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Geologically, Tibbetts Brook cuts through soft Inwood marble on its way from Westchester County to Spuyten Duyvil, the waterway that flows between the Hudson and East Rivers. Before diving into a concrete conduit, it feeds the Van Cortlandt Lake. The resulting freshwater wetland is ecologically valuable, providing a home for many plant and animal species.

The value of wetlands is that they slow erosion, preventing flooding by retaining storm waters, filtering and decomposing pollutants, and slowing global warming by efficiently converting carbon dioxide into oxygen. New York City once contained 224,000 acres of freshwater wetland, however over the past 200 years, increasing demands of a growing metropolis have resulted in most of this land being filled or dredged. Only 2,000 acres of freshwater wetland remain in the city today.

Wetlands origins

The formation of wetlands can be traced back to the most recent ice age. A massive ice sheet called the Wisconsin Glacier advanced on New York City 75,000 years ago, pushing rock, soil, and boulders ahead. When the ice melted 17,000 years ago, water flowed to the sea, creating streams and rivers that slowly carved through rock. Large glacial fragments broke off, melted, and left depressions called kettles. If layers of fine silt and clay were deposited on the bottom of the depressions, the kettles collected water and ponds formed. Where waters were shallow or flowed slowly, seeds and spores were able to take root and flourish. Generations of plants grew and decomposed, building peat–rich sediments. As wind and water eroded the soil, the steep slopes grew gentler, slowing the passage of water. Plant communities diversified under these favorable conditions, attracting animals that fed on the plants. The sophisticated food web that developed brought advanced predators to the wetlands: snapping turtles, wolves, several species of hawks, and humans.

Wetland characteristics

Water levels in wetlands are variable, influenced by the underlying rock and soil makeup, rainfall, season, and ground water inputs. Despite these variable conditions, freshwater wetlands teem with life. Water lilies, swamp loosestrife, and algae grow at different water depths, almost closing the open water by midsummer. Wood ducks and mallards feed and nest among dense stands of aquatic plants such as cattail, buttonbush, arrow arum, and blue flag.

Observing wildlife behaviors, such as feeding, nesting, roosting, courtship, and territorial displays, exhibited in this environment is astonishing. As in any diverse neighborhood, there is plenty of “street life”: eastern kingbirds sing from the tree tops, like parents leaning down from their perches to call their young home; mallards glide out from swamp loosestrife stands and preen themselves in the crimson reflection of swamp rose mallows; shy, brightly–colored wood ducks swim in the shadows. Green herons and great and snowy egrets search for fish and frogs. Red–winged blackbirds cry from cattail heads and painted turtles sun themselves atop muskrat lodges, ignoring the belted kingfishers feeding above them and raccoons swimming by. Most of these creatures are carnivores, feeding on insects, crustaceans, and small fish. Mammals, with the exception of muskrats, hunt in the wetlands but live on its uplands or edges. Edge–living creatures form a wetland’s most stable community. A drought may eliminate the world of deep–water creatures and shallow water may be flooded, but the edges remain.

The once–extensive wetland forests that ringed the lowland were razed to build the nation’s first municipal golf course in 1895. Enlarged from 9 holes to 18 in 1914, Van Cortlandt Golf Course was once famous for its many unplanned water traps, relics of its wetlands origin. A narrow belt of wetland forest still survives along the Putnam Trail around the open water, where pin oak and red maple trees grow above Solomon’s seal, Virginia creeper, marsh fern, and sensitive fern. Predators like barred owls and red–tailed hawks sometimes hunt in Van Cortlandt’s wetlands.