Great Swamp undeveloped and unbroken provides beautiful view sheds and enhanced quality of life for people in Putnam and Dutchess Counties.
Wildlife Habitat
A wide variety of habitats and animals have been discovered in the Great Swamp, including rare and endangered ones.

History and Geology

Over a billion years ago, the Hudson Highlands were a towering mountain range of folded rocks formed from the edge of an ancient sea. Many seasons and great ice sheets wore them down to the rounded hills we see today. The ridges are made of hard gneiss and quartzite, but softer marble or calcium carbonate lies beneath the valley floor. The marble makes the water in the Great Swamp "sweet" (of a higher pH) rather than acidic, creating some unusual habitats for plants and animals.

Threats to the Great Swamp

Although protected by local, state, and federal regulations, the Great Swamp is constantly threatened.
Invasive Species
Quick-growing non-native species of plants (such as purple loosestrife and phragmites) threaten to choke out natives such as cattails, reducing biodiversity and destroying fragile habitat. In the forest understories and open meadows, autumn olive, Japanese barberry, multiflora rose, garlic mustard, and other aggressive plants have invaded.

Destructive insects and diseases have also been introduced and threaten the health of the Great Swamp watershed-- the Hemlock wooly adelgid and others. Citizen scientists are needed to keep a vigilant eye on the forests, report plant, insect and disease intruders, and when possible, help eradicate them.

Stormwater runoff can carry harmful substances into the Great Swamp from roads, driveways, construction sites, salt piles, farms, waste water and septic systems, mines, and other sources. Even too much lawn fertilizer, or fertilizing lawns just before a rain, washes harmful chemicals into the waters of the Great Swamp and threatens its health. By building in the wetland or adjacent areas, the ecological and hydrological functions of the Great Swamp are threatened and the recreation value is reduced. Development in this sensitive area must be very carefully controlled, planned and executed to allow the swamp to continue to provide the values it provides.