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
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
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.
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.
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
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.
A wide variety of habitats and
animals have been discovered in the Great Swamp, including rare and endangered
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
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